Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Need Tuning after dyno run
leeklm:
Getting closer, i think... I opened idle down channel from .05 to .06, and confirmed the idle tubes at .036. Running the car up on jack stands (we still have a foot of snow...) and I am no longer lean. If anything, it seems to idle a little rich, and not quite as smooth as I would like. Not bad, just not great. With the 73 jets and 44 rods, I need to have the APT out only about 1 turn, which then seems to run good at part throttle using the tip-in procedure. Turning the mixture screws in will shut the motor down, so they seem to be working as they should. With the holes drilled out to .095, I am running the needles about 4 turns out.
My next jet sizes in the shop were 70, so I tried those, and was able to back the APT screw out about 4 turns for good part throttle running. It "felt" leaner, but hard to say with no load.
I then proceeded to try another (cliff recipe) qjet that was running like a champ on a stock 400, along with a Holley 750. Funny thing is, they all idled and ran very similar at part throttle!
I will wait for spring before I do much more so I can road test to get a better feel on how things are running under load. Maybe it is closer to right than I think it is?
Cliff Ruggles:
70 main jets aren't enough for that carburetor number, it will be lean at heavy part throttle....Cliff
leeklm:
Thanks, I did assume it would be too lean under load with the 70s, but wanted to make a big change to see how it impacted my idle and part throttle testing.
I will go back to the 73s you sold me with the kit for street testing.
By the way, what float setting do you like for this carb? My earlier Qjet was about .25, but the build sheet on this one calls for something closer to 17/32 if I recall correctly.
LM
Cliff Ruggles:
9/32 to 1/4" is best, depending on the actual fuel pressure.
Many late model carburetors used a lower setting, when the factory used higher pressure fuel pumps and return systems to keep the fuel circulating for improved cooling....Cliff
leeklm:
While reading the idle circuit chapter again in Cliff's book, I understand the challenge of getting the proper air/fuel mixture and amounts to the engine. Obviously my 455 is a big change from the 301 my donor carb originated...
Although I did add .098 idle bypass air holes in the baseplate, I still seem to struggle with an rich idle condition. My engine builder used a typical Holley 500 2bbl while running on the test stand, and we were pulling a solid 20" of vacuum. My current vacuum with either of my qjets or holley 750 is about 16", and not as smooth of an idle (again, not "bad", but not perfect). Part idle runs excellent, and can be easily adjusted with the APT.
At idle, I can pull the 5/16" PCV line and the idle speed picks up considerably, and idles better! A vacuum leak this large tells me I need more air at idle, correct? While closing down the idle mixture screws, I can restrict fuel to the point of killing the engine, but my vacuum reading never picks up. Again, more air is needed?
From reading the book, it appears that holes in the primary plates are rarely needed when you have bypass air, so I am hesitant to go down that path, although I do have an identical parts carb for new plates... Also, while this engine is at 468 cubes, it has a pretty mild cam and approx 9.3CR, so nothing even close to radical.
I am stuck, what do you guys suggest for a next step to lean out my idle circuit? Or, am I out in left field here?
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