Motor Stumbling

Motor stumbles/hesitates most notably at 35 mph. Also, light stumbling at higher speeds and on hills. Fuel system is clean. New carburetor, manifolds, distributor, plugs, fuel pump… whole new motor! New driveline and bearings. Differential appears to be fine. Checked for vacuum leaks, electrical malfunctions, carb adjustments, timing, etc. Two mechanics have looked, and neither of them can find the problem. Former motor did this as well. Any ideas?

How about the fuel filter?

How about the year of the truck?


Manual or automatic tranny?

Inspected for debris and replaced anyhow.

This truck has a four speed manual transmission.

I inspected for debris and replaced the fuel filter anyhow. No change.

Is it safe to assume that a new clutch went in when the new motor did?

You’re saying “Motor stumbling” and I’m wondering “clutch slipping?”

Good question. Yes, a new clutch and a needle bearing replaced the old bushing style pilot bearing.

The only thing left that I could think of would be to jiggle wires - poor connection in an old wiring harness? A ground that wasn’t removed, cleaned & reinstalled w/ the motor swap? Distributor & plugs are new - igntion coil & ground?

I have tried this and even hot-wired around everything. I did notice that when I removed and plugged the vacuum advance, the stumbling got worse. It may be possible that there is a distributor advance linkage problem or such. The stumbling is at all speeds and during deceleration but most notable at about a steady 35 mph. I have installed new coil, points, condenser, spark plugs, and s.p. wires. At one point in time, the new condenser went bad and this resulted in really bad stumbling. Maybe something is causing the condenser to fail prematurely? Thanks for your advice thus far.