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Discussion Starter · #1 ·
How can I tell if my drive shaft is out of balance? When I drive my car at 50 to 75 mph It has a shake to it but if you go 80 or more its gone any speed below 50 wont shake. Also how do I check the U joints to see if they are not bad.
 

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check to see if it has thrown any of the balance weights!Just had it happen on one of mine.Had it redone and it was night and day diff.Tom
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Is this a long term problem, or something that just recently developed?
 

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Discussion Starter · #4 ·
Its been going on for about one year not sure if its because I used a light flywheel but the stock one was much more heavy this one is 17lbs. How many weights are there I see the one near the back towards the diff.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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If it was a flywheel problem, it would likely be dependant on engine RPM, not MPH. And it would probably continue to increase with RPM, rather than disappearing at very high RPM (if that equates to the MPH you cited). Did the problem appear with the installation of the new flywheel?

What rear axle ratio are you running? The 4th Gens had a problem with the critical frequency of the stock steel driveshaft. Below 70-80 MPH, with the stock rear axle ratios (2.73, 3.23, 3.42) there was seldom a problem. But install 3.73 or numerically higher, and the DS RPM got into critical frequency range at 70-80 MPH, and started to vibrate. It was so bad GM issued a technical service bulletin (TSB) that got some people a new, aluminum DS if the car was still in warranty.

I suspect the number of balance weights will be dependant on the factory detected imbalance of the DS. Does it have a harmonic damper ring at the front of the DS, just behind the u-joint? 4th Gens had one. If all else fails, easy for a driveline shop to check.

Have you checked the tranny mount? .... torque arm front mount bushing?
 

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Wow, I deal with CRF daily on vertical motors! Strange to see it referenced on a drive shaft, but cool all at the same time. Maybe a Ujoint trying to seize up as well?
 

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you will be able to tell if it threw one as the shaft is painted after the balance.There would be a unpainted area where the weight was.Did this come after you changed flywheels?Tom
 

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Discussion Starter · #8 ·
Its all covered in rust its stock rear end I have not moded any parts of the car but transmission mount is good I replaced it the same time the clutch was replaced but would un balanced drive shaft with bad u joints cause the whole car to shake from 50mpg to about 84mph? ill get it looked at but also I don't think the harmonic balancer would cause the whole car to shake like that right? This issues happened after the flywheel like months after.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Have you actually done anything in response to the suggestions? Have you followed the diagnostic chart? If there are steps in the chart you are not equipped to perform, have you looked for a shop to run the tests?
 
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