99Hawk120

I just put a mechanical quadrajet on my 3.8L V6 and I'm having a rough time getting the idle mixture to behave. The carb I'm trying to use is a 17057274.

The problem is that whenever the throttle is cracked open enough to get the idle RPM at an acceptable level, the carb runs extremely lean and cannot be richened up with the idle mixture screws, not even if you back them out to the poitn where they almost fall out.

If the throttle is closed enough so that the idle mixture screws control the mixture properly, the car idles at 500rpm in drive and only slightly higher in neutral.

I set the throttle stop at the maximum amount it would let me and still have fuel control, and then took the carb off and inspected the throttle plate position. The throttle plates are hardly open at all! However, if I open them even a little tiny crack more, it starts to uncovers the transition slot. That's apparently the position that I lose control over the idle mixture. There don't appear to be any other slots in the throttle bore being uncovered for quite a bit more travel.

What I don't understand is why. I was under the impression the transition slots are supposed to feed MORE fuel as they pass below the throttle blade. Why does mine go lean when this happens?

The other thing I don't get is the idle mixture holes in this carb are quite a bit bigger than any other one I have. It should run richer, not leaner, I would think--but that's obviously not the case.

What I haven't been able to tell yet is if the lean condition is caused by the throttle being opened, or is just being caused by the extra air entering the engine. I'm thinking of drilling small holes in the throttle blades to see what happens. I have spares in case it doesn't accomplish anything.

five7kid

iTrader: (14)

Try setting the primary rods/power piston stop up a little higher. That'll bring the power taper of the rods into play sooner.

Might have a weak power piston spring, too. A stiffer one will allow the power circuit to come in with more vacuum.

Most q-jets were calibrated for use with a thermovac air cleaner. I'm assuming you've got an open element on this engine. Off-idle lean is very common with that change.

99Hawk120

It's not an off idle lean spot, so much as it is a lean spot caused by opening the throttle to set the idle speed. It will happily sit there and be lean all day long.

As far as air cleaner, normally I run a dual snorkel (sans snorkels) on this car with the THERMAC connected.

It's definitely something carb specific. Same jet and rod combination in a different carb works fine. I'm almost suspecting a vacuum leak I have as yet been unable to find.

rhuarc31

Could have a plugged idle circuit preventing full fuel flow. Also check to make sure your secondaries are positively closing. Third check for play in your throttle shafts which may be causing a vacuum leak, and makes it almost impossible to tune out. Fourth check for a warped baseplate preventing sealing and causing a vacuum leak which makes it very difficult to tune as well.

99Hawk120

I never bothered putting that carb back on. My guess is it was a vacuum leak, as the one that's on there exhibits similar weird behavior if I do something stupid that causes a small leak.