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Vibration in 1975 L48


Bill G

My wife and I found a beautiful 1975 at a Chevy dealer in a small city here in Washington State over the Labor Day Holiday. I have wanted a Corvette all my life and I thought my time had finally come to get one.

It had 40k original miles on it. After driving it for a while, we inspected it again and bought it. I did notice that the air conditioning pump was off it and so was the pollution pump. The dual exhaust was new from front to back and the cat was removed. Seemed funny that someone did this to such a otherwise near perfect looking car... The dealer mentioned that they had replaced a syncro in second gear and that they put bearings in the transmission. They also mentioned that the car had been stored for a number of years. (Uh oh)

It has a vibration that starts at about 3200 RPM and peaks at about 3750 RPM and then tapers off at about 4200 RPM. I thought that we would look into that later.

We drove it for about two weeks putting about 500 miles on it. I happened to look at the left rear tire and noticed it was wet on the inside. (darn it, the wheel cylinder is leaking)

I had already planned on taking the car to a local shop that works only on Corvettes. They have a excellent reputation for the work they do in frame off restores and mechanical work. They did an inspection and determined that the car needed lots of work. Mostly the result of the extended storage. I felt sick but we decided to have it done. We also decided to replace any parts that looked worn and that could give us trouble in the future.

Here is what was replaced... brakes (including lines and master cylinder), power steering pump and cylinder, harmonic balancer, water pump, all the shrouding around radiator, radiator rebuild, all the pullys on the engine, the fan and fan clutch, thermostat and housing, shocks, all six u-joints and a lot of miscelleanous stuff. (ouch) Oh yeah, the ball joints, idler arm and a lot of steering parts were replaced.

Vibration is still there (darn it) Must be the clutch assembly. After a lot of debate, we replaced the clutch assembly and had it balanced to the flywheel. I thought the L48 had a 10.5 clutch, but this one had a 11 inch. It did not appear that it had ever been changed. Also fixed the seals in the transmission that the dealer messed up. And, while we were at it, we also replaced the valve covers with cast aluminum valve covers. EVERYTHING we replaced on this car was with new GM parts.

Darn vibration is still there but not as bad. Still very irritating and I can feel the vibration in the clutch pedal. And yes, it will vibrate at the RPM's I mentioned in neutral with the clutch in or out.

Here is what I am wondering... Did someone replace the clutch and flywheel causing an imbalance with the engine? Should I contemplate buying a flywheel for a 75 L48 and replace the clutch assembly again to match it? I am not sure where to go with this and everyone seems stumped.

Thanks in advance for any help or suggestions. - Bill
Hi Bill,
As you've noticed, vibrations are one of the hardest problems to solve. You mentioned that the clutch was balanced to the flywheel. Since all 350's are internally balanced everything should have been balanced to .5 gram. Was this done? If so then the flywheel isn't the problem.
Will it vibrate sitting still at 3500 with the clutch in or out or only when driving? Can you feel any vibration in the shifter?

Just a thought?

Is there any possibility your engine is actually a 400 small block ... and that the harmonic balancer was replaced with one for a 350?

Or vice versa? Either scenario would produce serious vibration / imbalance problem

The vibration is there in neutral, clutch in or out.

I forgot to mention that this is a numbers matching car. The mechanics that worked on the car believe that it is like it came from the factory. Even the u-joints and brakes were original.

I still wonder about the clutch and flywheeel though.
Did you ever determine what the cause was for the vibration on your vette? (i have the same problem)
I also replaced the clutch, but it's still there.
Any advice would help.
I posted the following message here about a year ago...

The car now has 52,000 miles on it. The vibration was still present and I have just lived with it waiting to get a clue to its source. The last two months, it got worse and started showing up at 3000 RPM. In addition, a noise like a water pump going out surfaced. To make a long story shorter, the new water pump was on its way out. When they replaced it, the vibration went away at all RPM ranges. Boy am I happy!

What are the chances that a bad water pump that caused a vibration was replaced with a new pump (not a rebuilt) that was also bad? Close inspection of the last pump showed that the flange on the front of the pump was slightly warped. Not enough to see it in the fan, but enough to set up a vibration. I thought I would post this in case someone else has a similar problem with a vibration.

75 Coupe
Interesting post results thanks for sharing. I heard of motor mounts or joint causing this too.

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