Too much timing?

[TJ's GMC]

So how much is too much on these big engines? Reason I ask is I had mine set at 8 degrees advanced and the motor was good there, but for some reason I am not getting any Vacuum advance. I can tell this by using a timing light and disconnecting and reconnecting the vacuum hose and it makes absolutely no difference at idle. My 292 I6 would sound completely different when I did that. So my engine has been relying on mechanical advance...which seems to be fine as it's very responsive and pulls nicely.
Today I bumped my timing up to 12 degrees advanced and the motor sounds even better...more responsive all around and seems to idle smoother. I haven't driven it yet, but was wondering...you guys think 12 is to much? I know the factory setting is 6 degrees, but I'm getting no vacuum advance at idle. Carb is a 600 cfm edelbrock and works fine. Engine vacuum is 18 what ya call its. lol

[Clarke]

TJ, check out my thread named "Stock 305D Dyno Results". (Sorry I don't know how to insert a tread link inside a post.) I'm still running mine the same as noted within the last post. I do a lot of towing too and it runs great; no pinging, etc.

FYI, I've had my truck for six or seven years and I have the same 'lack of vacuum at the distributor at idle' you've noted. I don't believe I have ever brought it up here, so I'm curious to see what you find out. My intake vacuum is 20.

All other engines I have worked on (I6 & 4 bangers), there is a noticeable difference when disconnecting the vacuum advance line.

[George Bongert]

So how much is too much on these big engines? Reason I ask is I had mine set at 8 degrees advanced and the motor was good there, but for some reason I am not getting any Vacuum advance. I can tell this by using a timing light and disconnecting and reconnecting the vacuum hose and it makes absolutely no difference at idle. My 292 I6 would sound completely different when I did that. So my engine has been relying on mechanical advance...which seems to be fine as it's very responsive and pulls nicely.
Today I bumped my timing up to 12 degrees advanced and the motor sounds even better...more responsive all around and seems to idle smoother. I haven't driven it yet, but was wondering...you guys think 12 is to much? I know the factory setting is 6 degrees, but I'm getting no vacuum advance at idle. Carb is a 600 cfm edelbrock and works fine. Engine vacuum is 18 what ya call its. lol


Greetings TJ! The diaphragm in your vacuum advance is either cracked, or has a hole in it which will not allow it to work. I don't think 12 dgrees advance is excessive, however, I would replace the vaccum advance and reset the timing to 6 to 8 degrees BTDC.

[TJ's GMC]

Thanks for the replies guys! 12 degrees ended up being just a tad too much as I noticed cold starts became more involved. lol So I backed the timing down to somewhere's between 8-10. I'll be sure to check the dizzy and see what's going on, FWIW, it is a brand new one from Autozone.

[TJ's GMC]

TJ, check out my thread named "Stock 305D Dyno Results". (Sorry I don't know how to insert a tread link inside a post.) I'm still running mine the same as noted within the last post. I do a lot of towing too and it runs great; no pinging, etc.

FYI, I've had my truck for six or seven years and I have the same 'lack of vacuum at the distributor at idle' you've noted. I don't believe I have ever brought it up here, so I'm curious to see what you find out. My intake vacuum is 20.

All other engines I have worked on (I6 & 4 bangers), there is a noticeable difference when disconnecting the vacuum advance line.

Alright, so I did a complete read up on that thread and that was some great info. Thanks for taking the time to write that up! Anyhow, When I had my timing set at 12 degrees advanced the first thing I noticed was quicker throttle response for sure. Motor even seemed to pull better, BUT when trying to cold start I was more involved and even when the engine was somewhat warm it was a bit harder to start. So I came home and backed the timing down to around 10. Need to drive it again and see what happens. Motor will probably be close to the same as before as 2 degrees more advanced doesn't make much difference. When advancing the timing I didn't get much of an RPM increase or decrease. I Do still need to check my vacuum advance as it made absolutely No difference at idle. My 292 would idle at 10 degrees without vacuum and 25 with vacuum....my 305 V6 idles at 10 with or without so something must be wrong.

[abus319]

The vacuum advance issue has been puzzling me as well.
I checked the 478 with a hand operated pump before I fired it up, and found that it didn't start to operate until about 24 inches. That got me curious so I checked the 305's and found about the same thing. Basically they never operate.
I hope someone can shed some light on this

[Clyde]

On the 305 initial timing is set at 7.5 degrees, the vacuum advance has a total of 18 degrees, starts to move at 5-7 inches of vacuum and achieves full advance at 12.25 - 13.5 inches. The centrifugal advance has a total of 30 degrees, 0-2 degrees at 800 rpm's, 11 degrees at 2000 rpm's and 30 at 3400 rpm's thus giving a potential of 55.5 degrees under full vacuum and an rpm of over 3400. On the 351 centrifugal advance is 26 degrees vacuum advance is 18 degrees and 5 degrees initial for a total of 49 degrees at 3500 rpm's. Hope this clears up any confusion, this is from the factory GMC shop manual.

[TJ's GMC]

Thanks Clyde! I'm going to check that dizzy on the engine and see if the pump or something is jammed. Cause I'm not getting advance at all. Recently I looked that the pump's arm while reving the motor and there was no movement at all.

[Clarke]

TJ, I haven't noticed any change on how it starts. I drove it several years with the timing set per spec and after I advanced the timing it's much more responsive, yet starts the same. I drive mine during all seasons, even if it's 15 degrees or 100 degrees.

I swung by O'Riellys on my way home tonight to get a new vacuum advance, but they couldn't find the part number, so I need to do some research. If I still have the orig one, I wouldn't be surprised if the diaphram is stiff.

When I was a kid rebuilding honda car engines, an old timer taught me to advance the timing a few degrees, because it would make the engine start quicker. I don't know if it's true or not, but I did it anyway. Typically, I stick to the specs, untill I started playing around with this 305.

[TJ's GMC]

I was told to retard the timing for easier starts. :lolsmack2:

Have the initial timing around 10 now and it seems to be a little easier starting. Still got nice throttle response. Weather is real bad so I won't be out there. Ya know I remember putting the vacuum advance off of my 292's dizzy and onto the 305....and it kept advancing and retarding the timing all by itself....I could watch the arm move back and forth and back and forth and the motor never found a happy medium. But this brand new dizzy in.....nothing....no movement period...I can rev the engine all the time...nothing.

[Clarke]

TJ, wild and crazy stuff!

[TJ's GMC]

TJ, wild and crazy stuff!

For sure. lol I'm gonna go out in the morning if the weather is better and do some checking.

[TJ's GMC]

Ok folks, so I have an update. Turns out my new distributor's vacuum advance pump has a hole in it or something as I can blow air through it. So I took the pump off of my 292's dizzy and I now have advance. On my edelbrock there's two vacuum ports...one is timed vacuum and the other is full constant vacuum...I tired the 2nd port first which put the idle advance at around 16 and Man....you talk about that motor really becoming smooth and quiet...it always had a cool lope to it, but once the timing advanced that far the motor was super smooth and quiet. Took it for a test run and it was much peppier and pulled a bit better( I need to get my manifold machined for that 4 barrel because the 4 down to 2 ain't doing it no more.) I then tried it on the 1st port and the engine seemed to like that best. The 1st port offers No vacuum advance at idle, but once you rev it the advance comes in....so I bumped the timing up to 10 degrees and ran it....seemed to be happier. I will be uploading a video that I'll post this afternoon guys. See ya.

[TJ's GMC]

Here's the video guys, still doing tuning:

https://www.youtube.com/watch?v=vUX6qsNt2dA

[Clarke]

Thanks for sharing TJ. Mine idles a little rough too and I don't have any vacuum advance at idle, so this will motivate me to fix it. :)

[TJ's GMC]

Thanks for sharing TJ. Mine idles a little rough too and I don't have any vacuum advance at idle, so this will motivate me to fix it. :)

Be sure to post your findings! :)

[Clarke]

Be sure to post your findings! :)

Update: I mentioned before that ever since I've had this truck, it doesn't matter if I disconnect the hard vacuum line to my vacuum advance; the timing remains the same and so does the idle.

Thanks to TJ, I decided to investigate instead of ignore it. Anyway, I disconnected the hard line from the back of the carb and hooked up my vacuum pump to the hard line. I tested the vacuum advance several times and obtained consistent readings. At 5 psi, I noticed the rod in the slot starting to move, and it would hold vacuum. At 14 psi, the rod would stop moving, and it would hold vacuum. I could pump it up a little higher, but the rod quit moving after 14 psi.

So I removed the reducer fitting from the back of the carb to inspect and it was clear. I also looked in the hole in the back of the carb with my inspection mirror and there were no obstructions.

Do my test results seem normal? Any suggestions on additional inspections? Maybe I need to dig a little deeper into the distributor? The mechanical advance weights/springs on the top side (under the rotor) are clean and move easily.

[George Bongert]

Update: I mentioned before that ever since I've had this truck, it doesn't matter if I disconnect the hard vacuum line to my vacuum advance; the timing remains the same and so does the idle.

Thanks to TJ, I decided to investigate instead of ignore it. Anyway, I disconnected the hard line from the back of the carb and hooked up my vacuum pump to the hard line. I tested the vacuum advance several times and obtained consistent readings. At 5 psi, I noticed the rod in the slot starting to move, and it would hold vacuum. At 14 psi, the rod would stop moving, and it would hold vacuum. I could pump it up a little higher, but the rod quit moving after 14 psi.

So I removed the reducer fitting from the back of the carb to inspect and it was clear. I also looked in the hole in the back of the carb with my inspection mirror and there were no obstructions.

Do my test results seem normal? Any suggestions on additional inspections? Maybe I need to dig a little deeper into the distributor? The mechanical advance weights/springs on the top side (under the rotor) are clean and move easily.

Greetings Clarke!

There's no need to dig into the distributor. That's not where your problem is. The fact that your vacuum pump causes the vacuum advance to move, and that it holds vacuum tells you that your vacuum advance is OK and not your problem. If you connect a vacuum guage to the fitting at the back of the carb, dimes to donuts you will not get a vacuum reading when the engine is running. Check the gasket between the carb and the intake manifold to see if the area in that gasket to that fitting is blocked off. Without being there and not being able to see things firsthand, that would be my suggestion at this point, because there is something that is stopping you from having vacuum to that fitting. Hope this helps!

[Clarke]

Thanks for the feedback George.

I'll hook my vacuum gauge to the port on the back of the carb to see if there's anything at all. If not, I'll pull the carb off and check the base as you suggested.

I had the carb rebuilt two years ago, so I would have thought it would have been addressed at that time, but no difference.

New base gaskets were installed at that time. I will check the oreintation of the gaskets in comparison to the bottom of the carb. If I have any questions, I'll post them.


Thanks again,

Clarke

[TJ's GMC]

Clarke:
That does sound normal. I didn't have a gauge on mine, but mine does the same. Give it a bit of throttle and the advance arms starts to move....give it more and to a point it stops. And since I'm running a 4 barrel that was on Ported Vacuum. I do need to get another video up. lol Been sick and busy so I haven't gotten around to it.

[George Bongert]

Clarke:
That does sound normal. I didn't have a gauge on mine, but mine does the same. Give it a bit of throttle and the advance arms starts to move....give it more and to a point it stops. And since I'm running a 4 barrel that was on Ported Vacuum. I do need to get another video up. lol Been sick and busy so I haven't gotten around to it.

Greetings TJ!

Sorry to hear that you've been a bit under the weather. And I'm sorry to "rain on your parade" so to speak, but you should have full vacuum at idle where your vacuum advance is connected to. That's why it is recommended when setting your ignition timing that the vacuum advance hose be disconnected from the advance and plugged (temporarily) with the butt end of a drill bit, or some other object to prevent the engine from having a rough idle while setting the timing. If you have vacuum, and your vacuum advance is working properly, you should notice a perceptible increase in engine idle RPM when the vaccum advance is reconnected. You definitely should not have to increase your engine RPM to get the vacuum advance to "move." You should also have no less than 15 inches (PSI is a misnomer here) of vacuum at the fitting that your vacuum advance is connected to. And not to make anyone sound dumb here, but the mistake is made by many people refering to a vacuum reading as "PSI." This was one of my Automotive Technology teacher's pet peeves--refering to vacuum as "PSI" instead of the proper terminology of "15 inches of vacuum."

[TJ's GMC]

I REALLY need to get another video up. LOL.
My 4 barrel carb has two vacuum advance ports. Ported and Manifold. Ported allows the engine to idle at initial timing and Manifold vacuum allows the engine to idle at full vacuum advance. I believe I Did post a video on this thread showing the differences. I hooked the advance pump to Manifold advance and the idle rpm increased noticeably and the engine was Much smoother, but I did notice some hesitation coming off idle. Now that probably could have been fixed by retarding the timing a bit. Where I have the motor currently is 10 degrees initial with the advance pump hooked to Ported timing. No advance at idle, but full vacuum advance while cruising down the highway. I may try manifold advance again and retard the timing back to factory specs and see how it likes it, but so far it's been happy. :)

[Clarke]

Thanks George, for the clarification on the proper terminology. As I was catching up on messages, I realized I was making the same mistake, and know better. :)

[George Bongert]

Thanks George, for the clarification on the proper terminology. As I was catching up on messages, I realized I was making the same mistake, and know better. :)


No problem Clarke! I think you will find that your engine will run much smoother and operate more efficiently when your vacuum advance is working properly! :thumbsup:

[Clarke]

Update:

With the steel vacuum line (for vacuum advance) disconnected from the back of the carb, I connected my vacuum gauge to the back of the carb and here are the results:

0.8 inches @ 500 RPM
2.0 inches @ 800 RPM
5.0 inches @ 900 RPM

When I tested my vacuum advance the other evening, it started to move at 5.0 inches.

My vacuum reading off the back of the intake manifold (at idle) is consistently 19 inches.

Based on what I provided above, sounds like I'm losing vacuum in the carb?

[George Bongert]

Update:

With the steel vacuum line (for vacuum advance) disconnected from the back of the carb, I connected my vacuum gauge to the back of the carb and here are the results:

0.8 inches @ 500 RPM
2.0 inches @ 800 RPM
5.0 inches @ 900 RPM

When I tested my vacuum advance the other evening, it started to move at 5.0 inches.

My vacuum reading off the back of the intake manifold (at idle) is consistently 19 inches.

Based on what I provided above, sounds like I'm losing vacuum in the carb?


Hello again Clarke!

What you are describing would be the "ported" vacuum connection that TJ has referred to. On some carbs there are multiple vacuum ports, and most notably on GM (Rochester) carbs, there is a vacuum port on the throttle body, and that is the one that the vacuum advance is connected to since at that location you will have full intake manifold vacuum. Since the GMC Big Block V-6's don't use (for the most part) GM Rochester carbs, your best bet is to tap into the intake manifold to get full vacuum to the vacuum advance. Check your carb to see if there is a port in the throttle body that you can hook up to. You mentioned that your intake vacuum is 19 inches at idle which is good, and if you can get that full vacuum to your vacuum advance, your engine will purr in appreciation! :happy:

[Clarke]

Great idea George!

I'm still running the Stromberg carb and I didn't find any other ports to tap into, so I will follow your suggestion and tee into my intake port. It will be interesting to see how different it runs at low RPM.

Thanks a lot George... I learned a lot with your assistance.

And, thanks TJ for bringing this topic up.

[TJ's GMC]

There should be two ports on your carb Clarke. One ported and one manifold. My original stromberg has two.

[Clarke]

TJ, I looked again and only saw one port. I attached two photos... I guess you have a different carb?

[TJ's GMC]

Boy that's interesting. Mine does have two, but maybe it wasn't what I thought. My 305E is a 67 series from what we were able to tell and it had the governed points dizzy on it...two vacuum ports on the vacuum pump....one in the front and one in the back. How it was supposed to work I'm not sure. lol I'll get some pics of my carb for ya later today. The two ports on the carb were both hooked to the pump.

[TJ's GMC]

Here ya go Clarke. Mine actually has 3! lol!

[Clarke]

Here ya go Clarke. Mine actually has 3! lol!
Yes, that is interesting. Looks like the carb portion is the same, but your lower attaching plate has two additional ports.

Thanks for sharing.

[TJ's GMC]

Yes, that is interesting. Looks like the carb portion is the same, but your lower attaching plate has two additional ports.

Thanks for sharing.

Might also be cause mine was in a larger series truck. Not sure. But the carb should be the same, just a different base. :)

[bobdylan]

I have a 305 e Stromberg, they have one port, runs to the distributor.

[TJ's GMC]

Wish I didn't get rid of that old dizzy. lol Might have been able to help shed some light on this subject.

[TJ's GMC]

Was driving the ol truck around today. V6 Really seems to be happiest with 10 initial and ported vacuum advance. Everything else I tried just Doesn't seem to be as nice. So I'm sticking with this setup. By the way....I had the ol thing up to 70 MPH for a Short time. LOL Bout 3300 RPM or something like that....super scary though...ain't doing that again. I like that motor and want it to last for awhile. 2500 seems to be it's cruising limit so I don't push past that....Most of the time I'm at a Peak of 2400 which is around 50. Get a bunch of tailgaters, but I don't care...it's My truck and I'm not gonna push it past it's limit cause some idiot is in a hurry on a weekend. lol Cold starting has been more involved, but then it suddenly comes to life. The advanced timing has also helped make warm up times shorter. Used to have to let it idle for like 5 minutes, but now I let it high idle on choke for about 3 and just go easy on it until I get to the high way. Doesn't stumble or back fire when it's cold anymore. She's been a good runner for sure. Confidence keeps going up.

[Clarke]

I left my timing at 16, which is what I set it up a year or so ago. I plugged the vacuum port on the back of the carb and connected a rubber vacuum hose from the intake manifold to the distributor. The truck idles smooter when I hook the full vacuum up to the distributor, as the timing is further advanced.

Once I get above 800-1000 rpm, it seems to be the same as before.

I consistently run mine at 3K rpm and shift around 3,400 rpm when hauling a load. I still might install a gear splitter one of these days. Shifting from 3rd to 4th it drops 900-1000 rpm and when you're hauling a load, it struggles to maintain.

[George Bongert]

Greetings Clarke!

16 degrees BTDC (Before Top Dead Center) initial ignition timing is a bit too much in my opinion. I would set the timing to factory spec, and run the vacuum advance with full intake manifold vacuum. That will be where you will get your best performance. You will lose power with ignition timing that is too far ahead as well as with timing that is too late. It will interest you to note that ignition timing that is too late (retarded) will contribute to engine overheating problems. That is why the people who designed the engine designated that the initial ignition timing be just so many degrees (factory spec) BTDC.

[TJ's GMC]

X2 with everything George said. When I bumped the timing up to 10 degrees and then hooked the advance pump to Manifold timing...the engine idled Super smooth...as seen in the video I posted....But, it had some hesitation coming off idle and under wot it would back fire(Partly a 4 down to 2 adapter problem as well). I then backed the timing down to 6 initial and manifold advance...still seemed a bit rough...harder starting and not as smooth....I then brought the timing up to 10 initial and ported advance....Bingo...found that sweet spot. Engine is harder cold starting, but when it's warm it only takes 1/2 a crank, and the engine pulls very well through all the ranges and when cruising I have full advance. I do have to wonder if the roughness in your engine has to do with valves or a vacuum leak?

[Clarke]

First of all, this is not my area of expertise, so I appreciate your feedback. Given the subject of this thread, I'd like to explain how I arrived at 16 degrees (in lieu of the factory spec of 7.5 degrees).

Typically, I follow specs closely, but there was a previous thread where someone was talking about advancing the timing, which I've heard so many times over the years, but never much detail on how much advance.

Many old timers I've spoken to over the years say they never use a timing light; they adjust the distributor (advancing the timing) until the motor starts to run rough (detonation), then back it off a little. (What's a little? I don't know? LOL!) I tried that with my 305 and it started running rough at 20 degrees, so I backed if off 4 degrees.

Correct me if I'm wrong, the engine achieves the greatest amount of power when the combustion process happens when the piston is at it's highest position (least amount of open area), just as the piston is starting on it's "down stroke". This is why when you advance the timing from factory spec, the engine idle increases. As you are reducing the amount of open area during the combustion process, the increased energy pushes the piston down, resulting in more power.

In the previous thread I mentioned above, I noted I completed several dozen timed runs, which consisted of cruising in 4th gear (I think 40 mph), then flooring the gas pedal and holding it until I hit 70 mph. I experimented with 7.5, 10 and 16 degrees. The times between 7.5 and 10 degrees timing were very consistent, but with 16 degrees timing the times were consistently faster. I duplicated the timed runs the following day with all three timing settings in reverse order and the results were very close to the day before.

Again, I appreciate the feedback. I obviously do not want to cause engine damage if the above justification is not correct.

[TJ's GMC]

These engines have super low compression so I don't think you have to worry to much about detonation with 16 initial. I heard that same saying from the old guys as well. lol Setting by ear I had mine originally at 4 degrees. Then I bumped it up to 8 and felt a nice increase....then 10 and felt little more, higher didn't seem to make All that much difference so I stopped. I'm considering the HEI from HEIDIZZY on ebay. I'm gonna try and do an acceleration video so we can do some comparing. Hopefully this weekend.

[ilvracn]

the factory setting of 7.5 degrees was made to be safe under all conditions. if your pulling a 10000 lbs load on a 90 degree day you may want to back the timing off a little. for normal driving 16 deg should be fine as long as you dont hear any pinging. more likely, low rpm high load. fuel will make a difference too, lower octane means you run less timing.
rick

[Clyde]

Remember these engines were designed to run the low octane gas of the day which was 92 octane according to the owners manual, today this is high octane.

[George Bongert]

These engines have super low compression so I don't think you have to worry to much about detonation with 16 initial. I heard that same saying from the old guys as well. lol Setting by ear I had mine originally at 4 degrees. Then I bumped it up to 8 and felt a nice increase....then 10 and felt little more, higher didn't seem to make All that much difference so I stopped. I'm considering the HEI from HEIDIZZY on ebay. I'm gonna try and do an acceleration video so we can do some comparing. Hopefully this weekend.

Greetings again to all following this thread!

Personally, I would stick pretty close to factory settings, and certainly nothing over 10 degrees advance. Given factory specs, you are already 2.5 degrees above what is recommended, and I don't believe you will damage your engine with it set at 10 degrees initial advance since it's not that big of a jump. However, there is one thing to consider in advancing initial timing too far, and that is kickback against the starter which can, and has resulted in damage to starters:goodluck:, such as a broken starter nose cone. This is something that must be considered no matter what engine you are working with. Again, I wouldn't recommend anything above 10 degrees BTDC.

[David R Leifheit]

First of all, this is not my area of expertise, so I appreciate your feedback. Given the subject of this thread, I'd like to explain how I arrived at 16 degrees (in lieu of the factory spec of 7.5 degrees).

Many old timers I've spoken to over the years say they never use a timing light; they adjust the distributor (advancing the timing) until the motor starts to run rough (detonation), then back it off a little. (What's a little? I don't know? LOL!) I tried that with my 305 and it started running rough at 20 degrees, so I backed if off 4 degrees.

Correct me if I'm wrong, the engine achieves the greatest amount of power when the combustion process happens when the piston is at it's highest position (least amount of open area), just as the piston is starting on it's "down stroke". This is why when you advance the timing from factory spec, the engine idle increases. As you are reducing the amount of open area during the combustion process, the increased energy pushes the piston down, resulting in more power.



First, the "old" method of adjusting until it runs good is, in my opinion, only valid for engines of our era. Once computers got involved with the engines, you lost control of the timing. What this method does do is take wear into consideration. As the timing chain stretches the timing does change.

Second, While you are correct, the most power should be gathered at top dead center, you have adjusted the firing to be 16 degrees before top dead center, meaning it is firing well before TDC. According to the information on Jolly's site, 450 RPM idle and 5 degrees BTC.

Is your engine old or rebuilt with all new innards? If all new then with that much advance (triple stock) I would suggest something is terribly wrong. If it is an old, worn, engine then it is possible that the wear is the reason for the advanced timing. Which suggests, to me, a rebuild will be in your future.

One thing people do need to do when working on an older engine is to not rely on the tools (timing light) but rather on what the engine is telling you. You can time it all day with the light, and it will run rough if it is really worn. I have seen "kids" get frustrated trying to work on non-computer all manual older systems. They can't even figure out how to set the points (which could also affect the timing).

Having said all that...
If it works for you at 16BTC, then that is good for you. You could be compensating for wear, or have something else going on. If you are confident that everything else is as it should be, then enjoy your truck.

[Clarke]

My engine has 57K orig miles on it. A couple years ago, I gave it a complete tune up. The compression was really good and consistent within a couple psi between cylinders. Majority of the valves were within tolerance, a few were .001-.002 out of tolerance, so I dialed those in. The vacuum was/is 19 inches. It was running good before the tune up and still is.

It was after the tune up when I started experimenting with the timing and it seems to have a litte more power. I drive it one or two days a week all year ago and put the cheap gas in it wherever I need it. I do a lot of towing too. I've never noticed loss of power, pinging, or inconsistencies at any time year around. Therefore, I think it's okay, but based on the feedback I think I'll dial the timing back to 10 degrees. I was running it at 7.5 degrees before and it was fine too. I was just trying to get more efficency/power out of it.

Thanks again for all the feedback.

[TJ's GMC]

Your welcome! This has been alot of fun for me. lol The ol V6 has been running so good in my GMC that I've been needing something to do! haha! My engine is the same....mileage is unknown as it is Not the original V6 from that truck, but the compression was at 120-125 in all cylinders and the cam looked fairly good last I had the valley pan off. Put some engine restore in the oil and now I need to find some ZDDP for the oil and it should be good for, hopefully with the easy driving I'm doing, another 100,000. The oil was so dirty when I got the truck that I had to change it 3 times withing 40 miles before it was finally staying clean, but rather have clean oil than a flat cam or anything else.

[George Bongert]

Your welcome! This has been alot of fun for me. lol The ol V6 has been running so good in my GMC that I've been needing something to do! haha! My engine is the same....mileage is unknown as it is Not the original V6 from that truck, but the compression was at 120-125 in all cylinders and the cam looked fairly good last I had the valley pan off. Put some engine restore in the oil and now I need to find some ZDDP for the oil and it should be good for, hopefully with the easy driving I'm doing, another 100,000. The oil was so dirty when I got the truck that I had to change it 3 times withing 40 miles before it was finally staying clean, but rather have clean oil than a flat cam or anything else.


Greetings TJ and all the other club members following this thread!

This has been a very interesing subject with a lot of good comments, and recommendations. Regarding the subject of a ZDDP additive for the oil, I would like to offer a suggestion. Amzoil Synthetic Motor Oil. I am running Amzoil Synthetic Oil (already contains ZDDP) in my 2001 Pontiac Bonneville (3.8 Litre V-6). My brother also runs Amzoil Diesel Synthetic Motor Oil in his 2001 Chevy 2500HD Duramax Diesel, and swears it's the best oil he's ever used. It's a bit more expensive, but I took into consideration the benefits of synthetic oil plus the added benefit of the ZDDP. I know some folks who say you can run synthetic basically forever, however, I still recommend following the manufacturers oil change schedule. One nice thing about some these newer cars (like mine) is that the computer will warn you that you need to change your oil soon. I change both the oil, and the filter (no sense in having clean oil in the crankcase when there's still dirty oil in the filter) when the computer indicates that there is 5% oil life left. Of course, after the oil has been changed, the computer has to be reset (very simple procedure) to reset the oil life countdown to 100%.

[TJ's GMC]

Have heard alot of good about Amzoil. Joe Gibbs oil is what my machinist sells for a good price. I have been using Rotella with no issues thus far. Plan to add the zinc at the next oil change which will be soon if I head to troutdale in the truck this summer. Now I gotta figure out something else I can mess with on this thing! haha!

[1969_CM2590D]

I'm a little tardy on this post, but over this Labor Day weekend i finally got back to my '69 305E 2500. The truck has 41K+ on it with manual everything. Coming form Alabama it was one of the last pickups to have no emission controls. With all that in mind, it's incredibly simple and brings a whole new meaning to the term serpentine belt.

I did the 4412 swap with the 1937 adapter when I got the truck the truck a few years ago. It never would idle right often not holding an idle as well as a hesitation beast on giving it the gas. I get frustrated over time. As time went on, I realized I had misread the instructions for installing the carb and tightened things to ft/lbs instead of in/lbs. A few moths ago I ordered new gaskets for the adapter and carb for when I could get to it, putting off fears I might have warped anything.

Saturday, I put the new gaskets on and once the dry carb got gas, things improved over 1000%. Toning the idle down from what was probably 1000+ RPM, it held its idle much better at the specced 500 RPM.The truck idled for over a half an hour. In the driveway, I gave it the gas and it accelerated smoothly. I let i sit for a few hours and with a flick of the key it fired up. Same thing the next morning. So a lot of problems solved. It still had a slight miss, though, so today I timed it by by feel, turning the dizzy until things were much smoother and reset the idle speed.

With all that in mind, I took it into town for a road test. No stalling and a decent idle, but going up a hill under load there was hesitation. so I got back home and looked into setting the idle mixture. Holley says to use the manifold vacuum for that- so there was one error I'd been making. I have been using the ported vacuum on the 4412 fer setting it up- Holley instructs to use that, but just for the vacuum advance. The advance was checked a while back and was OK. I came across this thread which got me looking into using manifold vacuum for the advance. More to look at as time goes on.

With my non-emissions 305E what I've come across here and elsewhere on the net tells me I should be using manifold vacuum, not the emission controlled ported vacuum. I came across this post here http://www.hotrodders.com/forum/ported-vacuum-95255.html and I'll copy one response below which was pretty enlightening. Comments?

[1969_CM2590D]

Quote:
Originally Posted by badhabit
.DO NOT HOOK YOUR VAC ADVANCE UP TO NORMAL VACUUM--- this will cause the idle to race and really run flat, because your advance is all in at idle-- not as you accelarate.

This will only happen if you select the wrong VA can. Having full vacuum advance at idle will increase the VE, reduce the EGT's and make the idle smoother. A former GM engineer wrote a nice piece on vacuum advance:

Quotation thanks to JohnZ

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[TJ's GMC]

I tried my engine(305E with a 600 cfm 4 barrel and modded magnum manifold) on Manifold vacuum advance and it would hesitate off idle after idling for so long. Thing is, weather on Ported or Manifold....both are useless under load. Under load your relying on the weights in the dizzy for advance. The vacuum timing only comes in while cruising at a constant set speed to aid in power and mileage. Therefore, not Enough or to Much can have an affect on power and mileage. Which is why I backed mine down to 8 initial and left it on Ported timing. I did do 12 initial once, but it felt like to much and didn't really make a difference, so I left it at 8-10. 4-6 is is factory for stock, but with the extra fuel I have found advancing a little helps. Also the addition of an HEI makes a difference in itself. That and hot wires and plugs will aid in power. I still need to open the plugs in mine a tad more, but I haven't as it runs like a top and I drive it Everyday...so if it ain't broke I don't fix it. lol If I had a dyno that would be something I would play with and get some paper results. :thumbsup:

[bigblockv6]

TJ, when you decide to open up the plugs, I have found the correct plugs with larger gaps. I'm running AC-R44XLS6 plugs that take .060 gaps for HEI applications in my 478M.:upyes:

[TJ's GMC]

TJ, when you decide to open up the plugs, I have found the correct plugs with larger gaps. I'm running AC-R44XLS6 plugs that take .060 gaps for HEI applications in my 478M.:upyes:

That's one big sparky! lol

[krazzz]

Quote:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
[/B]

Thank you for the wonderful write-up 1969_CM2590D. I think I will take your suggestion and go with manifold vacuum. What do you recommend I set the initial timing to? The factory spec is 7.5 on mine. And to be clear, I still set the timing with the vacuum advance disconnected, correct?

[jerrspud]

Thank you for the wonderful write-up 1969_CM2590D. I think I will take your suggestion and go with manifold vacuum. What do you recommend I set the initial timing to? The factory spec is 7.5 on mine. And to be clear, I still set the timing with the vacuum advance disconnected, correct?
this is a 6+ year old tread
I just adjusted the timing on my 305E. I think I set the initial timing (yes with the vacuum disconnect) to around 10 degrees and then used the manifold vacuum. The advantage is that you get timing added to your idle after you start the engine, but you won't have hard start issues due to too much advance when it's turning over.