hey vettman did the problem go away. i have the same symptoms you have. i have a 79 4 speed and vibrates at around 65 and gets worse as the speed goes up. the tires are brand new and i had an alignment done last year
thanks

Harmonic Balancer almost definatly won't be the problem if you've been driving it the same way for 7 years. Mine just fell apart and it was free spinning around the shaft right against the timing chain cover, and you can see the worn spot on the timing cover where the metal to metal was ripping it open. So after 7 years...you'd have one heck of an oil leak.

Scot

have you had the drive shaft spun balanced

Thanks again for the replies. Well I took it to a brake shop but couldn't find the problem. Yes I had the driveshaft and half shafts spun and check. They were good. Also had the tires check (put on a new set from a buddies car) no change. I now have my car at a retired GM Mr.Goodwrench guy who worked at a Chev dealer for years and he only worked on Vettes and took his training in the assembly plant. He thinks it could be the output in the trans or the diff. but did say it was minor other than pulling out the trans. If he is right he said it is common in C3's and he is going through all the TSB's and CSR's that he kept from my year. Will let you know. Anyways while it's there he is fixing a lot a minor things. This guy just loves to work on Vettes and since he retired he misses it. He worked on them for 40 years and has two himself. Not bad for a guy in his late 60's. I got two more months before the cold weather so I'm not giving up yet. Will keep you posted

Mr. Goodwrench is the kind of mechanic you need for this problem. I think he is close. The vibration could be coming from the transmission input shaft-a worn pilot bushingis probably the culprit. This allows movement of the shaft and an imbalance of the clutch disc at high speeds. It is lessened when you release the clutch as it tries to find its center of rotation. This is also why your shifter makes noise. While you have the tranny and clutch out, have him check for bellhousing runout. This is a common Chevrolet problem that has plagued manual shift GM products since they started making them. When any part of the rotating assembly is crooked,or in a bind, you will have unwanted vibration.
Good luck, and keep us posted.
c3

Robert,

I was just browsing the C3 site and noted your vibration problems from back in the summer. I had the same trouble and found the tailpipe was against the tranny crossmember. I relocated the exhaust away from the frame member, vibrations disappeared.

Dave

Hey Guys and Gals, well its now going into Nov and I'm still at it. The retired guy thought he solved the problem but it's still there. I don't want to tell him since he did a great job on everything he did and basically charged me way below minimun wage. I now have it back at the brake shop and going over a check list of more items to look at. It's up on the hoist and I'm going to start it up and run it while in the air and see what vibrates. Question I'll ask for the 4 speed C3's is how many shifters vibrate (rattle) and if they did how did you get rid of it. I'm getting to the point of living with the vibs but have to get rid of the rattle. I put a new OEM shifter in but still had the rattle. What I do now is keep my hand on the damn thing to stop the rattle. Hey I started in the beginning of summer and it's going to snow soon. This car has never seen snow and I'm looking at finishing in the spring.
Thanks for the support.

i still got my vibration too in my 79 4 speed. Ive heard that a sticking valve could also cause a vibration or a misfire or bad timing. Ive checked the timeing but now i have to do a compression test to see if a valve is sticking. So check to see if any valves are sticking

Well one more update and I think I'm done. I think that I'm 90% there. My car was at the brake shop (buddy of mine) and I told him of the latest replies from the site. He looked at autom8r and guess what he found. Yup the pipes were rubbing on the cross member. Can't belive it but most of the vib is gone. My shifter still has a rattle at about 45 to 50 in 4th but that all and I think some bushing should hopefully take car of that. Can you believe this and how many others have the same problem. I won't tell you how much it cost me over the past few years and mainly this year. All it took was about 20 minutes to reposition the pipes and clamps and it was done. I took my car up to between 115/120 mph for about a minute and it ran great. Haven't done that for over 10 years. Thought it was going to fall apart before. I'm now working on a small thermostat problem and then it goes to sleep for the winter. Come spring I'll be getting at my shifter and getting rid of the small rattle (I hope). Thanks for all your support and help.