Carb adjustment
Thread Starter
Burning Brakes
Joined: Aug 2011
Posts: 828
Likes: 286
Carb adjustment
Hello Everyone
Hope everyone is having a good summer. Tinkering and taking out the car in nice weather. 79 L48. Car starts fine, idles good, accelerates good but has one issue. After it is warmed up there is a problem with a very slight pedal.
If I am driving along and not touching the pedal at all or giving it more then 5% throttle it is fine. Between 0% and 5% throttle there is a metal tapping sound. That is the best way to describe it. I am guessing a lean tip in but am not sure. It does not happen when the car is warming up.
Would a good guess be deposits since it is not driven that much? When it comes to quadrajets i have VERY LIMITED experience but am not afraid to open it up. Short of a full rebuild I was wondering if there is anything to look for or adjust. Any help is appreciated.
Hope everyone is having a good summer. Tinkering and taking out the car in nice weather. 79 L48. Car starts fine, idles good, accelerates good but has one issue. After it is warmed up there is a problem with a very slight pedal.
If I am driving along and not touching the pedal at all or giving it more then 5% throttle it is fine. Between 0% and 5% throttle there is a metal tapping sound. That is the best way to describe it. I am guessing a lean tip in but am not sure. It does not happen when the car is warming up.
Would a good guess be deposits since it is not driven that much? When it comes to quadrajets i have VERY LIMITED experience but am not afraid to open it up. Short of a full rebuild I was wondering if there is anything to look for or adjust. Any help is appreciated.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,016
Likes: 4,371
From: Marlton NJ
Sounds like it is going momentarily lean, and you are getting spark knock or "rattle".
It is happening under very light load, but it could be somewhat dangerous to the pistons, so you want to minimize it and cure the problem.
Honestly I would set up the timing curve first, then tune the carb.
Have you tuned the distributor curve?
Look up LARS method & instructions.
Basically:
It is happening under very light load, but it could be somewhat dangerous to the pistons, so you want to minimize it and cure the problem.
Honestly I would set up the timing curve first, then tune the carb.
Have you tuned the distributor curve?
Look up LARS method & instructions.
Basically:
- Change distributor springs to get full advance all-in by 3000 rpm
- Set mechanical advance there to 36 degrees, & lock down distributor
- Just make a note of what your setting is at idle for future reference, hopefully 10-15 degrees. Or come back for advice.
- Then hook up vac advance to manifold vacuum.
- Restrict the vac can to 12 degrees extra vac advance at idle.
- You will need a vac limiter for that. The distributor cans usually have way too much advance for the "new" gas we run now. This is usually where the "rattle" comes from.
- Then tune the mixture in the carb.
