Vibrations at 2800 RPM's?

[alaska60]

This has had me puzzled for quite some time now. I have a 60 with a 383 stroker chevy V8. It has a toyota 5 speed with a ranger torque splitter in between. The tranny mounts are stock on the cross member and in very good shape... in fact they're brand new as well as the motor mounts too. The vibration has nothing to do with the drive train as it does this vibration in neutral in the driveway as well as on the road moving. it only happens between 2800 and 3300 RPM's.
I have replaced all mounts on motor and tranny. any ideas?????

 

[doctorjj]

Did you have the rotating assembly balanced? Do you trust your machinist?

 

[alaska60]

The motor was built by a very good machinist. it ran great with no vibrations until about 3 months ago. it hasnt changed, still the same vibration. like the tree said to the logger......."I'm stumped"

 

[NocalFJ60]

I would use a stethoscope or long screwdriver with your ear on the end and try and isolate where the vibrations is coming from in the engine.

Also a good engine guy with ears might tell you in seconds what it could be listening to it.

 

[alaska60]

Harmonic balancer perhaps? can these balancers fail at a given moment?

 

[ntsqd]

Is the frequency of the vibrations directly the same as the engine RPM's, or some multiple of the RPM's?

The balancer's outer ring could have slipped, known to happen. Easiest way that I know of to check is to put a timing light on it and see where the timing is. If it is way off, as it would likely be if the ring slipped, then you'll know.
Another (best) way is to bring the engine to TDC by means other than checking the timing marks (remove spark plug & check for air blowing out, remove rocker cover and watch rockers as spinning thru by hand, etc.). Then see where the timing mark is.
If this is the case, do a net search on "Damper Dudes" in Anderson, CA. They rebuild harmonic dampers.

If there were any balance weights on the pressure plate it may have thrown one or more of them. Should be a healthy ding on the inside of the bell housing somewhere if this happened.

Check the edge of the clutch disc for completeness. It may have fragmented and tossed some of the friction material. It may have also tossed a hub spring. Either of these are pretty rare, but are possible.

If you'd recently done a clutch job I'd guess that the flywheel resurfacing was done on a slight angle b/c of some junk on the grinder's register surface (BT,DT...).

 

[alaska60]

NTSQD,
Thanks for the info.... i will look into this potential disaster this morning. Also thanks for the lead on where to go if this is the fact too.
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