Too much timing?
Thread Starter
Racer
Joined: Jul 2008
Posts: 250
Likes: 6
Too much timing?
1981 Corvette no computer. New Engine New HEI distributor with vacuum advance . Advance is at its lowest setting. Hooked to manifold vacuum. Car idles great runs great even at WOT but on the interstate slight knocking on acceleration only. I am thinking I have too much advance. Total timing is 36 deg at 3,000 RPM. Initial is around 17. Any thoughts or suggestions to my issue?
Melting Slicks
Joined: Mar 2008
Posts: 2,989
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From: Meriden CT
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Your initial/mech'l sounds fine but to check, disconnect your vacuum advance and take a test drive. If it doesn't ping under acceleration, then you have too much vacuum advance at part throttle. Retard your vacuum advance.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Your timing is fine. It's your vacuum advance curve that's too long. Limit your vacuum advance to 10 - 12 degrees. Most likely, your vacuum advance is pulling in about 18 - 20 degrees of vacuum advance, and that's your problem. 
Lars
Lars
Thread Starter
Racer
Joined: Jul 2008
Posts: 250
Likes: 6
This is exactly what I was thinking. I did disconnect the vacuum when setting the timing. The knock is heard at light throttle condition, everything else is fine. I was thinking that my curve needed to be adjusted. I am all in at 36 deg BTDC at 3000rpm. So i need to keep it connected to manifold port? At its lowest adjustment vacuum advance is around 10.
When I had the CCC system all I had to do was set initial timing and I was done so all of this is new to me and adjusting the curve is new to me as well. When changing springs would I need heavier springs?
Thanks for your help!
When I had the CCC system all I had to do was set initial timing and I was done so all of this is new to me and adjusting the curve is new to me as well. When changing springs would I need heavier springs?
Thanks for your help!
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Race Director
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springs
This is exactly what I was thinking. I did disconnect the vacuum when setting the timing. The knock is heard at light throttle condition, everything else is fine. I was thinking that my curve needed to be adjusted. I am all in at 36 deg BTDC at 3000rpm. So i need to keep it connected to manifold port? At its lowest adjustment vacuum advance is around 10.
When I had the CCC system all I had to do was set initial timing and I was done so all of this is new to me and adjusting the curve is new to me as well. When changing springs would I need heavier springs?
Thanks for your help!
When I had the CCC system all I had to do was set initial timing and I was done so all of this is new to me and adjusting the curve is new to me as well. When changing springs would I need heavier springs?
Thanks for your help!
Thread Starter
Racer
Joined: Jul 2008
Posts: 250
Likes: 6
Burning Brakes
Joined: May 2014
Posts: 884
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From: Rhine Georgia
Hooked to manifold vacuum
I was mistaken I use the AR31 can connected to manifold vacuum not the AR12.
Last edited by Bloodzone; Mar 1, 2017 at 04:07 PM.
Thread Starter
Racer
Joined: Jul 2008
Posts: 250
Likes: 6
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,067
From: Virginia USA
I have the same problem. No matter where I set my initial/mechanical timing my cruise timing (vac can and I like the throttle response of an aggressive timing curve) goes way too high and I get partial throttle ping. Lars makes a vacuum timing limiter and I'm waiting on his reply to get one.
Right now I'm running without vacuum advance and it's fine. The only issue doing that is a decrease in gas mileage and it runs 10 to 20 degrees hotter depending on OAT. I don't want to run without vacuum advance over the summer so I'll install Lars gizmo. I thought about getting an adjustable can but I don't trust them just as you are finding out.
Right now I'm running without vacuum advance and it's fine. The only issue doing that is a decrease in gas mileage and it runs 10 to 20 degrees hotter depending on OAT. I don't want to run without vacuum advance over the summer so I'll install Lars gizmo. I thought about getting an adjustable can but I don't trust them just as you are finding out.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
As Lars said, it's your vacuum advance. I made a limiter for mine out of a thin piece of strap steel. One end has a slot for the vacuum advance rod to go through and the other end is slotted for the single screw at the end
of the vacuum mount. You can then adjust it to the amount of advance that you need.
Mike
of the vacuum mount. You can then adjust it to the amount of advance that you need.
Mike