71 Chevy truck cam saga.

Well I got the truck back from the shop and all seems pretty good but one small thing. There is a very slight harmonic vibration somewhere in the driveline that wasn't there before. It comes and goes in one second intervals. I am thinking U joint or torque converter needs rotated on the flywheel. Any other thoughts?

Possibly the carrier bearing. Could also be tire out of balance or round, motor mounts, trans mount (from most to least likely)

Is it speed related, or RPM related? If speed, it"s probably something in the drivetrain. If RPM, it"s probably something in the engine. If you have a 400 ci chevy engine with a 350 counterbalance, that can cause an RPM related vibration, because 400s are externally balanced, whereas the 350s are internally balanced. Also, I think, but am not sure, that the 400"s auto flexplate is different from the 350"s. Of course, the inverse of these two are true also. If it"s speed related, it could be a bad uni joint, wheel bearing (not likely), unbalanced or bent driveshaft, or something I had when I swapped a 400 Chevy into my "72 Vega wagon: It would vibrate like hell when I was going about 25 mph, then disappear. Drove me nuts, until I took the driveshaft/universal joints apart, and rotated the driveshaft a quarter turn. Vibration disappeared. It seemed to want the rear uni on a different rotational location from the front one.
Whatever, we need to know if it"s speed related or engine RPM related.
BTW: The Vega ran like hell.
Gary

Rex2 yes, you are right, the chevy sb400cid has extra balance weight on the flexplate,as does the 400cid flywheel with extra balance weight cast into them. Jay

The 350 and 400 flex plates are the same. The harmonic dampers are different. I'm putting a 383 (400 crank in 350 block) together to replace a 350 in my 98 Silverado. Because I'm building a core engine and don't have the engine out of the truck yet, I had to buy a new flex plate for the machine shop to do the custom balance on the rotating assy. It was quite a bit cheaper to go with a 400 damper than have a huge amount of weight added to the crank. Weighting the crank might be better for high rpm's, but this will be a trailer towing/daily driving engine.
Like Gary says, we need to know if the vibration present only when driving, or can it be recreated in neutral with engine speed?

Some of the trucks that model had a center drive shaft bearing and if it is assembled wrong it will vibrate badly.

350 AND 400 FLEXPLATES are not the same. 400 flexplates have added weight on them for the externally balanced 400 motors. All 1986 and later smallblocks were externally balanced. By than the 400 was gone.

I crawled under it today and checked the drive shaft. It seems ok and snug. The carrier bearing does have a slight bit of movement but not much. I drove it tonight and it does it the worst at about 60MPH and the vibration period lasted longer when I went down a hill. The engine is all 350 with no 400 parts. I want to get this taken care of soon as I will be driving it to Oklahoma next week. I will check to see if it does it in neutral tomorrow.

Bob, any play in that bearing is too much play. I'd plan on replacing it before going to OK. It just might be your problem, and if not...it could be soon.

First thing we ever did in the tire shop I used to work at when something came in with a vibration problem was to check the inside of the wheels for mud buildup.

If that's ok, assuming the tires are ok and properly balanced, then your thoughts of a u-joint or as mentioned the carrier bearing are the next(and most likely) places to look. A miss in the engine that only shows up under load could possibly feel that way too. Try holding the brakes with it in gear and giving it gas(powerstalling the engine) and see if anything shows up there under load).

Turning the converter on the flywheel should not have any effect on balance, and in fact some of 'em will only go on one way.

You might also look at the pulleys on the front of the engine as it's idling to make sure none are bent or possibly the bottom pulley or fan/waterpump pulley did not get fully seated when they put it back together.

One final thought, if somebody else did the work, is that possibly they could ruined the balancer when the pulled it off and stuck one off a 400 back on there in it's place, although this would most likely be much more noticeable than what you're talking about.

Ok Guys, Here is the latest poop. I took it back over there and the guy and I went for a short ride, he had it figured out pretty quick. When I put the engine back in he thought the motor mounts were a little iffy but he didn't replace them. (I wish he would have called me, I would have said change them out)Anyway, the tabs on the lower part of the mount are just barely hitting the frame brackets. It would vibrate some at high RPM in neutral but not much. Going down the road at about that RPM it was much more noticable. My guess is the added torque from driving is making one side contact a little harder and transmitting the slight vibration from the engine and amplifing it in the frame and body. The vibration went away as soon as I put the trans in neutral. The movement I noticed in the carrier bearing was the rubber support boot that was moving, not the bearing. ( I have never had one apart to know how they went together). The driveshaft is in excellent shape front to back. So New motormounts are ordered and hope that does it.