Tab,
Check the simple stuff that has been mentioned before. Check for loose flywheel and clutch bolts, motor mounts, tranny mounts. Check for a loose water pump bearings, pulley or fan blade. Make sure the alt or generator pulley and mounts are tight. If these items are correct and not causing the problem, then delve deeper into the motor.
Now for some of the more involved and "out there" ideas:
The possibility that the front balancer and pulley are now different is a big clue. Ask the shop why they are different. If the parts are different than what the engine had when you brought it to them, then remove the balancer and have it rebalanced.
Its kind of like a tire/rim balancing. If a tire is out of balance, it'll hop up and down, right? The wheel weights on a tire/rim assembly are there to achieve "zero balance". This is so there is no heavy spot for centrifugal force to cause vibration. Same thing on a 235 front balancer. (I think)
Some balancers on other types of engines (V8s, etc) had weights in them to offset the crank weight. That way, the total system achieves "zero balance" I don't think the 235 is like this. I just think its a dampener, not really a "balancer" per se. So if the unit on your engine is not at "zero balance" it may cause some vibration due to offset weight. I may be wrong, but if so, somebody will correct me.
The front balancer/pulley assembly is supposed to "dampen" some of the torsonal (twisting)action and vibration harmonics that appear in a crankshaft when running.
If the unit has been custom balanced for a different 235, it might be out of balance on your engine. If the balancer is defective or "slipped" it may also malfunction and not cancel the crank harmonics that appear at certian RPMs in all engines. A 235 balancer is a press fit on the crank snout. There is no bolt that holds it on. So if the unit is loose and wobbling on the crank, then that is another common problem. It is very important to have a good balancer properly installed on the engine to dampen vibration.
I think a 235 is an internally balanced engine. All that means is the crankshaft counterweights are supposed to offset the weight of the piston/rod assemblies and the crankshaft throws. However, the rotating assembly should always be rebalanced during a rebuild. The new pistons, rings and bearings are going to weigh different than stock specs, throwing the "factory designed balance" off.
The flywheel is dowel-pinned to go on only one way. This makes sure that the timing ball in the flywheel always indicates TDC. I don't know if the flywheel is offset weighted or not. If not, you can have it "zero balanced".
If the flywheel is supposed to have some offset weight, then I don't know how you correct this, since the engine wasn't balanced during the rebuild. You wouldn't know how to balance the weight on the flywheel to cancel the offset weight in the engine, because it wasn't measured during the rebuild.
Checking the balancer, flywheel, and pressure plate may solve the problem. If not, then you'd have to look for an unbalanced component inside the engine.
Sorry for the novel. If I've gotten a technical detail wrong, somebody please correct it.
good luck
chip
