Q'jet idle elusive - need insight

[CCCC]

As done many times, cleaned and re-set this Q'jet on a 1970 SB. Went through all of the steps and settings that usually result in nice idle and great running. Replaced gasket between the top and middle section and used a slightly less thick gasket to manifold.

Started and ran great at higher RPM but, at first, would not idle AT ALL without setting manual choke closed a bit. Looked for air leaks and plugged the vac openings, and once it is warmed up am able to get it to faster idle, but rough. Idle mixture screw adjustment has no effect. Spraying carb cleaner around outside fittings/gasket area does not show much effect.

Is this problem possibly a gasket issue? Have now tried everything I know to fix the idle (including reading a lot of past posts on here) and will appreciate any insight on possible cure before I take it off and start over.

[trac209]

You could have a look and make sure back throttle bores are not hung open slightly.

[HO455]

If both of the idle mixture screws are not working it is a sign the primary throttle blades are open too far. Try bumping the timing up and then lowering your idle speed. Then see if your mixture screws start working.

[Tquillen]

Have you by chance made any engine changes, like a cam change? Sometimes, with decreased engine vacuum, the throttle blades have to be opened quite a bit (as previously stated). If this is the case, drilling a 1/8" hole in each primary blade will allow the blades to be closed back up, which will increase the signal across the idle slot and make the idle screws operable again.

[CCCC]

Thanks for observations so far. Have made zero engine/timing changes from when this thing was purring. This carb just came off this engine for clean/set due to ethanol issues - and changed nothing with the throttle blades. BUT - will check both blade openings, and will consider the timing bump if nothing more simple/obvious fixes the problem. Will be back after I check those and check/maybe change the gaskets.

Other ideas out there?

[HO455]

If your sure about the throttle plates. Then you may very well a gasket issue. I would pull the carb apart and compare your old gaskets with the new ones. You may find a difference. Don't overlook the base gasket (see photo, these two styles are not interchangeable) If the problem was limited to one side I would suspect that you had a plugged passage.

[dmjlambert]

I don't know if you want to go down this road, but...
On my 1969 CST/10, after fighting with my Quadrajet for a while, I ordered one from National Carburetors. It is a beauty, very clean and nice finish, looks like new. Bolt on and go, no adjustments needed on mine except to choke. Lifetime warranty. Built for modern gasoline. Surprisingly inexpensive at around $180 to $220 depending on your specific truck model. I sent the bad one back for a core refund. Man, that was a good deal.

[CCCC]

Originally Posted by dmjlambert

I don't know if you want to go down this road, but...
On my 1969 CST/10, after fighting with my Quadrajet for a while, I ordered one from National Carburetors. It is a beauty, very clean and nice finish, looks like new. Bolt on and go, no adjustments needed on mine except to choke. Lifetime warranty. Built for modern gasoline. Surprisingly inexpensive at around $180 to $220 depending on your specific truck model. I sent the bad one back for a core refund. Man, that was a good deal.

dmjlambert - thanks. I do have a clean/freshly rebuilt Q'jet on hand (from a different shop) -because most of my trucks have this carb and I never know when one of my own cleaning/kit/adjust jobs is going to fail. I've been very fortunate with fixing Q'jets over the years, but this cleanup may be a flop. If I can't get it straightened out, I'll put the fresh one on and re-do this one from bottom to top.

But, first, HO455 gives me some hope with his comments on gaskets, so I'm going to check those for correctness.

[68Gold/white]

www.cliffshighperformance.com is the Quadrajet answer man. Send him an email. His parts are the best!

[Tquillen]

Cliff is the MAN with quadrajets and associated parts!!

[davepl]

If I hear 1968 or 1969 and Q-Jet and Vacuum leak, I stop reading and post. I post the following answer:

"Whacky 68/69 gasket stack"

Read this, it's likely your issue (if I may be so bold as to guess!):

http://www.corvetteforum.com/forums/...d-problem.html

In short, you're almost certainly ingesting air/exhaust from the crossover into the carb vacuum. Closing the choke a little richens up that mixture, which helps, but it's still inert/leaky.