Loss of vacuum and power on 1964 gm 348W

Asked by Sep 20, 2019 at 02:03 AM about the 1964 Chevrolet C/K 30

Question type: Maintenance & Repair

My 348W, GM 1964: Edelbrock 4BB carb rebuilt,
original distributer and governor rebuilt: when
shifting at low speed/high acceleration,  usually
from 2nd to 3rd,  I lose vacuum pressure and the
engine loses power. I must kick in the clutch and
keep pumping the accelerator to get power back,
all the while dropping down to a crawl on busy
hiways. I have the same scenario happen on hills
and long grades, and sometimes while sitting at a
stoplight. I am pushing 12ton. The tranny is 5spd.
So why am I having this issue?

1 Answer


At high acceleration, there's very low vacuum in the system anyway, so I don't think this would be the problem. In that instance, maybe the fuel mixture is just leaning out, or there's a weakness in the ignition that is just noticeable under heavy acceleration. Might be as simple as a clogged air or fuel filter, a defective fuel pump, an exhaust restriction, or a weak coil. Maybe even the old Edelbrock carburetor.

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