Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Poor Cold Running - Sputters, Backfires, Hesitates, Dies

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kuhnzoo:
I appreciate the response. The rebuilder told me he was increasing the jet sizing because the replacement Goodwrench 305 motor had more HP than the original 305. And, no, there was no provision added to adjust the APT.

I believe part of my problem was also excessive vacuum advance timing. I have since added a limiter plate to the vacuum adv can to reduce total vacuum advance to 12 degrees (I read most HEI cans are about 22 degrees) - confirmed with timing light. That has helped with driveability when warm.

I have two other peculiarities/curiosities with this carb.

The first has to do with the carb vacuum ports. They do not match my underhood label, so I doubt this is the original carb (not really surprised given the age and carb #). There is no "H" ported vacuum port (front driver's side) on this carb. My "B" vacuum port (where the label shows my distributor should be connected) reads manifold vacuum. I thought the distributor was supposed to have a ported vacuum source from factory? And I have a "K" ported vacuum port in the front of the baseplate (just to the driver's side of the manifold vacuum port for the PCV) that I have not found any info about. The other existing ports match the underhood label.

The second is that the idle speed screw can be completely backed out and I cannot reduce the idle speed (in D) to 600 rpm even with the timing set at 4 BTDC per the label (I prefer to run initial timing at 8 BTDC). It did this with the standard size jets also. Idle mixture screws were set with a vacuum gage. Vacuum at idle is about 19 inHg. There are no more vacuum leaks as all hoses and defective/leaky vacuum devices have been replaced, removed or capped. Never had a carb with this problem. Only way I can reduce idle speed is too lean out mixture screws or retard timing, neither of which are proper solutions.

Am I trying to fit a square peg in a round hole (wrong carb for application)? Or perhaps there is more wrong with this carb than I realize? I am sorry to dump all this out there. I do not have time to become a QJet expert (and trust me, as a mechanical engineer, I am very curious about all this). I would have preferred to send this carb to your shop for remanufacture, Cliff, but your published turnaround for a remanufacture is 12 months and I did not have 12 months to wait.

Thanks again for any and all replies. I do appreciate everyone that has taken the time to read and/or reply to my post.

Rob

kuhnzoo:
Just curious if anyone knows anything about what seems to be an odd arrangement of the vacuum ports on this carb? I can't find any info about this port labeled "K" in the baseplate.

Anyone have ideas about the high idle speed? My concern with this (besides the speed being too high, of course) is that sometimes the engine "diesels on" after shutting off the igntion. This is somewhat embarassing.

Thanks, Rob

kuhnzoo:
OK, I took the top off the carb to check out the primary hanger arms. They were not even as judged by my eyes, but only slightly off. I adjusted the arms to where they looked even.

Also looked at primary rods and jets for sizes. Jets = 72. Rods = 51M. I have no idea if these are what I should be using for my engine?

With the airhorn off, I drove out the plug above the APT screw and tapped the hole for a threaded plug. Just need to get a threaded plug for the hole now.

Adjusted the APT screw 2.5 turns up from fully seated as a starting point.

Just need to install on the truck and start adjusting APT to hopefully eliminate the off-idle and off-cruise hesitation/poor throttle response. If I get this resolved, I still need to figure out why I can't get the idle speed down to the proper RPM.

Thanks again for any responses.

Rob

429bbf:
i would check if your rebuilder got one of the throttle plates not square. if you still have the carb off .tip it over and see how much light you can see with the plates closed. you said they put bushings in so they had to have the plates off.also i would check to see if theres a little play in the rod going to secondaries . with the pri. closed .id also look at the fast idle for any rubbing or binding.it all seems so simple but once in a while we all miss the obvious.ps. make sure the secondaries all the way shut. (fwiw )

kuhnzoo:
Thank you for the suggestions. I did give the throttle plates a "once over", but I will double check for good measure. And I will check out the rod going to the secondaries, look for any hang up on the fast idle cam side and make sure the secondaries are fully closed.

Anyone have any feedback on the primary rod/jet combo I mentioned? I can't remember the secondary hanger / rod combo, but I am not so concerned with that for now since it runs fine at WOT (which is almost never anyway for this stock engine).

Thanks again.

Rob

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