6066 (1960-1966) GMC Truck Club Bitcoin now accepted here! 
Pay Dues
Pay Dues or become a Site Supporter
 



Go Back   6066 (1960-1966) GMC Truck Club > 6066 GMC Truck Club Forum > GMC V6 and V12 Engines


GMC V6 and V12 Engines Engine repair and rebuilding

Reply
 
Thread Tools
  #21  
Old March 10th, 2016, 01:48 PM
George Bongert George Bongert is offline
Senior Member
 
Join Date: Jan 2013
Location: Oshkosh Wisconsin
Truck: Former owner Repowered by GMC 305 V-6, 1963 Chevy 30 Series SM420 Long Bed (9') Step/Fenderside
Posts: 210
Rep Power: 141
George Bongert will become famous soon enoughGeorge Bongert will become famous soon enoughGeorge Bongert will become famous soon enough
Default Re: Too much timing?

Quote:
Originally Posted by TJ's GMC View Post
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."
Reply With Quote
  #22  
Old March 10th, 2016, 04:24 PM
TJ's GMC's Avatar
TJ's GMC TJ's GMC is offline
Senior Member
 
Join Date: Mar 2015
Location: Cave Junction, Oregon
Truck: 1964 GMC 1500 LWB Wideside 305E V6.
Posts: 1,341
Rep Power: 399
TJ's GMC is on a distinguished road
Default Re: Too much timing?

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.
__________________
"Excuse the rust I use my truck"
1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

1964 Chevy C20 292/SM420
1966 Chevy C10 292 hotrod 6/TKO600


My youtube channel aka Military Chevy:
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Reply With Quote
  #23  
Old March 11th, 2016, 04:07 AM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default Re: Too much timing?

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.
Reply With Quote
  #24  
Old March 11th, 2016, 08:07 PM
George Bongert George Bongert is offline
Senior Member
 
Join Date: Jan 2013
Location: Oshkosh Wisconsin
Truck: Former owner Repowered by GMC 305 V-6, 1963 Chevy 30 Series SM420 Long Bed (9') Step/Fenderside
Posts: 210
Rep Power: 141
George Bongert will become famous soon enoughGeorge Bongert will become famous soon enoughGeorge Bongert will become famous soon enough
Default Re: Too much timing?

Quote:
Originally Posted by Clarke View Post
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!
Reply With Quote
  #25  
Old March 12th, 2016, 04:31 PM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default Re: Too much timing?

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?
Reply With Quote
  #26  
Old March 12th, 2016, 09:15 PM
George Bongert George Bongert is offline
Senior Member
 
Join Date: Jan 2013
Location: Oshkosh Wisconsin
Truck: Former owner Repowered by GMC 305 V-6, 1963 Chevy 30 Series SM420 Long Bed (9') Step/Fenderside
Posts: 210
Rep Power: 141
George Bongert will become famous soon enoughGeorge Bongert will become famous soon enoughGeorge Bongert will become famous soon enough
Default Re: Too much timing?

Quote:
Originally Posted by Clarke View Post
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!
Reply With Quote
  #27  
Old March 13th, 2016, 05:34 AM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default Re: Too much timing?

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.
Reply With Quote
  #28  
Old March 14th, 2016, 03:52 AM
TJ's GMC's Avatar
TJ's GMC TJ's GMC is offline
Senior Member
 
Join Date: Mar 2015
Location: Cave Junction, Oregon
Truck: 1964 GMC 1500 LWB Wideside 305E V6.
Posts: 1,341
Rep Power: 399
TJ's GMC is on a distinguished road
Default Re: Too much timing?

There should be two ports on your carb Clarke. One ported and one manifold. My original stromberg has two.
__________________
"Excuse the rust I use my truck"
1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

1964 Chevy C20 292/SM420
1966 Chevy C10 292 hotrod 6/TKO600


My youtube channel aka Military Chevy:
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Reply With Quote
  #29  
Old March 16th, 2016, 02:30 AM
Clarke Clarke is offline
Senior Member
 
Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
Posts: 217
Rep Power: 128
Clarke is on a distinguished road
Default Re: Too much timing?

TJ, I looked again and only saw one port. I attached two photos... I guess you have a different carb?
Attached Images
File Type: jpg IMG_1439.JPG (317.3 KB, Multiple views, 12 clicks)
File Type: jpg IMG_1441.JPG (305.8 KB, Multiple views, 10 clicks)
Reply With Quote
  #30  
Old March 16th, 2016, 05:14 PM
TJ's GMC's Avatar
TJ's GMC TJ's GMC is offline
Senior Member
 
Join Date: Mar 2015
Location: Cave Junction, Oregon
Truck: 1964 GMC 1500 LWB Wideside 305E V6.
Posts: 1,341
Rep Power: 399
TJ's GMC is on a distinguished road
Default Re: Too much timing?

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.
__________________
"Excuse the rust I use my truck"
1964 GMC 1500 305E/sm420 4 barrel intake mod and dual exhaust.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

1964 Chevy C20 292/SM420
1966 Chevy C10 292 hotrod 6/TKO600


My youtube channel aka Military Chevy:
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Reply With Quote
Reply

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Timing Archiver Previous Forum Posts 1 May 28th, 2012 02:04 AM
Timing Archiver Previous Forum Posts 0 August 16th, 2008 01:17 AM
Timing Archiver Previous Forum Posts 0 August 16th, 2008 01:08 AM
RE: timing Archiver Previous Forum Posts 0 October 23rd, 2002 01:07 AM
Timing is everything Archiver Previous Forum Posts 0 March 4th, 2001 04:37 AM


All times are GMT +1. The time now is 11:08 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2013, Jelsoft Enterprises Ltd 681711554703|1710454101|0