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]

Originally Posted by 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

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]

Originally Posted by 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.

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.

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.