Can't find my old post but I had asked awhile back what the OEM jet sizes would be for a 4bbl Q-Jet on a 1971 350 auto. Recently added a TA manifold, 284 cam, heads ported etc., basically a mild build. Would there any worthwhile improvement found in upping the jet sizes from OEM?? If so to what?
If your original carb was stock calibrated for the 350 then yes youll definitely see some benefit, power-wise, in upping the jetting to suit your new combo. Stock I believe it was 68 jets, about 43B rods and BS 2nd rods. I would step up to 73 Jets, leave the 43B rods, and add some CT 2nd rods. those are pretty mild changes, gets you close to the 455 carbs stock jetting. Really the way to get the most out of the carb is to completely recalibrate it for your setup. That means the works....jets, rods, idle tubes, air bleeds, pulloff opening rate, etc etc. If you get Cliff ruggles book its a really good source of info on recalibrating the Qjet. The mods make a HUGE difference. good luck!
gas mileage could suffer, if you go too rich you could get poor ignition, sluggish response, black smoke out the tailpipes. Fouled plugs. its going to take some tinkering to find the "sweet spot" that your motor likes. upgrade in small steps.
Rule of thumb is if you go up 2-3 jet sizes, go up one size on the primary rods. Headers will require that usually.. You can try a one size larger on the jets and change the secondary rods to richer ones. See if it helps.
Just sent a rebuilt unit back that wouldn't hold idle. Seemed to be something internal. You could turn the adjustment screws all the way in, cover the throat with your hands and the car ran. Take you hands off and she stalled right out unless I set the idle at 2000. Even the brakes were hard! Rebuild had 73 primaries for performance but I was thinking of downsizing on my carbs return. For now I put my old stock q-jet back on and the cars runs fine but seems to be starving with mild engine modifications such as TA 284 cam, TA manifold, ported heads, Petronix ignition, power curving etc.
It alters the cruise mixture and WOT. I believe all the rods have the same tip diameter, but the upper portion of the rod is different. All this information is in the Q-jet books by Doug Roe and Cliff Ruggles. If you want to work on your carburetor, they are a must.
Good info, I was under the impression the secondary jets couldn't be changed but didn't think of the rods. Thanks
