I have a bit of vibration between 25-mph and 40-mph. Any other speed and the ride is as smooth as can be.

Only vibrates when under power, if coasting with clutch depressed, no vibes.
Also vibrates on steep downhill at these speeds with foot off accel and engine "braking".

This is a new acqusiton, extreemly clean restoration, very tight front end including king pins, but the front end does that groan when starting out from dead stop with wheels turned sharply. To my horror, when lubing the front end, the zirk fittings were both missing top and bottom of the right king pin (amazing because of the otherwise splendid attention to detail on this resto), and I could see the kingpin through the threaded hole, obvously dry.

When I received the car, the sway bar was disconnected at the right end, I used spare parts to reconnect it.

Transmission also reported as being rebuilt,and from sterile appearance of gaskets, this seems to be true.

One of the u-bolts was not tight on the rear leaf spring (its a Duett), "has the rear end shifted?"

The u-joints appear tight, there is a bit of lash between the ring and pinion of the rear end, but not enough to make any noise during driving.

I leaned the stromberg carb (NOS) for emmissions inspection, so I was running lean, but have restored the orginal needle setting.

Good compression 135-140, however strange is that checking for non-firing cylinder by pulling off one wire at a time seems to have no effect on idle (this is a first).

So... is vibration at this speed a typical symptom of mis-alignment of the rear end, or of a broken pressure plate spring? Is there something I am overlooking?

Thanks for the interest and assistance, Leroy

Thanks, swapped the driveshaft with the one from the parts car, and vibration has stopped.

Oy, sounds like that Duett is a goner Sell it to me!!! ;^)

Depends on what sort of quality the vibration has. If it is a light buzzy sort of vibration it could be something like the exhaust system buzzing against the body/frame somwhere when the drivetrain is torqued. Or something around the engine.

It does sound more like something with the driveshaft to me though - those are usually more like shudders than knockings. Perhaps the rear axle is twisting under torque and getting the u-joints out of alignment. Or a loose u-joint, or loose center support bearing or loose/missing support bearing bushings. Typically, by the time something on the driveshaft is loose enough to shudder it will be noticably loose when you get under there and yank around on it.
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I'm JohnMc, and I approved this message.

Leroy:

It sounds like you have a very nice PV. If the vibration has a rattling sound, then you might want to check your exhaust supports and/or clearances.

Joe M

The usual cause of viberations under power or engine braking is some where in the drive line usually the u joints , hanger berring or clutch. The most common of these is u joints. Some times these can not be found with out taking the drive shaft out and trying each joint by hand.

I second this possibility. U-joints can cause all kinds of strange symptoms if one of the cups is binding. Removing the driveshaft and flexing each of the joints by hand will uncover any problems.

On the subject of whether the rear axle could have shifted due to a loose u-bolt, I doubt it. The axle tube has a flat-sided pad welded to the underside, and the pad has a hole it in that fits over a pin bolted to the spring assembly. There is no possibility of the axle shifting relative to the spring either rotationally or laterally.

On old cars I've seen that pin-headed bolt break off, allowing the axle to slowly scoot forward or backward. The car then crabs down the road. It is usually plainly visibly awry before causing any sort of problem.
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I'm JohnMc, and I approved this message.

Yes, right! I had a 1950 Dodge Bus that sheared off the rear spring locator pin - my fault since I backed up onto a loading dock by forcing the wheels up over a 2' high wall! The poor bus really went sideways going home that day. I immediately found the problem and a spring company fixed it for under $30.

I'm feeling pretty sure that your problem is with a u-joint. It's time to take the driveline apart and inspect each u-joint for wear of dryness.

Regardless of what you find, replace all your u-joints (2 or 3?) with the ones with the zerk fittings so you can grease them.

It just occurred to me that you may want to jack up the front end and check the front wheel bearings for play, although your description doesn't really lead us that way.

It's a good thing to check anyway!

Thanks for the feedback guys. Although no one can make a definative diagnosis based on the limited info, the group consensus (drive shaft) is a great help to me as I had assumed from getting under there and twisting on either side of the joints that I could rule this favorite suspect out. Pointing me to something this simple has at least cheered me up.

Also, I infer that the driveshaft has to be under load to cause this vibration, since it does not vibrate under free-wheel conditions. This makes some sense as without some load it might not stress the u-joints into an out of center condition.

I will also try to take diagonal measurements from the axel to the front end to see it the rear end has gotten sideways.

Regards, Leroy

Before you take the driveshaft apart, you might wanr to pick up a couple of drive shaft bearing bushings, part # 653192-5. These are cheap (about $4 from the local Volvo dealer) and could be a cause of the problem.
Chris