Vacume advance issues?

[Suregrip391]

1970 Challenger RT/SE, bone stock 383 auto survivor car. Changed the cap, rotor, points, condenser, wires, and plugs. Set the timing at 16/32. Starts and runs great! Without the vacume advance hooked up that is! Once I hook it up to the ported side of the Edelbrock and start driving (under light throttle only) the engine will shake, miss, and studder till I get moving along and it smooths out and runs just fine again. Runs fine without the vacume advance hooked up but not so nice with it. Never had this issue before.

Any ideas for me? This is also a new carb on it also. It's an Edelbrock AVS2 600 or 650, don't recall as I type. Firing order is correct.

[DeathProofCuda]

Have you tried using a timing light to see what happens to the timing when you tip the throttle and begin applying vacuum to the advance canister?

[Racer57]

I have a 383 and it does the same thing as yours with the vacuum hooked up with the same timing.  So I left it unhooked. 

[Skdmark]

Have you tried adjusting the vacuum advance?  It doesn't change the amount of advance just when it kicks in.
Insert a 3/32 hex key into the vacuum to adjust. CCW higher vacuum to advance.

[tman]

Maybe this article may help

https://www.moparaction.com/2020/08/16/advance-to-go/

[chargerdon]

Stock mopar vacuum advance canisters are set to add 7 degrees of advance and that works well when the static is set to 12 or less.   

With static set at 16, when you go off idle and are a low RPM that bumps the timing up to 23 degrees, plus a little of the mechanical so probably up to near 30 which for most cams is simply too much at low RPM.   Remember, stock recommendation for static is only 12 degrees.   So the combo of 16 static plus 7 is too much for a mild cam.   Hence it doesnt run right. 

Me with my 408 and Lunati voodoo 703 cam i do it differently.    The cam requires way higher advance than the stock 12...It IDLES best at around 22-24.   Also, light throttle likes total timing in the 20 plus range.  However, if i set the static at 20 or higher, it gets very difficult to crank..(kickback) when warm.    Darn.   

So, i did three things.   One, i tune the vacuum canister to give a max of 5% advance.   
Two, put in a blocker plate to reduce the mechanical advance to 16 % and set the static advance to 18% so that 18 + 16 gives 34 total at cruising and WOT.   BUT then i plug the vacuum advance into the MANIFOLD side of my Edlebrock carb.   The manifold side gives vacuum while at idle, but, as soon as throttle is opened it goes away.   Result is that when cranking to start the 18 degree doesn't slow down the cranking...but, then as soon as it starts and is at idle i get another 5 degree to bring me up to 23 degree and it idles good.   Then open the throttle and the advance goes away, and it runs very well.   

[DeathProofCuda]

Stock mopar vacuum advance canisters are set to add 7 degrees of advance and that works well when the static is set to 12 or less.   

With static set at 16, when you go off idle and are a low RPM that bumps the timing up to 23 degrees, plus a little of the mechanical so probably up to near 30 which for most cams is simply too much at low RPM.   Remember, stock recommendation for static is only 12 degrees.   So the combo of 16 static plus 7 is too much for a mild cam.   Hence it doesnt run right. 

Me with my 408 and Lunati voodoo 703 cam i do it differently.    The cam requires way higher advance than the stock 12...It IDLES best at around 22-24.   Also, light throttle likes total timing in the 20 plus range.  However, if i set the static at 20 or higher, it gets very difficult to crank..(kickback) when warm.    Darn.   

So, i did three things.   One, i tune the vacuum canister to give a max of 5% advance.   
Two, put in a blocker plate to reduce the mechanical advance to 16 % and set the static advance to 18% so that 18 + 16 gives 34 total at cruising and WOT.   BUT then i plug the vacuum advance into the MANIFOLD side of my Edlebrock carb.   The manifold side gives vacuum while at idle, but, as soon as throttle is opened it goes away.   Result is that when cranking to start the 18 degree doesn't slow down the cranking...but, then as soon as it starts and is at idle i get another 5 degree to bring me up to 23 degree and it idles good.   Then open the throttle and the advance goes away, and it runs very well.   

Except that your engine is essentially a vacuum pump, so that as soon as you start cranking the engine with your dizzy hooked to manifold manifold vacuum I'm betting that your vacuum advance kicks in.  Also, manifold vacuum does not go away as soon as you open the throttle.  It will drop and will go to zero at WOT, but it doesn't go away when the throttle is partially open.

Great article @tman   Thanks for posting 

[Dobbers70]

I'd like to add another thought on the canister....the condition of it and the diaphragm. I just had issues with my car before I swapped out the distributor. Sounds like it's the original.

[chargerdon]

Your right, at part throttle the vacuum does not fully go away only partially...BUT...part throttle means the engine is not under load and can handle MORE advance...  If you take the 18 static, and 16 mechanical gives 34...and at part throttle another 3 or 4 from the vacuum canister..go give a total of about 38-40  and while that sounds like too much...with a hot cam and NOT under load...it will run just fine.   If not then you can make the adjustments to static and mechanical advances should u hear any pinging.

Look, with a stock cam shaft and low compression engine this WOULD NOT be what is wanted...but...with a hot cam and my computed 9.75 static compression...it is works best...for me anyways.   The ported side of the carb is used to combat the pollution problems of a low compression cam.   Totally different.     

PS...after about 2,000 miles my spark plugs look great...clean with a light gray color...i also used an Endoscope and looked in my Number 3 cylinder and couldnt believe how clean the cylinder was...no carbon build up at all.     

[Suregrip391]

All, I'm sorry for not responding back! I forgot I posted this here. I appreciate all the responses. I'll provide a quick update in hopes this will help anyone else. As it turns out, I pulled the factory distributor and installed one of unknown condition (came in a box of parts), it looked clean and ready for use so I tried it. Car ran like new! I got ahold of Ray at Halifax shops and he agreed to look over my factory distributor and said that the advance plate was gummed up and not advancing properly. Got it all cleaned up and ready to go! I sent it to him last Thursday, he got it on Saturday, called me on Sunday and shipped it back Monday. Quick service and professional. Car runs great. I set my initial timing to 14 degrees, that gave me 30 dwell and .018 gap on points.

[DeathProofCuda]

Seems odd that if the advance plate was gummed up and not operating properly that the car previously ran well without the vacuum advanced hooked up    But, hey it's running well now and that's the important part.  Glad that you got it figured out and thanks for reporting back. 

[pschlosser]

Does the vacuum advance pod hold a vacuum?  If it leaks, it may cause your symptoms.  I'm sure you've checked this by now.

[Suregrip391]

No leaks, mine was good.