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Discussion Starter #1
Hi Guys - I'm trying to diagnose a surge/miss in my car. When the car is under a light load (high vacuum), it surges and misses. Car idles fine, and runs great otherwise. I suspect the vacuum advance is advancing the timing too much.


Mechanical timing is set at 12 degrees initial, 34 degrees total at around 2800 rpm. I checked the timing while idling with the vacuum advance connected, and it's at 40 degrees at 870 rpm. It'll increase to nearly 60 degrees if I rev the car up to roughly 2500 rpm while in neutral. That seems like a lot. Question - how much advance should the vacuum advance add to the total advance?
 

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26-28 degrees of vacuum advance is too much. You need to limit it to about 12. You may want to look up Lars Grimsrud and ask him for his most recent versions of his papers on vacuum advance canisters and timing. He and Henry Olsen also make a limiter plate that keeps your VA can from pulling too much advance. He put one on my distributor when he rebuilt and recurved it and it works great. Before that I had small bushing that I pressed on the end of the VA arm that limited my VA to 12. Too much timing advance will cause detonation, not a good thing.
 

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Discussion Starter #4
Thanks for the input guys. I recently installed a CDI ignition system, and during that installation I removed the electronics module from the distributor and wired the CDI system to the magnetic pickup. I noticed the surge/miss problem after installing the CDI. The distributor has an adjustable vacuum advance can, but no amount of adjustment will change the timing. So, I suspect my distributor modification have have something to do with this. Guess I'll pull the distributor and take a look.
 

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I had the same issue on my Camaro, but this was before I installed a MSD box. I solved my problem by jetting up the primaries on my 650dp two sizes. I went from 67 to 69. This change helped my surge big time, but I could still feel it when my foot was barely touching the gas pedal.. I have a MSD ready to run. I have 15 initial and the 21 degree bushing for a total of 36. Trickflow recommended 36 with my heads and cam. I slowed down my advance curve using the springs in the distributor. Before I had all my timing in around 2500 or so. I changed it to bring it all in closer to 3000 and now I don't have any surging at all.
 

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The 350 ZZ4 in my 1968 Camaro came with a 20 degree vacuum advance which was about 10 degrees to much. I found that GM has a 10 degree vacuum advance that solved my problem. The car now runs smooth and the mileage has increased. This was an easy 10 minute fix, I took the cap off and did not have to remove the distributor.
 

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The model I bought was 1973682. I found a NOS on eBay for $15.00. You might be able to cross that number if you can't find one.
 

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Discussion Starter #9
My car has an Accel distributor with an adjustable vacuum advance. According to the instructions, the advance can be reduced by inserting an allen wrench in the vacuum connection, and turning counter-clockwise. I tried adjusting it last night, and I can hear/feel a click when I turn the adjustment counter-clockwise every revolution. It acts like it is at the end of the adjustment travel. Does anyone have any experience adjusting one of these? Maybe the instructions are incorrect.
 

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That's a pretty huge timing jump from your vacuum advance at idle. As you have ignition mods ( ignition box) maybe I am all wet but is ther a chance that you have the hose for the vacuum advance connected to the wrong vacuum port on the carburetor. There are two ports, one for straight manifold vacuum and the other is metered vacuum that does not allow vacuum until around 1,500 RPM which is when it begins to activate your vacuum advance. As far as adjusting the advance canister counter clockwise adds more advance and if you can feel clicking then you are out of threads and the screw and receiver are no longer connected. Try re engaging it with a couple of clockwise turns.

The best way I've found to watch the vacuum advance is to use a timing light with the vacuum canister disconnected to establish base timing at idle then increase throttle to see how much mechanical advance you have. Then connect your vacuum advance and run through the same drill only this time you will see when and how much your vacuum advances timing. 34 to 36 degrees all in timing at road speed is your target. The vacuum advance may bump it up a bit but certainly not up to 60 degrees at idle. Not sure how your engine is running without severe detonation with that advance. Just my opinion. Best wishes for a successful outcome and don't fry that engine with those advance numbers. :smile2:
 

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Have you took it for a ride with the vacuum advance hose plugged ?

I ran my 406 locked out at 36 and the only issue I had was keeping my foot out of it :eek:
 

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Discussion Starter #12
Thanks for the input guys. I tried using the adjustment on the vacuum advance can, and that had no effect. I ended up making a small L-shaped vacuum advance stop bracket, and installed it in the distributor using one of the available screw holes that had been used to mount the electronics module. The bracket limits the travel of the base plate holding the magnetic pickup that is moved by the vacuum advance canister. After a little tinkering, I got it adjusted so that the vacuum advance adds 10-11 degrees. The car runs perfect now.

It might be the most elegant solution, but it works.
 

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A real lean cruise air fuel ratio will also give you that surging, put them both together and it is more sensitive.

I run 22 initial, 11 vacuum and 40 total for the street, pump it up to 44 for the track, this beer can motor loves timing.
 
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