Truck jumping/clunking occasionally when putting in gear?

[maddog1331]

Hi all, I am new here, bear with me ok

I recently bought a 1971 chevy half ton with a 350 engine and (I think) a 350turbo trans. Occasionally it will clunk when I put it into drive or reverse, and occasionally it will lurch forward or backward when I put it into drive/reverse.

Any ideas on this? It has been years since I have done mechanic work to amount to anything (and never did anything on automatics), but am slowly getting back into it. I am not sure if this would be the universals (I did a quick check on them, couldnt tell any problem but I did not take off the drive shaft...probably should have taken it loose?), or maybe something with the torque converter?

The clunking and lurching both happen at the same time (I think...I notice this happening but don't quite remember if I have ever had one problem without the other).

Thanks for any information!

[67ChevyRedneck]

Do you have a tachometer in the truck? Your timing/idle may be set too high, so when it is put into gear, it jumps.

That's my guess...

It could also be bad universals (or a combination of the two) but a bad universal shouldn't cause a "jump" when it's put into gear, it should just make a clunking sound.

[rambler]

clunking is usually the u-joints,, have you checked the tranny fluid? maybe time for a change... but need more information as well, it could also be too high a idle, if you just bought it, I would check/change all the fluids.. many stories have been told about people putting "miracle" fluids in the tranny and engine to get it to move, then new owner discovers how bad it really was..

good luck, but you did a good purchase, so enjoy and let us know what you find out.

[maddog1331]

Thanks guys I have yet to change the transmisison fluid, I plan to this or next weekend and change the filter out....I know that is supposed to be done occasionally on automatics, I just have NO experience with them. I had a 1976 elite with an automatic about 15 years ago, other than that no experience with them. EDIT: I have checked the transmission fluid, and have added some. It looks clean if that helps.....the pan itself is a bit dirty, there is an obvious leak down there but nothing major.

I paid I think 850 for the truck, it needs some work but it works for me

There is no tach in the truck, though I have noticed that the truck seems to idle at different speeds sometimes. I put a new elelbrock on there awhile back and really have not completely tuned it like it needs (good enough for driving overall though), I might have the idle a bit high. Also have to get the timing set....runs fine enough for me but it is not exactly right.

I'll pay closer attention from now on and see if it seems like there is a rather higher idle when it makes the jumpclunk. I really need to finish up on getting the carb/idle/timing set. So many things, so little time lol.

[67ChevyRedneck]

I thought of one more thing. Does your truck have a single one piece drive shaft or a two piece drive shaft? These trucks are pretty old and on the two piece drive shafts the carrier bearing tends to go bad (it's the bearing in the middle of the two driveshafts). That could be your clunking too. If you get under the truck, and can literally wiggle the drive shaft, it's bad.

[maddog1331]

Hey, good point....it is a long wheel base, with 2 piece drive shaft and hanger bearing. I'll check it out this weekend. The odd thing is that it doesn't do it all the time, but maybe after I look at it I'll get a better idea of what's up.

Thanks man, I appreciate the info!

[ChevLoRay]

I had a '76 Blazer that had a driveline clunk that ended up being the slip yoke needed greased. Previous Owner had spent almost $2K on it to try and resolve it. It had 12-inch wide wheels that were holding 12.00x15 Swamp Thing tires.

So, that's one idea. If it is only a sometimes thing, there has to be a variable condition that is contributory. Idle speed; slack in the driveline have been mentioned. Wear has occured, somewhere.

But, check the slip yoke in the tailshaft. Is there a leaking seal there? Are the spring mounts secure?

Solving it could be easy or elusive, as the case was for the previous owners of my old Blazer.

[maddog1331]

Thanks man. This weekend I'll have some time to really check it out, hopefully I'll find the problem.

[heavy71chevy]

When my 71 still drove it did the exact same thing...It never seemed to hurt nothing tho...drove me crazy...now I know what im looking for..thanks guys