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 Quadrajet Idle Needles no Control 
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Joined: Tue Dec 01, 2015 11:26 am
Posts: 3
I have a project I inherited from another guy in my shop I'm trying to sort out, the carb appears to be an early 80's non-electronic model with APT and is a front inlet design. It has no part # on it anywhere so there is no good way to identify it otherwise. The carb also runs external vacuum for the power piston if that tells you anything about age. I have built dozens of these over the years and have never run into anything like this. The idle down-tubes are at .035, the upper idle air bleeds are at .075 and the mixture discharge ports (where the idle screws go) are at .085. I ussually don't mess with the lower idle air bleeds or the tubes which flow down into the base-plate with the mixed air/fuel. The primary jets are 64's and the rods are 47Ps. Everything else on the carb works fine but I can not get it to idle at all on the primary circuit and I'm only getting idle control with the idle screws nearly all the way out. Is this carb just sized too small or is something else going on? It is making me nuts, I've never had one of these that wouldn't idle no matter what else was done?

Tue Dec 01, 2015 11:36 am
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
The external vacuum reference for the power piston identifies it as most likely a carb for the Turbo Buick V6 or perhaps Turbo Trans Am with the 301 V8. The external reference power piston version was also used on some NA 301 Pontiac engines, some with the same carb number as the Turbo Buick.

Are you sure there isn't a number stamped on the vertical boss adjacent to the secondary throttle lever?

The idle circuit calibration is the same as a Buick carb I have in my notes, but the main jet you have is .010” smaller. Unless that is a small venturi single bleed carb the .064” main jet is way too small and is probably too small even in one of those. The last turbo carb I worked on had a lot of the T-slot showing below the closed butterfly. If yours has a lot of slot showing below the butterfly it is probably getting all the idle fuel it needs from the slot and that renders the mixture screw somewhat redundant.

If the curb idle discharge hole in the primary bore is too small and too much slot is exposed the adjusting screw doesn't have enough flow area to make much difference, relative to the are of the exposed slot.

I think part of your problem is the main jet is too small and I'll bet a lot of T-slot is below the butterfly.

(A) Is it a large or small venturi, (B) what is the primary main air bleed configuration, single or dual, and (C) what size is the curb idle discharge hole in the primary bore?

Wed Dec 02, 2015 2:48 am

Joined: Tue Dec 01, 2015 11:26 am
Posts: 3
1) Large Venturi
2) Upper and Lower Main Air Bleeds, .040 and .050 respectively
3) .075 on the curb idle discharge hole

I went back and looked and you are correct, a lot (nearly half) of the T-Slot is exposed. We ended up going to 74's on the jets and it still seems to be on the lean side at idle and part throttle.

Wed Dec 02, 2015 3:31 pm

Joined: Tue Dec 01, 2015 11:26 am
Posts: 3
I'm beginning to wonder if the larger jet vs the very small main upper and lower air bleeds will cause an issue with the car running well?

Wed Dec 02, 2015 3:45 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
When you say "idle down tubes are .035" do you mean the Idle Down Channel Restriction, the little cup pressed in the body at the top of the idle down channel to the base, or is that the size of the idle jet in the idle tube, the approx. 1" long brass tube with the collar on top and the jet (pinch) at the bottom you would have had to extract from the body to measure?

What size is the hole in the little brass cup?

.075" is small for the curb idle discharge. .086" or so should help, but take care with that because if the idle mixture screws are the later short and very fine thread variety (compared to older carbs) they bottom on the spring when the discharge hole is enlarged too much.

The main air bleeds are OK. The main jet and rod (and power piston spring) will need to be selected to please the engine, in any case. .074" MJ with those bleeds may be a little rich.

Are you using a WBO2?

You might find it useful to move the idle air bleed from the bore wall, where it is just above the IDCR, to the lid. Plug the bleed in the wall and drill and tap the lid for a 6-32 set screw so you can tune the idle air bleed size.

This thread describes the procedure and has some pictures. viewtopic.php?f=26&t=430&start=0

Wed Dec 02, 2015 4:22 pm
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