Been having a time coming up with a carb that doesn't have some sort of problem.
I have 3 different original carbs (Two 7027036, one 7027153) &, all pretty clean, put kits in two.
All three work fine above idle speed.
Two have rough idle problems which I think are related to worn throttle valve shafts.
The third carb will idle fine while the engine is cold, but when the engine reaches around 150 degrees, it stalls.
It can be restarted, but runs very rough and only if the RPM's are kept high.
I plan to have one of these carbs restored, but the last one I mentioned is perplexing.
Obviously, I'm not a carb expert, but I still can't imagine why it idles fine until it begins to warm up.
Possibly temperature has nothing to do with it and it's just coincidence that the problem occurs at 150 degrees.
Any ideas from someone who knows more than I do?

 

Sounds like a vacuum leak. Since the carb's functions are controlled by vacuum, it is important to minimize leaks. Perhaps try eliminating possible leaks- like remove the power brake vacuum line and plug/ cap it. Small vacuum lines are new? Choke pulloff is testing ok- stays pulled when vacuum is applied? Spray carb cleaner or similar flammable around intake gasket, carb base gasket, throttle shafts, etc. see if it gets drawn in at a leak and changes engine speed/ behavior.

The temp related thing is weird. Choke comes to mind- that is temp related.

Are the carbs' well plugs sealed with epoxy or at least checked for leakage during the rebuild? All passages cleaned and probed during rebuild? The idle tubes are hard to remove and ordinarily do not get removed, but the book author says you have to extract them to ensure that no grunge remains under there in that idle passage. On my last carb [800 cfm for 403] I removed those tubes and found it very clean underneath even though the carb had been outdoors [underhood] for so long the accelerator pump had to be removed with pliers.

 

Thanks for your input, Chris.
I've pretty well covered everything you've mentioned.
I always epoxy the secondary metering rod wells anytime I have a Quadrajet apart.
Like you, the temperature thing makes me suspect the choke, but the choke flap is wide open when the stalling occurs.
Even though I've checked the intake for leaks as you described and it tested OK,
I still suspect this might be the problem since I recently had the engine out for resealing and gasketing.
Hate to do it, but, I'm considering pulling the intake and redoing the gaskets.
I'm thinking the intake gasket(s) might be leaking on the bottom side inside the valley
where they can't be checked.

 

Do you still have the ST300 trans in it? Is the switch pitch functioning properly, also make sure your advance plate isn't sticking in the distributor. For the carb, if you look down into the carb and operate the throttle by hand, do you have 2 good streams of gas spraying from the accelerator pump.

 

I've upgraded the original Jetaway to a 700R4, so the switch pitch is not an issue.
Went through the distributor (as well as everything else!) when I had the engine out,
but, I haven't actually checked it since I got the car running again.
Yes, the streams of fuel seem to be normal.

 

I find that if you need large throttle opening to keep the carb running, the idle system is usually too lean due to vacuum leak or clogged idle fuel passages. You then have to open the throttle blades far enough to run off the primary main metering. In these cases the idle mix screws dont have much effect.

Another possibility is that youre too rich at idle. When cold, the engine will like the extra fuel, same as when the choke is closed. Once the car warms up allittle, the fuel evaporates better and the mix is too rich for the engine so it stalls. The too rich symptom would produce a wavy uneven idle and soot out the tailpipes. The waviness would build and the engine would stall out. The over rich could be caused be a heavy/old float, primary or secondary nozzle drip, a stuck power piston etc etc

just some ideas to consider.
whatever the source of the problem, a pro Qjet rebuilder will be able to have whichever carb you choose running like new.

www.everyday-performance.com

 

Ken,
Thanks for the suggestions and the reference.
I've been researching rebuilders to decide who to use.

I considered a bad float. In fact, I got out a new float, but haven't installed it yet.
I just can't imagine the float working OK for 10-15 minutes, then suddenly absorbing a lot of fuel
on a very consistent basis, then, after the engine cooling down, working OK again.
I may go ahead and change it out just to see if that cures the problem.

I'm still suspecting an intake leak, although when I've checked it, it seems OK.
I'm thinking it may be leaking on the bottom side in the lifter valley where I can't check it.
Thanks again.

 

if you are going to have one rebuilt give Custom Rebuilt carbs a call. I had them rebuild a 66 carb for my BTR 468 build.



http://customrebuiltcarbs.com/index.shtml

 

Ed, just came across this thread.
I did a lot of the same things you did.
Changed out like three carbs.
I eventually found my rotor was one
of the cheapie's. Replaced it with an
older A/C Delco unit with the brass
riveted electrode, and it helped the idle a lot.
I am going to try to get the one Eric suggested
from Napa with the longer electrode and see if
that helps even more. Just a thought. Mine still
has a little hard start after warmed up and running at temp.

 

Thanks for that tip.
I hadn't considered the rotor since all that is new, but, it's a "brand-X" from Advance.
Maybe I should find an A/C Delco and see if that helps.
Do you have a part number for the NAPA one you mentioned?
Points, condenser, distributor cap, plugs and plug wires are also new.

The other 2 carbs started hard cold overnight, first start of the day, but,
they started easily for the rest of the day.
Although I epoxied the secondary metering rod wells on all 3 carbs,
I think at least 2 are still leaking off somewhere overnight.
This could be the reason for the hard starting first start of the day.

 

Check the thread 'Dist rotor gap UPDATE'
from brown7373. Good info on the subject.

 

Check the power piston spring. If spring is too stiff, it may lift the power piston off it's seat and cause it to run too rich (low vacuum and/or vac leaks at throttle shaft as mentioned before).

 

I ran into this once on a non olds powered car. it had a vacuum leak that was huge. turned out a plug was missing in the intake.

Just a thought