Ok, guys. Here is the story. I am brand new to the threads, but not new to wrenching. I recently aquired a non running 1986 Firebird with a 5 speed. The car was not running when I got it. After getting it running and doing some research I discovered that it has a 1975 model 350 in it. I also noticed that it has a nasty vibration in it. It is not in the transmission and can be felt at any RPM. It can even be felt when the engine is cranking. I replaced the flywheel in an attempt to remedy the problem but there was no change. I am tossing around a few ideas and would really appreciate some suggestions as to a possible cause. The engine runs smooth and has plenty of power, but the car itself will rattle your teeth out. Thanks guys.

 

is the engine internally balanced?

 

The front balancer n flywheel need to be match depending on balance, GM change from internal to external balance a few times.

mix up the parts n bad things happen.

 

The engine should be internally balanced. After doing some research and comparing numbers I found that all the 2 piece rear main engines were fully internally balanced with the exception of the 400 until 1986. When they switched to the 1 piece rear main, the flywheel and balancer had to match to get the final balance. I am leaning toward the balancer having a problem, but it looks like it is in good shape. I suppose it could have twisted on the hub. Any other ideas?

 

are the motor mounts good?

 

Yep. The motor mounts and tranny mount check out. All are in good shape. I really do appreciate all of the suggestions. I think on Monday I am going to swap the harmonic dampener and cross my fingers.

 

Mmmmm if it's a '75 motor it could very well be the flywheel. It would have to be a retrofit aftermarket 'wheel to fit the bellhousing (as '75 truck motors came with 168 tooth pieces, and the firebird should have a 153). If this is the case, perhaps they installed the wrong one and it has a bobweight on it. Or maybe they got really freaky and redrilled the stock flywheel :x.


EDIT: I say redrill because the '75 2piece RMS flywheels have a much wider bolt pattern than newer 1pieces. IIRC the 1piece started in '84. Could be misremembering.

 

Sounds like someone put a 305 crank in a 350. Or the crank is broke.

 

Another shot in the dark, watch the front balancer when its running, sometimes folks put them on and think its all the way on but its not and it will wobble after a while n then trash the mains.

 

GM made 153 tooth FW to suit a 2 pce seal crank
# 14085720

 

All SBC ( ex 400 ) use the same neutral balancer regardless of crank style ; only 400's had a weighted one

Even if the bond broke and the outer ring rotated in relation to the hub it should not affect balance much on a neutral balancer

 

I am really beating my head against a wall here. I replaced the flywheel and clutch with a 153 tooth from a 1984 which was a 2 piece rear main. I will know for sure about the dampener today. It doesn't seem to be wobbling at all. The next place my mind goes is to think that maybe it is a 383 and I need to get a 400 flywheel and dampener................. Isn't the unknown wonderful????

 

with that said, there is only 1 way to know for sure what you have.........yank it and open it up. Yes, that means a new gasket set but you might find other problems and also you'll find out what engine you really have.

 

No luck with the dampener......It was worth a shot. About the only thing I can imagine is that it is a 400 crank. The vibration is present even when the engine is cranking.

 

Id say time to take it all apart, though id run it with out the pressure plate or trans hooked up, for shiets n giggles to see if it stays or is worse.

 

Thanks for all the help and suggestions, guys. I think I'm going to jerk the motor and swap it for a known 350 while I go through the one I have. I was really hoping that it didn't come to this, but I guess I'll know for sure now. Thanks again.