Hello,

I am hoping there are some seasoned veterans out there can lead me in the right direction. I put a 350 ci motor in my 65 shortbed. I had the long block done at a machine shop and I installed it with a th350 trans. I have the engine running and have driven it a couple times but I can't figure out why the motor vibrates when I let off the gas. I took it in to the shop and they put the fine tune on the valves, carb, timing etc. as well as a new harmonic balancer. The motor runs pretty godd I got it up to hwy speeds but I dont want to damage any bearing etc. I noticed it does this more when in park and less when in D and on brake, but still all the while its running.Any one run into this before??? Could it be flywheel, torque converter, fan...I dont know. PLEASE any input would be appreciated.
Thomas  

For starters did you replace your motor-mounts? This can be good source of vibration. Check them and let me know, if that's not it, then we'll work from there.

John G.  

Hey John Thanks for responding, yes I did replace motor mounts as well as installed a tubular trans crossmember from CPP. I checked all mounts and didn't see any problems. It kind of feels like a centrifugal imbalance not necessarily mounting issue if that makes any sense. My flywheel has a counter weight on it could that be it, or does it need a fan with a clutch on it? I've rackkin my brain  

Okay, I really doubt the fan could be out of balance enough to cause that kind of vibration. You can pull it off and run it without to see what it does, but I doubt that's it.

I'm not sharp enough to tell you exactly, but I know externally-balanced and intrernally-balanced mix-ups can cause this kind of thing. Hopefully someone jumps on here and helps you solve it. I will ask the guys at Super Chevy or PHR and see what they think, and get back to you.  

Thanks John! I really appreciate your help. I hope someone out there has run into this before. This is my first major addition to this truck I hope it turns out well. I hope to hear some good advice!!

Regards

~Thomas