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#1 |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Vacuum advance issues
Hey folks,
I recently put a 307 in my '67 C10. I put on a 2bbl rochester carb, with an HEI distributor. It consistently hesitated acceleration, and wanted to die when cold. I eventually found out through a mechanic friend that the timing is way off because there's no port vacuum on the (older) carb, and that the manifold vacuum was messing up the timing on the HEI. I had retarded it myself so it wouldn't ping, but when it's set right, it pings a lot under acceleration and freeway speeds. He said I can disconnect the vacuum advance and just change the mechanical advance curve until I find something that works. I bought a timing kit and plan to reduce the advance, but I wanted to know you're opinion of just using mechanical advance, or if I should just get a new carb with port vacuum, or anything else! My goal is to have the most efficient option for better economy. Will mechanical advance alone run the engine efficiently enough? |
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#2 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,217
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Re: Vacuum advance issues
You can run without vacuum advance. It is for economy basicaly but does help driveability in some cases. Weather it is hooked to full vacuum or ported should not effect the pinging problem. The advance should drop back under load as vacuum drops. You might need an adjustable vacuum chamber so you can set it to come on at a higher vacuum & limit the amount. How about the centrifical advance? Often they are stiuck on an HEI. You may need stronger springs. You will need to experament with the tining & advance as every engine has its variations.
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#3 |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
After disconnecting vacuum, I set my initial timing at 8 adv. 3000rpm takes me to 27 adv total. I drove it and it pings at WOT and under heavy acceleration.
I hooked up manifold vacuum, and have an idle timing of 32, and 52 at 3000rpm. It seems to idle better but driving it pings easily. I read that manifold vacuum is necessary)(http://rockridgefarm.com/vettdoc/Timing_101.pdf) "...you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively." But I've heard from others including that mechanic that you need ported. Also, here (http://www.73-87.com/7387garage/drivetrain/hei.htm) gives tips on setting timing with an adjustable vacuum can. I'm so confused!! What is right?!? |
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#4 |
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Location: Victoria, B.C
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The easiest and perhaps the cheapest way to beat this is to get another 2bbl carb with ported vacuum on it and swap it out to try it.
You can probably pick up a reasonable one for a good price off of Ebay or equivalent.....lots of Rochester stuff on there. Good Luck Coley ![]()
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....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500 |
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#5 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,217
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Re: Vacuum advance issues
The 307 was designed as a low emission engine. The factory initial timing was about 2 degrees. You will need to set the timing back far enough to prevent ping as it will destruck the engine & then tune from there. As I said brfore engine vary. Some work better with the vacuum on the manifold & some with ported vacuum. The larger trucks didn't have vacuum advance & they run fine. They are not concerned about part throttle cruiseing economy because they pulling all the time. You will have to use trial & error. There has been a lot of discution on this subject in these posts & other websites also. The car manufactures have made them both ways.
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#6 | |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
Quote:
I think I've decided what to try: install an adjustable vacuum advance and play with all three settings (initial, vacuum, and mechanical) until it runs well without ping. The tutorial I mentioned above seems to make sense to me and will try to follow that. This whole thing is an adventure for me! Keep any other advice coming! Thanks |
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#7 | |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
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#8 |
Registered User
Join Date: Dec 2011
Location: Creighton Sask. Canada
Posts: 4,121
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Re: Vacuum advance issues
I would pickup a carb with ported vacumn and run both this will give you your best ecomomy, but if not wanting to change carb then eliminate vac advance and just run a bit lighter spring on mech advance to get between the best of both. The vacumn doesn't work or induce advance until you are under part throttle operation and helps a bit to stop initial bog or lag off of bottom end only then mechanical advance takes over. I run a mechanical adv only with big cams and change springs till I get it where I want it.
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#9 |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
I worked on the timing today and was able to get a decent result. I set the initial timing to 4 adv, then put in heavier springs. With mechanical advance only, the engine pinged under heavy acceleration in the lower RPMs. I discovered my HEI distributor already had an adjustable vacuum advance, so I adjusted that until I got advance that didn't cause ping under load. I will keep trying to dial it in, but right now am more satisfied with how my truck runs.
This is certainly an adventure! I didn't know timing was such an art ![]() |
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#10 |
Burnouts are therapeutic
Join Date: Jun 2007
Location: lancaster, south carolina
Posts: 852
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Re: Vacuum advance issues
I went through this last year with my pontiac, it had been upgraded to hei before I got it. I adjusted the timing and vaccum adv. several times and got it better but it never went away fully under load.
I realized they had used the old coil wire to power the hei which is lower voltage, I ran a wire from the fuse block that was a keyed 12 volt and it corrected all my issues. You need to make sure you have 12 volts going to the hei, this may not fix your issues, but it would be the first place I would start. good luck
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1969 CHEVELLE SS 1972 C10 ALMOST DAILY DRIVER 1968 C10 RUSTBUCKET (MISSION IMPOSSIBLE) 1972 PONTIAC LEMANS CONVERTIBLE (WIFE'S CAR) "My wife and I were happy for twenty years. Then we met." - Rodney Dangerfield "The difference between genius and stupidity is that genius has its limits." - Einstein |
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#11 | |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
Quote:
I hope I can get rid of all the ping. Right now it runs great and as long as I don't gun it (at lower RPM) there shouldn't be an issue. I have to figure out how to connect my tranny kick-down to the carb (cable is too long) so low RPM load can be avoided. Also, this 307 of mine has 283 heads so it has ~9.9:1 compression. I want to try 91 gas to see if that fixes the problem. |
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#12 |
Registered User
Join Date: Oct 2010
Location: Ocean Springs, MS
Posts: 1,746
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Re: Vacuum advance issues
With 9.9 compression iron heads you'll probably have to run premium gas or very very little advance. Then you won't have much power.
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Gary 1971 Chevrolet C/10 1951 GMC 100 1977 GMC C15 1955 Chevrolet 3100 |
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#13 | |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
Quote:
The truck is running very well now! I feel much more satisfied while driving it that I have it running more efficiently. Thanks for all your advice. |
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#14 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Vacuum advance issues
I thought 307s were very low compression... doubt it should ping on 87 octane. You really want vacuum advance, it's not just an emissions thing. Without it you're going to have very retarted timing at idle which will heat up valves, heat up the engine, and suck for driveability. You might be able to get it to work with a lot of base timing and less mechanical advance, but then you'll be fighting part-throttle issues.
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#15 | |
Registered User
Join Date: Dec 2008
Location: San Francisco, CA
Posts: 144
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Re: Vacuum advance issues
Quote:
Yes I agree vacuum advance is valuable. This mechanic told me to just run without vacuum, and now after my research I wholeheartedly disagree with him. |
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#16 | |
Registered User
Join Date: Dec 2011
Location: Creighton Sask. Canada
Posts: 4,121
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Re: Vacuum advance issues
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#17 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,217
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Re: Vacuum advance issues
There sure is a lot of variations of opinions on this.
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#18 |
Registered User
Join Date: Dec 2011
Location: Creighton Sask. Canada
Posts: 4,121
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Re: Vacuum advance issues
You are right about that, all I know is what I have done working in the shop for 20 years on other peoples hotrods, what works works time after time, a lot of people know what they heard or read, but real world makes a diff. All you can do is try, make sure your vac adv hose is off and plugged when setting the initial mech advance with the stock springs, just remember to consider anything else in total truck that may influence a load or pinging situation when driving like brake drag low tires, engine heat zone like at operating temp not above, anything else definetily helps, even quality of fuel. You will get it, just takes time and figuring it out.
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Tags |
307, advance, carburetor, hei, timing |
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