Vacum advance and knocking?
Thread Starter
Racer
Joined: Jul 2001
Posts: 341
Likes: 0
From: macon, ga
Vacum advance and knocking?
I replaced my cam a while back and at that time i found out that my mechanical advance was also stuck, so i installed a new dist. i set my advance at 36 deg at 3000 rpm w/ the vacumm pluged. when i plug everything back together my base timing is 8 and any time you rev the engine past 2000 rpm the timing will go to 52 and stay there no mater how high you rev it. is this how its supposed to work? second question. when the base timing is at 8 i have a knock at idle. it almost sound a loose rocker arm but two or three times louder. when i advance the timing to 12 it almost goes away. at 20 it is gone. any ideas what it is?
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Re: Vacum advance and knocking? (ykp53)
I have mine set to 35-36* @ 2500 rpm and used an adjustable vacuum can to give me a total of 52* @ full vacuum. 50-52* total, mechanical and vacuum is what I have seen recommended.
No idea about the knock.
No idea about the knock.
Melting Slicks
Joined: Feb 2001
Posts: 2,115
Likes: 0
From: VA
Re: Vacum advance and knocking? (ykp53)
I am having the spark knock problem too. I put in a new HEI distributor with mechanical advance. My total advance is 36 deg and the initial is 20 - 22 deg. I was told by the shop that put in the distributor that if I hooked up the vacuum advance I would have an even bigger problem. I don't have a problem at idle but I get the knock on acceleration. If I back off of the advance then I have a problem with it starting after the engine reaches operating temp. :confused:
Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
Re: Vacum advance and knocking? (ykp53)
52 should be OK for cruising speed. Now remember the vacuum advance goes away as you increase load on the engine. It should go away as you suddenly increase RPM (increase load/open the throttle suddenly) and go back down to 36 or so at WOT (whatever you have your total set at) and then settle back up to 52 as your load decreases and vacuum is increased. 52 at 2,000 may be a bit to advanced and your curve is comming in a little bit to quickly. Are you saying it stays at 52 even when you punch the throttle? If so then you must either 1.) Have something stuck in the vacuum advance mechanism that keeps it from closing when vacuum decreases or 2.) Your vacuum advance mechanism is too sensitive and opens at a very low vacuum rate.
Check out your vacuum can, make sure it is functioning currectly (take the distributor cap off, attach a vacuum guage to the can, and watch it operate, see what pressure level it opens or closes at).
This may explain why it is pinging at idle as well, maybe your vacuum can is opening at idle and advancing to far. Do you have it connected to ported or manifold vacuum? Although it doesn't explain why if you increase the timing the pinging goes away. Hmmmmm.
Check out your vacuum can, make sure it is functioning currectly (take the distributor cap off, attach a vacuum guage to the can, and watch it operate, see what pressure level it opens or closes at).
This may explain why it is pinging at idle as well, maybe your vacuum can is opening at idle and advancing to far. Do you have it connected to ported or manifold vacuum? Although it doesn't explain why if you increase the timing the pinging goes away. Hmmmmm.
Safety Car
Joined: Feb 1999
Posts: 3,631
Likes: 14
From: Fountain Hills AZ
Re: Vacum advance and knocking? (ykp53)
If the knock decreases with more timing it probably isn't pinging, especially since it does it with no load. I would expect that your idle speed is increasing as you advance the timing. Are you resetting your idle speed back down? If not, it could be that the sound is just becoming less noticeable as the rpm increases. Piston slap and some other things will do that. As engine speed increases the "knocks" become closer together until you can no longer hear the individual knocks even though they're still there.
