Beating my head against the Q-jet wall and getting it all bloody!

[fanmanj]

Ok, guys... My truck runs fine, at anything but idle... Romp on the gas, no bog, plenty of power. Try to let it idle, the idle goes up, down, sounds like a misfire at times. I've rebuilt the carb, cleaned all passages and had the idle mixture screws set anywhere from 3/4 out to 21/2 out, no difference. I disabled the fast idle cam, and if you close the choke to half the idle speed increases... That tells me its lean... Arrgh! I'm really at the end of my rope... I think the carb is stuffed... Anyone have a spare 17085226 quadrate laying around? I think as cruddy as this carb was, it may be time for another...
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[cjlinkster]

Could you be having a vacuum leak somewhere? How about at the base of the carb and intake?

I have owned just a couple of Chevy's with Q-jet, and I have had good luck out of them.

[fanmanj]

No vacuum leaks, the intake gaskets are new, new vacuum caps, new vacuum hoses, new carb base gasket... I'm starting to think my idle passages are blocked up in the carb... I really wanna stick with the q-jet because I want the best mpg, and I know they can be a really good performing carb.
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[Restrorob]

Need more info;

Did you just install this carb or have you been running it for a while and it started acting up ?

What engine and stock or modified ?

There's really no set number of turns to open the low speed adjusters, Best results is using a vacuum gauge and adjust each as close to 20" engine vacuum as possible.

Can you kill or nearly kill the engine by running both adjusters in all the way ?

Did you check the throttle shafts for wear while you were rebuilding it ?

Sorry for all the ?'s but I'm a diehard Q-jet user of many years. The big brown truck dropped one off today that's getting a custom build for my 502, I'm done with the sorry Demon.....

[my4by2]

Did you reseal under the fuel bowl after the rebuild? If not it can leak down and screw up your idle air/fuel mixture

[fanmanj]

I just installed this carb. It is the stock one that came off of my '86 305. The engine is a stock roller cam TBI 350. The carb was in rough shape. It was flooding terribly when it was on the old engine. I cleaned it the best I could. The float chamber and jets were disgusting. I had it on the old 305 and ran it a couple of times, it acted just the same. I've actually got it torn apart right now, and I checked all the passages... Everything checks out... It does have a little slack in the throttle shafts, but not bad enough for this.. The bowl plugs don't appear to be leaking, but I'll slather them with some epoxy just to be safe... I also wanna go ahead and grind the stop on the secondary air valves so they open 100%.
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[rayfinseats]

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Could you be having a vacuum leak somewhere?

Usuallly when the mixture adjuster screws don't help and it idles up and down...

[ray_mcavoy]

Hi fanmanj,

I don't know if this applies to your particular model # carb or not, but I remember reading an old service bulletin that said there were a couple different design Q-Jet baseplate to float bowl gaskets. And accidentally installing the wrong style will create an internal vacuum leak and resulting rough idle.

If you've still got the carb apart, check to make sure all the holes/openings in that gasket are lining up properly with the corresponding openings/passageways in the baseplate and float bowl.

[cjlinkster]

Perhaps this artical will help you with the Q-jet:

http://www.73-87.com/7387garage/drivetrain/qjetidle.htm

[TheWhiteElephant]

My guess is also with the throttle shafts at the base plate. If memory serves, that is a symptom of too much play in the shafts at the baseplate, a common issue with the qjet.
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