MattKelly7520

Having issues with my 1980 vette. It's a 4 speed manuel transmission. While I'm driving and shifting the car into 2,3, and 4th gear. The car bucks and makes a clunging nose like I'm shifting at the wrong time and then continues to accelerate like normal (This was never a problem in the past) My father was the original owner and the car only has 18,000 miles on it. Could it be the transmission fluid? Or something else?

71VetteLover

I suggest revving it to at least 2500 rpm before shifting into a higher gear.

Wee

Either the snubber bushing or the trailing arm bushings are shot....Does it want to fishtail when you shift as well....?

Brian

MattKelly7520

It doesn't fish tail, it just bucks back. This bucking doesn't occur until I've been driving the car for like 15-20mins. I can drive it around for some time and shift with no problem. Once it gets warmed up, it begins to happen

Wee

That sounds like it could be the posi disc sticking in the differential....Has the fluid been changed and if so did the additive get added with the new gear oil...?

Brian

MattKelly7520

I'm going to have to guess it has not been changed. The car was kept in a garage for 20 years before it started being used again (2010). I recently changed the oil (every summer), brakes, brake calibers, suspension, and had the starter rebuilt. That fluid is probably from 1980 when it rolled off the assembly line lol

BlackC3vette

Is the bucking and clunking only when your shifting and the clutch disk is engaging the flywheel?

7T1vette

Clutch problem; posi-trac problem; U-joint problem.

That would be my list to check.

MattKelly7520

Any idea how I can check to see if the U-Joint is worn out?

BlackC3vette

U-joints problems usually are accompanied by vibrations which is not mentioned. The clunking is coming from the transmission area or the rear end?.

MattKelly7520

I can feel the clunking towards the rear of the car (drivers side). Only when shifting after like 10-15 mins of driving. Sometimes occurs in reverse as well

7T1vette

If you get 'clunking' when turning at slow speeds, that would lend itself to a posi-trac problem. If moving straight ahead, not so much.

If it happens just AFTER a gear shift, it would most likely be a clutch or transmission problem, IMO.

Bad/dry/damaged U-joints would bind and clunk/jerk when moving slowly and vibrate when going faster. If you THINK it might be a U-joint problem, don't do much driving until you have it investigated. If a half-shaft joint fails when you are moving, there can be LOTS of secondary damage to the underbody/suspension.

DUB

My vote is looking a the U-joints....especially how you had mentioned that after driving for a little bit of time it begins to show up.

I also would have advised to look at your differential front mount bushing...like 'Wee' did in post #5......but if there is no noise when you start to drive it...forward gears and reverse...then that is mute. If it were bad...it would happen right away.

I seriously doubt it has anything to do with your clutch disc...because 'bucking' may be due to not having it revved up enough when you are shifting. But if the 'clunking' sound occurs when you press in on the clutch and then when oyu release it teh sound occurs...it can be due to a bad or worn/dry U-joint as like many others have mentioned.

DUB

BlackC3vette

I think you need to get the vette up on jack stands to check out those U-joints. Put it in neutral and spin the drivers side rear tire and see how smooth it is.

MattKelly7520

I just order 4 new U-joints! I'll going to jack the car up this weekend and change them! Keep you updated. Thank you!

MattKelly7520

I just ordered new u-joints. Going to replace them this Saturday! I'll keep you updated! Thank you!

MattKelly7520

Changing them this Saturday! Thank you! I'll keep you updated