Q jet question

[Nordic]

I have a 350 sbc with a TH350 and 308 gears in the diff. The engine is stock 1978 that has been refeshed. I am running a rebuilt Q-jet and it use gas like it was still $0.30 a gallon! Truck idles smooth and pulls a steady 21 inches of vaccum. Under acceleration(not wot) it seems like it is triing to hard and not going any where quickly. Timing is set right and no pinging. Am I missing some thing with the carb?

Thanks
Bruce

[mknittle]

Might be better off with a vacuum secondary holly. Q-jets have mechanical secondarys that are huge. Q-jets are also a pain to tune to be efficient.

[Speedbumpauto]

If you look down the vent hole between the primary barrels(as I remember) you can see the top of a metal T. The vertical side of the T is a piston with a spring under it and at each side of the horizontal T is a needle.(2, one on each side) The metering rods(needles) move up and down in the primary jets to control the mixture. Engine vacuum pulls the piston down against the spring and moves the fat part of the metering rods into the jet to keep the mixture lean for cruising and that's where it spends most of it's time. When you step on the gas, the manifold vacuum goes away and the spring forces the piston up and then the skinny part of the metering rod is in the jet which richens the mixture for acceleration power. ONE of the common problems with that carb is the piston sticks in the down position where it spends most of it's time and the vehicle has a hard time accelerating. The other is it sticks in the up position and you get horrible gas mileage but the engine runs OK. To the moral of the story. You should be able to carefully stick a small screwdriver down the vent hole and push the piston down against the spring and let it come back up if it's working properly. If there's any doubt, you have to take the top off the carb and check. The thin metering rods are easily bent and they are very finicky about fitting properly down into the primary jets and assembly line rebuilders can easily screw them up. The other problem with those carbs is the throttle shaft bores just get worn out and very few rebuilders tackle that problem. They just give you someone elses problem. If you can wiggle the primary throttle shaft up and down where the throttle cable hooks up, it's time for a new carb. The base plate can be bushed but the cost is usually too much unless you need a numbers matching carb for restoration. Sorry about the length of this, but that's the story.

[xaircav]

He's right about the Q Jet primary enrichment system. If yours passes all the tests try a weaker power piston spring.