428 Carb Set up

[Mr.Fixit]

"Well, I changed the pv to a 2.5 and drove it around for a while. The stumble around 2000 rpm was much improved, but a new problem developed. When you are cruising in third gear at 2500 or so and then quickly push the gas pedal down, there is now hesitation in the 2800-3000 rpm range that was not there before-it almost sounds like a lean miss. If you gently roll on the throttle, there is no hesitation."

Idle was improved cause PV is no longer dumping fuel at idle. The hesitation sounds like an accellerator pump tuning issue. That will result in a lean miss. Accel pump overcomes the temporary lean spot when you mash the throttle, until fuel is pulled from the boosters. If it hesitates right away, try a bigger squirter, if it hesitates after a half second delay, try a smaller squirter to lengthen the pump shot's duration. a 50 cc pump may be needed, (but no need to start there).


"I need to get some longer tubing on my vacuum gauge so I can drive around and see what the vacuum readings are on the road, but I had the following thought: Is it possible that when I am cruising and when the vacuum is highest, the vacuum does not drop enough to activate the 2.5 pv causing a lean condition? "

having gauge visible will definately help you tune it. While cruising at a steady throttle position, the PV should not be opening, it is there for WOT fuel enrichment only. Your manifold vacuum should be nearly atmospheric (at WOT), but you can try a 3.5 PV, see if it makes a difference for that.

" And a smaller duration cam may be in the future . . "

I wouldn't start there, I ended up going to a smaller cam not because of carb issues (I got that sorted out) but because the loss of low and midrange torque. By the time the motor was in it's powerband, it was time to shift; car was quicker with a smaller duration cam. Replacing the carb as some suggested probubly won't help, that one will need setting up too.