General Category > Quadrajet Carb Talk and Tips
Lean at Idle
71vette:
I finally got my carb built (used a 1979 carb) following all the procedures in the book. I bought all the parts from Cliff. It was a ton of fun and it turned out great on the bench. I installed it this afternoon but it isn't idling well at all. It seems to be lean at idle because when you put your hand over the airhorn, it speeds up and seems to run better. I of course also have the nozzle drip because the throttle plates are open too much just to keep the car running.
I checked everything for a vacuum leak and found none. I kept the original carb untouched as it runs pretty good with that carb so whatever problem I have is with the new carb.
Can anyone give me some tips or things to check?
1971 Corvette, 454 manual, 9.5 CR with 180 lbs static compression, very mild cam with 218 duration on intake, 228 duration on exhaust with .510 lift.
Cliff Ruggles:
Did you measure the idle system specs?
Lower idle airbleed, upper idle airbleed, idle tubes, idle downchannel restrictions and idle bypass air.
It needs more idle fuel, which is relatively easy to accomplish by looking at how the carb is set-up, then using one of the "recipes" from our book.....Cliff
71vette:
Hi Cliff: Yes, I used recipe #1 from the book exactly. You and I had a few emails back and forth. I am the guy that asked about driving in the air bleeds vs. drilling the holes to the brass air bleed outside diameter.
Anyway, I have the main bleeds at .070, the idle tubes at .036, the idle down channels were already at the proper diameter .046, I drilled out the mixture screw holes to .090, set the float at .250. I also blocked off the choke vacuum hole by tapping and threading a screw coated with epoxy. It has#71 jets with your #44 primary rods. I used the light blue power piston spring. I left the by pass air stock because I think it measured .080.
I took a lot of time and care with the measurements including double checking the drill sizes with a digital micrometer.
Once I get beyond the idle system with the throttle, it seems to run fine. I can't find an external vacuum leak. I have adjusted the mixture screws to about 4 turns which seems like a lot to me especially with the increased hole size.
Since this is a newer carb on a 1971 factory intake, is there any mating issue I should look for?
I'm sure it is something I've done wrong but I don't know what it is.
Cliff Ruggles:
Nothing done wrong, the engine simply requires more idle fuel than you've allowed for it.
Also measure the idle airbleeds in conjunction with the "recipe". A larger upper of lower idle airbleeds requires sizing it down or compensating with a larger idle tube.....Cliff
71vette:
I backed out the idle screws another couple of turns (4 turns out) and the engine idled better but it still didn't idle well. I started looking for a vacuum leak using brake cleaner. I know I may be fooled but there is a big pickup in idle speed when I spray brake cleaners on the secondary throttle shaft on both sides of the carb. I have read where Cliff doesn't put bushings in but there is a pretty good amount of up and down and side to side movement of the shaft from both sides of the carb. Additionally, when I opened them up a bit and let them settle back, it idles better for a few seconds and then starts to get rough.
I realize that I might not have the secondary plates lined up but I was very careful when installing them and held the base plate up to light to make sure they were sealing perfectly. I checked my original carb and the secondary shaft is not nearly as loose as the carb I rebuilt.
I'm wondering if I've found the issue or if I am just being fooled?
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