I have a Qjet #17058213
What did it originally come from?

I'm trying to work out the vacuum for it. I think it's missing one of the "thing that the vacuum hose goes onto" (nipple?) dead center front on the throttle plate.

any help would...help.

The big 3/8" nipple for the PCV valve line?

No, that's there fore and aft.

see arrow
http://i206.photobucket.com/albums/bb59/jackassplus/100_2903.jpg

78 Chevy, federal emissions, manual trans.

Hmm. don't have as many ports as the Chilton manual has for 1978 Federal. It shows 6 ports.

I have 2 3/8" ports in front and in back, the one above the mixture screw in the picture. and one for the front vacuum break. (No rear vacuum break)

On your original carb did a medium-large vacuum line go there? Like 5/16", if memory serves. If so, it was a low vacuum emissions port for purging the charocal canister. No vacuum at idle, an only gradually ramps up as you lean into the throttle.

If so, I'm not sure it could be added to a carb that didn't have it originally.

It had a Street Demon on it when I got it, Never worked right. Hence the switch back to a Qjet. I got it from a '79 C35 GMC 454. Guess it probably wasn't the original for it either.

What is the difference between the AT version and the MT version?

Difference between a MT QJet and an AT QJet? No lever or stud on the bottom of the throttle arm for a kickdown cable is the usual difference, although some automatic transmissions don't use a kickdown cable, like the TH400.

Just for you Qjet guys http://www.nastyz28.com/~copo/qjet.zip