L48 idle problems
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Instructor
Joined: Oct 2004
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From: Hattiesburg MS
L48 idle problems
Finished rebuilding my Q jet and installed yesterday. Since this was my first carb rebuild I was really glad the car cranked and ran! Where I need help is the car idles like it did before the rebuild. Can't get it to stay running below about 1500 RPM and it bounces from about 800 to 1500. This is what made me think the carb needed to be rebuilt but it didn't help. It has been adjusted correctly (I think) according to everything I've read but I am almost ready to "stab it with a pencil" (quote stolen from another post)! Does anyone have any suggestions of what I can try? 
Team Owner
Joined: Jul 1999
Posts: 65,492
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From: Orange Park Florida
plug ALL vacuum leaks/hoses/sources of maybe a problem untill it smoothes out, check that bottom gasket, there are couple/3 differant styles, and you gotta have one that agrees between the carb base and all those over complicated channels cut in there, and the manifold base.....it's a guessing game far as I can see...you gotta hold it there in place and check what you looking at for the carb and manifold....
after that, it should idle better, then maybe go for too lean a condition...but that changing idle happens from a vacuum leak, engine revs, but dies down untill vacuum drops, idle tips come up from spring pressure, and jets flow more fuel...and rpm's go up again....crazy....had a Pontiac do that to me years ago, sat there scratching my *** wonder WTF was going on....looking for ghosts....
GENE
after that, it should idle better, then maybe go for too lean a condition...but that changing idle happens from a vacuum leak, engine revs, but dies down untill vacuum drops, idle tips come up from spring pressure, and jets flow more fuel...and rpm's go up again....crazy....had a Pontiac do that to me years ago, sat there scratching my *** wonder WTF was going on....looking for ghosts....
GENE
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From: Hattiesburg MS
The bottom gasket I used is a thick one (that is what was on there when I took the carb off). There is another thin one and it matches the holes in the carb and manifold. The thick one is open in the center and allows flow from front or rear of the carb to all holes in the manifold. Is this wrong? I am going to disconnect all lines from the carb and plug them all to see if I've missed any leaks. Will advise of results.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
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From: Sewell NJ
Originally Posted by Mr Fanuc
The bottom gasket I used is a thick one (that is what was on there when I took the carb off). There is another thin one and it matches the holes in the carb and manifold. The thick one is open in the center and allows flow from front or rear of the carb to all holes in the manifold. Is this wrong? I am going to disconnect all lines from the carb and plug them all to see if I've missed any leaks. Will advise of results.
Safety Car
Joined: Aug 2001
Posts: 3,685
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From: Waterford Mi
To check for leaks around the carb base try a lite spay of starter fuild to see if it causes a change in the idle speed. then spray around the edges of the intake manifold to see if a leak is there. Also check your cap and rotor, plug wires, and spark plugs.
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Instructor
Joined: Oct 2004
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From: Hattiesburg MS
Originally Posted by vettfixr
The Q-jets typically use a thick gasket which acts as a heat shield to the carb. You should stick with the thick gasket. When you rebuilt the carb did you seal the well plugs. These are notorius for leaking and should be sealed with epoxy. If you didn't there's a possibility that you are dripping raw fuel into the intake which is causing an overly rich condition regardless of how you adjust the idle screws. Take a look at one of Lars rebuild faqs and make sure you did everything you needed to do.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
You should be able to find what you need here.
http://www.corvettefaq.com/listing.asp?group=1&sub=8
If not, Lars posts here regularly and is the Q-jet god. Look back in the old posts and I'm sure you'll find him. Good Luck.
http://www.corvettefaq.com/listing.asp?group=1&sub=8
If not, Lars posts here regularly and is the Q-jet god. Look back in the old posts and I'm sure you'll find him. Good Luck.
