Carb problem...
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
Likes: 0
From: Collierville TN
Carb problem...
OK, I've read through Lars' papers but I am a bit stumped by this problem.
1974 stock L-48 with Quadrajet.
My engine is running really well now (after a lot of work) but I have one small carb problem remaining.
There is a certain throttle position just off of idle at which my engine runs pretty ragged. Not sure how best to describe it....a little bumpy...not smooth. Open the throttle a hair further and the problem disappears. The idle with the car in-gear is a little rough too.
Sounds like a problem in the idle circuit, perhaps inadequate fuel flow? Not sure how to troubleshoot or resolve it though. I've cleaned and rebuilt the carb, float level is set at 1/4", the power piston is free and moving, 19" vacuum at the manifold. Any suggestions would be greatly appreciated!
1974 stock L-48 with Quadrajet.
My engine is running really well now (after a lot of work) but I have one small carb problem remaining.
There is a certain throttle position just off of idle at which my engine runs pretty ragged. Not sure how best to describe it....a little bumpy...not smooth. Open the throttle a hair further and the problem disappears. The idle with the car in-gear is a little rough too.
Sounds like a problem in the idle circuit, perhaps inadequate fuel flow? Not sure how to troubleshoot or resolve it though. I've cleaned and rebuilt the carb, float level is set at 1/4", the power piston is free and moving, 19" vacuum at the manifold. Any suggestions would be greatly appreciated!
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
Its not making the transition from the idle circuit to the main circuit, probably because the main circuit has an issue, maybe because the idle circuit is a little lean. Go thru the idle mixture again. Also have you been thru the timing setup?
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
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From: Collierville TN
I've got the idle screws out over 5 turns each right now so if its still running lean I guess the problem lies within the carb...
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
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From: Collierville TN
Drifting
Joined: Oct 2004
Posts: 1,496
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From: Lafayette Louisiana
Don't do any of this by ear. Get you a vacuum gauge and a timing light. It sounds as if you are close, but the vacuum gauge will tell you what your adjustments are doing. Play with it until you obtain the max vacuum. As you get out of the sweet spot your vacuum will go down. Also, keep them equal.
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
Likes: 0
From: Collierville TN
Don't do any of this by ear. Get you a vacuum gauge and a timing light. It sounds as if you are close, but the vacuum gauge will tell you what your adjustments are doing. Play with it until you obtain the max vacuum. As you get out of the sweet spot your vacuum will go down. Also, keep them equal.
I'll hook up the vac gauge and the tach tonight and see if I can find the best vac + RPM position.
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Team Owner
Joined: Jan 2006
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From: Crossville TN
When you buy that vacuum gage, do yourself a favor and see if your manifold vacuum has any problems first. It sounds to me like you have a vacuum leak somewhere. You can adjust around it for normal idle with the idle adjustments, etc. But once you move the throttle, the vacuum leak is "doing its stuff". Remove all vacuum lines from carb and intake manifold (including power brake diaphargm, if you have one). Hook up the vacuum gage to any of those fittings and plug off the others. Start car, warm up to normal op temp and see what idle vacuum is. (It should be in the 17-21" Hg range). Then add one vacuum circuit back at a time to see if the vacuum level drops or not. If not, keep adding stuff back. When you get everything hooked back up, you shouldn't be reading lower than about 1" Hg less than what you got for "max. vacuum" without any systems hooked up. If you have leaks, you will know what circuit they are in....fix them. Then try to set idle speed and make final adjustments to carb/dist.
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
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From: Collierville TN
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
P.S. Don't forget the vacuum advance can on the distributor. The diaphragm in that can fail/rupture and you'll get the same result. Make it the first system you put back in so that the car runs smoother for the rest of the testing.
Thread Starter
Instructor
Joined: Apr 2007
Posts: 177
Likes: 0
From: Collierville TN
When you buy that vacuum gage, do yourself a favor and see if your manifold vacuum has any problems first. It sounds to me like you have a vacuum leak somewhere. You can adjust around it for normal idle with the idle adjustments, etc. But once you move the throttle, the vacuum leak is "doing its stuff". Remove all vacuum lines from carb and intake manifold (including power brake diaphargm, if you have one). Hook up the vacuum gage to any of those fittings and plug off the others. Start car, warm up to normal op temp and see what idle vacuum is. (It should be in the 17-21" Hg range). Then add one vacuum circuit back at a time to see if the vacuum level drops or not. If not, keep adding stuff back. When you get everything hooked back up, you shouldn't be reading lower than about 1" Hg less than what you got for "max. vacuum" without any systems hooked up. If you have leaks, you will know what circuit they are in....fix them. Then try to set idle speed and make final adjustments to carb/dist.