# Timing and vacuum port help



## Kenny-68 (Apr 14, 2021)

Hello all. Bought a 68 GTO and new to the forum. My GTO came with no history info. Engine appears to be original (#16 heads) save for the Edelbrock AVS2 carb (original style rebuilt q-jet on the way) and TH400. From reading posts here, I got a copy of Lars' timing procedure and used it. Dialed in total mech timing at 36 degrees at approx 3000 rpm. Came back to idle and engine sounds great. Checked idle timing (no vac adv) and reads 18 degrees. Seems high, but runs good and cool. Is 18 OK? Second issue. At equal idle rpm's (600) and in gear, the engine runs better with ported vacuum rather than the recommended manifold vacuum. With manifold vacuum, it almost dies on acceleration from stop. And stumbles at idle on a quick stop. I think I know what may be going on but wanted some input from the forum. Thank you.


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## michaelfind (Dec 14, 2018)

Welcome. I’m no expert on the issue but know others here will chime in soon with solid advice.


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## Lemans guy (Oct 14, 2014)

So you have a vac can that is pulling too much timing, yes 18 can be ok and 18 Centrifigal for 36 total. You may be read a dated timing appear, newer analysis led Lars to create the vacumn corrector that limits the vac to 10 degrees…E mail him @ [email protected] about $14 bucks……I will tell you about the can vac can later …I got to go now…I just told someone on another thread


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## Lemans guy (Oct 14, 2014)

You can run ported, but I would just run that until I could get the vac can right.

The correct distributor for that year was a points style not a wide body HEI. Lars corrector will fit the narrow body points style..

My favorite vac can for those is a B26 can…..Standard Motor Parts (SMP) makes em.

Rock auto part # SMP VC 181
Same can from O’Reily number is BWD V375.

Use that can and Lars corrector hooked to full manifold vac. See how that does.


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## michaelfind (Dec 14, 2018)

Yes, what LeMans Guy said. But be warned, I followed his advice and now I have a new problem. I can't ever get enough traction! 🤣


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## Kenny-68 (Apr 14, 2021)

Thanks Lemans Guy. I did read something about changing the vac can, but didn't know if that would be the issue here. If I am at idle with 18 degrees of mech advance and I hook the vac advance to the manifold, will the additional advance be the amount of adv the can provides?


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## Lemans guy (Oct 14, 2014)

Yes, if you have 18 degrees of base timing you don’t really want any more than 10 degrees of vac advance hooked to full manifold vacumn. That will give you 28 degrees of idle timing.

if the cam is medium to hot his will run good, if the cam is very mild this may be too much at idle.

I think what is happening is you are adding a vac can that likely has 20 + degrees of timing and adding that to 18 makes your car run awful. Timing can be too much or too little.

I have set up a lot at 18 base and 18 Centrifigal for 36 total. And then added ten vac for 28 at idle.

I don’t like to start with that much idle timing. But lots of racers go for that setting and some some new distributors come set like that. If you have a Petronix 2 or later module it will regard the crank timing by 4 degrees so you don’t get kickback on the starter when cranking.

So you can try 18 + 10 vac and see how you go.I am not a fan mod ported vac, but some guys run it, I think usually because their base timing is too high, rather have that in the mid twenties. But each AFR a tad different.


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