Carb Help
Thread Starter
Instructor
Joined: Mar 2001
Posts: 229
Likes: 7
From: west point ga usa
Carb Help
I have a problem with a quadrajet on my other toy an 81 Z28. To make a long story short I had a speed demon 750 and had a backfire which caused a short fire (about 10 sec). I figured bad carb and I purchased a quadrajet and installed it. Car still backfired through carb. Traced the problem to a broken advance weight spring. Car will only start with pedal depressed a little. It will not idle at all when the choke opens. If I take my foot off of the pedal the car will slowly decrease in rpms and die. No sputtering just a slow death. I found an old quadrajet and installed it and it runs fine. I took the quadrajet apart and did not find any problems, put it back on car and the same thing. It will start and run until the choke opens. I had a friend start the car and held the linkage open and had him let off the pedal, after that I closed the choke and left around 1/4" opening and it will run all day. The idle mixture screws have no effect at all. I can idle it up around 1500 and it will idle but a little erratic. By me closing the choke and it running tells me it is running lean. What causes a carb to run lean at idle? I plugged all vac ports and I don't think I have a leak. What should I check next?
Melting Slicks
Joined: Mar 2006
Posts: 2,465
Likes: 0
From: Orange Park Fl
Try spraying a stream of carb cleaner around the carb, especially around the base. With the wrong base gasket (a common problem), you can get air leaks there. If the idle speed increases when you hit a certain spot with the cleaner, that is where the leak is. Sometimes the throtle shafts on these carbs get worn and leak air also. God bless, Sensei
Melting Slicks
Joined: Mar 2006
Posts: 2,465
Likes: 0
From: Orange Park Fl
You didn't give too much info on the carb, but here are a few more possibilities. Bad rebuilds are common and you may have clogged passages in the carb. Also, later q-jets had restrictive idle ports that made it impossible to over-richen the idle. This can be fixed by taking the mixture screws out and drilling through with a 3/32 drill bit, but I doubt that is the problem. If it were mine I would probably disassemble, soak it in laquier thinner, blow compressed air through all passeges and re-assemble with new gaskets, but that's just me. god bless, Sensei
