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Vac advance debate!!

Drew67coronet

Drew67coronet

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Hello all, I'm back again with another problem. Ive got a bone stock 1967 273 2 barrel engine with a single points distributor With my vac advance hooked up my engine stumbles all the way up until I give it a good amount of gas then it runs like a raped ape.
If I unhook my vac advance the stumble disappeares. I tried to remedy the stumble by advancing the timing and that worked great but caused the engine to ping at all RPMs.
So no good there. I retarded the timing back to where I had it. And it runs great Im just wondering if maybe I need to adjust the timing more? My vac advance canister is not adjustable.
Thanks
Drew67coronet
 
Is it connected to a ported or manifold source? Have you confirmed that the advance canister is functioning properly by applying vacuum (engine off) with something like a mity-vac and observing the arm movement? You can always install an FBO limiter plate that will allow more initial but limit total. I have mine set at 18/18.
 
When connected to a ported vacuum source the vacuum canister adds advance just off idle/cruise and it's reduced as rpm increases/vacuum decreases.
I'm guessing your canister is non-functional.
 
Last edited:
Lionized said:
When connected to a ported vacuum source the vacuum canister adds advance at idle then it's reduced as rpm increases/vacuum decreases.
I'm guessing your canister is non-functional.
Click to expand...
Not exactly. With the slot covered by the throttle blade at idle, the advance won't receive a vacuum signal until it's cracked open, right when it needs the advance. No need to have extra advance at curb idle.
 
Lionized said:
When connected to a ported vacuum source the vacuum canister adds advance at idle then it's reduced as rpm increases/vacuum decreases.
I'm guessing your canister is non-functional.
Click to expand...
Think you have that backwards. When hooked to ported, it won't add anything at idle. When hooked to manifold vac, it will add timing upon engine start.
 
turbine68rt said:
Not exactly. With the slot covered by the throttle blade at idle, the advance won't receive a vacuum signal until it's cracked open, right when it needs the advance. No need to have extra advance at curb idle.
Click to expand...
I agree, I should not have written "at idle" and I apologize for any confusion.
 
turbine68rt said:
Is it connected to a ported or manifold source? Have you confirmed that the advance canister is functioning properly by applying vacuum (engine off) with something like a mity-vac and observing the arm movement? You can always install an FBO limiter plate that will allow more initial but limit total. I have mine set at 18/18.
Click to expand...
I have it hooked to ported and yes I used my vac pump to confirm canister doesn't leak it's also brand new maybe two weeks old
 
turbine68rt said:
Not exactly. With the slot covered by the throttle blade at idle, the advance won't receive a vacuum signal until it's cracked open, right when it needs the advance. No need to have extra advance at curb idle.
Click to expand...
I thought most used non ported post so no vacumn is needed at idle and only when RPM is added does the advance move. That is why you pull off the vacumn line when setting the timing. Am i wrong?
 
Lionized said:
When connected to a ported vacuum source the vacuum canister adds advance just off idle/cruise and it's reduced as rpm increases/vacuum decreases.
I'm guessing your canister is non-functional.
Click to expand...

That's unported, and is what Chevrolet does
 
Robert Schaad said:
I thought most used non ported post so no vacumn is needed at idle and only when RPM is added does the advance move. That is why you pull off the vacumn line when setting the timing. Am i wrong?
Click to expand...
That's sort of contradictory. By "non ported" are you referring to using manifold vacuum? In my case (68 AVS) there is only one source for advance and that's ported. Plugging the line removes any potential vacuum for the most accurate initial setting.
 
well I'm not saying what is correct or better, just saying Chevrolet uses manifold Vacuum LOL and what I understood you told initialy. This debate about whats better allways appears quite often
 
Last edited:
And on the other side of the coin, here is a video FOR manifold vacuum. Pick it up at 17:00, when they get into ported vs manifold.

 
Don Frelier said:
Timing is everything...
Click to expand...
I've been setting my timing up various ways nothing seems to work out. If I retard the timing the engine doesn't ping and it has good acceleration but stumbles and misses at part throttle if I advance the timing even just the tiniest amount it gets rid of the stumble but misses with the vac advance hooked up. I'm at a loss.
 
When did this start happening? Did the car run well and then you changed something?
 
69L48Z27 said:
When did this start happening? Did the car run well and then you changed something?
Click to expand...
It started happening once I replaced the old vac advance canister that was leaking with a new one since the other one was capped off I wanted to run vac advance for the optimal performance and efficiency
 
turbine68rt said:
That's sort of contradictory. By "non ported" are you referring to using manifold vacuum? In my case (68 AVS) there is only one source for advance and that's ported. Plugging the line removes any potential vacuum for the most accurate initial setting.
Click to expand...
OK my bad using terms that may be wrong. All Mopars I have owned require, NO VACUMN at idle to the distributor.
 
Just cuz we are in to the distrib, I couldn't get a 350 Olds to run and with a dwell meter I saw dwell variation where the needle wouldn't stay still. The cam lobes weren't true anymore. A new 49 dollar distributor and that car came on like gangbusters. Probably not your issue, just threw that out there
 
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