classic ford f-100 pickup truck at a classic car show

Q: I have a vintage Ford F-100 pickup with a transplanted 5.0L from a 1991 Mustang and the following mods: Ford Performance cam (220° duration/.498″ lift), Airflow Research CNC-ported 185 cylinder heads, Edelbrock Performer intake manifold, Holley 600 cfm carb, B&M flexplate with a 50 oz. balance, TCI C-4 Street Fighter transmission, and a TCI 10″ Street Fighter torque converter.

The water pump, pulleys, and damper are from a 1969 Ford 302. I am experiencing a sporadic vibration between 35-55 mph when driving and in neutral or park when the engine is revved. I can feel it in the gas pedal, steering wheel, and seat. The vibration goes away when I lift off the throttle. My mechanic claims to have sent the engine out to be balanced during assembly.

I believe something is out of balance or the torque converter is bad. Can you help?

A: We promise to try!

We think the problem is the balancer. In 1981, Ford changed from a 28 oz. to 50 oz. imbalance.

To fix the problem, simply use the original balancer from the 5.0L engine, or an equivalent replacement, and you should be good to go!

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.