Okay, I bought a motor that has about 5000 on the rebuild. Its a 77 400. I bought it from a guy who I trust, but he didn't build it himself. I am quite positive it has a performance cam in it, judging by idle and hp curve, and just by how high it revs, it revs real high for a 400, and has a lot more power than the 11.5:1 compression 460 I had in my other truck.

The problem is this, first time I took it wheeling after I got the motor in, I got on the gas hard (but not overrevving it at all), and it started backfiring and I had a ton of noise out of the pass side valvetrain. So i towed it back, pulled the valve cover and found 2 pushrods that were really mangled up, both on the rearmost cylinder.

2 things come to mind:

1. Are the valves to tight in the guides (cause of hte recent rebuild)?

2. Is the cam lift too much for the valve springs and or rockers.

Any other Ideas?

I really want to check out the second idea, I was wondering if there is an accurate enough way to check the cam lift without pulling the cam out?

Also, any other thought on what may have caused the 2 bent pushrods on the same cylinder would help.

 

It could be that rod is letting go. The heads may be finished unevenly also. Or the rocker arms may have been installed improperly. You need to check the entire engine over.

 

If the valves were sticking you would likely have a destroyed engine. (done that! )
Lift can measured with a dial indicator... measure the lift with the rocker off... that is the transit of the unloaded push rod.
Perhaps also the preload was over-looked on this one cylinder? but check them all. GL