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?

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.

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?!?

Re: Vacuum advance issues
 

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;)

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.

Re: Vacuum advance issues
 

I didn't know that and will take it into consideration. This 307 has 283 power-pak heads and a mild isky cam in it, so I'm not sure if that initial timing still applies.

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

Re: Vacuum advance issues
 

I have considered that and was going to do it until I saw the article saying you need manifold vacuum. It said that ported came around in the smog era so there would be less advance, less efficiency, hotter engine, just for hotter exhaust to reduce hydrocarbons. Man, that is some backwards thinking right there...

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.

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 ;). I hope my mileage goes up too because before I was very retarded (both ways).

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

Re: Vacuum advance issues
 

Yes, I read about that ahead of time before installing the HEI, and I do have an ignition hot regular copper wire going to the fuse box.

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.

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.

Re: Vacuum advance issues
 

Yeah, I haven't quite gotten rid of the ping, so I'll try premium gas. I was able to re-hook up my tranny kick-down so that'll help under load.

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.

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.

Re: Vacuum advance issues
 

As I mentioned above, I conquered the 307 doggie-ness by using 283 power-pak heads, giving a higher 9.9:1 compression.

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.

Re: Vacuum advance issues
 

The mechanic is right if it is an all out mostly race engine but not a street or part time srip engine, the vac advance is there for a reason, besides part throttle it helps a lot for economy because as stated you would be running it pretty retarded and then low power and kind of gutless. Start with heavy stock springs for initial advance then reconnect vac advance and readjust when driving, just make sure it is ported only not full vacumn that way it starts off with no vacumn adv until the throttle is just part way open then it helps as you accelerate so it is mech only until the throttle starts to open, if it was full vac then the advance would give extra timing at idle and cause ping not what you want as noticed.

Re: Vacuum advance issues
 

There sure is a lot of variations of opinions on this.

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.