: 425 Qjet jet size for dual exhaust



sixtycyclehum
06-02-10, 09:18 AM
Anybody know the stock jet size for a '79 Cad with original Q-jet? Mine has had dual exhausts installed before I got it- used stock Y pipe (due to crossmember, I suppose), then dual pipes, no cats. I doubt they re-jetted the carb, but would guess it may need it, might be a bit lean. Should I go up one size or two?

cadillac_al
06-03-10, 07:17 AM
I guess you're going to have to take it apart to know for sure. Most Qjets i have worked on have 72's or 74's. The dealers used to keep them on the shelf so it was no big deal to go buy some. I doubt if that's the case now. I don't get too concerned with jetting when I add duals, especially 70's cars, they seem to be rich enough. It's those 80's cars that are jetted lean and the old Chevy 305's did benefit from some bigger jets. I'd say if it seems to run fine I wouldn't worry about it.

sixtycyclehum
06-03-10, 11:04 AM
I was hoping to have jets in hand when I take it apart. I rejetted my '68 CDV 10 years ago and it had 71 jets in it. From what I have read the late 70's GM stuff was leaned out pretty high for emmisions, with EGR and whatnot. I've got a spare Q-jet from probably somewhere around 77-79, I'll crack it open and see what sizes are in it.

deVille33
06-05-10, 10:49 PM
You need Doug Roe's Bible to understand the internal working of the QJ. The size of the needles that fit into the jets have as much to do with performance as the jets do.

djcwardog
06-08-10, 05:50 PM
You need to check out also what Lars posts on the Corvette forums... He is a superb car tuner and really knows the Quadrajet. He writes about calculating the rod area (at tips for WOT and at the thicker base for cruise) and the jet area and starting with what came from the factory for those specs... On my '77 425 4-bbl, it had the stock M4ME carb and I found inside that it had the newer 0.034" tip rods (as expected for a M4M like ours...), size #52M and #72 jets. I left the rods alone and got it up to #75 jets before it really ran well. It was lean as Hell when I got it and worse after I added true duals, a 472/500 stock intake and MTS's MT-3 cam. Unless someone tuned that carb before you, I can (almost) guarantee that your car is running lean now and it will be worse when you allow it to breath a bit easier... See to this soon as sunken valves are easy to get if you run lean for too long - even on a freshly rebuilt motor...