holley 4150 mechanical double pumper, 67/73 jets, 400 sbc at 5500rpm.
do i have to change accelerator pump cams when switching from a 30cc pump to a 50cc pump?
i seem to be running lean at the top end.
thanks
The 50cc accelerator pump won't help the lean top end one bit.
lemme try to describe it better:
i would warm up the motor, and drive it to the track. when i accelerate hard, at the top end i sometimes experience what sounds like a metal string snapping, the engine hesitating, then running normally again. it'll do this once, maybe a couple or a few times. do you think this is running lean on me? or could it be an ignition problem?
It could be going lean, it could be detonating, it could be missfiring, it could be the oilpan is emptying out and you're hearing bearings and lifters.
Since the effects of the accelerator pump are all over with after 2-3 seconds, like I said, it isn't going to help.
My guess is your bowls are running empty, lean, detonation, not good. Put a fuel pressure guage on it that you can see from the drivers seat and make sure you have at least 6-7 PSI at all load/rpms.
Sounds like valve float or a bad valve/spring. To answer your question, 50cc pump uses the big brown cam. It is alot bigger than the assortment of cams they have for the 30's. The lever is different also. If you are running those jets in 400, you may be lean depending on how good the signal is to the boosters. Holley's are weird about vacuum and flow through the boosters. Good vacuum signal will require the use of smaller jets to compensate. Poor low speed vacuum will need bigger jets or a big shot on the front pump to "cover up" the flat spot. Then you will be semi-rich on the big end and screw up the torque curve. It takes alot of work on some carbs to get them dialed in right. Some take out the power valve to try and simplify the deal but I haven't had good luck doing this on street motors. That usually causes the motor to be rich or just right on the bottom but lean up top. Good luck.
yes on changing pump cams
No ,the accelerater pumps do NOT have any effect on the Top end and running lean. That is JETTING
Sounds like valve float or a bad valve/spring. To answer your question, 50cc pump uses the big brown cam. It is alot bigger than the assortment of cams they have for the 30's. The lever is different also. If you are running those jets in 400, you may be lean depending on how good the signal is to the boosters. Holley's are weird about vacuum and flow through the boosters. Good vacuum signal will require the use of smaller jets to compensate. Poor low speed vacuum will need bigger jets or a big shot on the front pump to "cover up" the flat spot. Then you will be semi-rich on the big end and screw up the torque curve. It takes alot of work on some carbs to get them dialed in right. Some take out the power valve to try and simplify the deal but I haven't had good luck doing this on street motors. That usually causes the motor to be rich or just right on the bottom but lean up top. Good luck.
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