C3 Vibration Problem

[Robert E.]

Hi all, I'm a brand new NCRS member and I'm happy to be aboard. Anywho, I'm hoping I can get some advice with my newly purchased 1971 LT-1. I purchased the car over the winter and I'm just getting the first few rides in this spring. The car came with a set of Goodyear F70-15's that were badly flatspotted, so I was forced to put on a set of B.F Goodrich Radials (much nicer ride). Now that the flatspotting is gone, I've noticed a bad vibration between 2700 and 3300 rpm while rolling or standing still!! What the heck can be causing it? Please don't tell me it could be an internal engine problem because it runs real strong below and above those rpm's and have very good oil pressure. Before I bring it in for service, I'd like to educate myself with as much info as possible. Any suggestions are welcome.

Thanks in advance

Robert

NCRS#41801

[William C.]

Re: C3 Vibration Problem

If it occurs at that rpm in all gears, the likely problem is the balance of the engine internally, or (more likely) the clutch and pressure plate assembly. The problem should have been there before the tire change, but it may have been masked by tire vibrations.

[Craig S.]

Re: C3 Vibration Problem

Robert - Welcome to the NCRS! I would first see if the vibration occurs at a stop, just revving the engine. If it does, you know you have a balance problem in the engine or flywheel and clutch. The first rule of thumb is don't trust the balance EVER on a flywheel or clutch/pressure plate asm. I put a new pressure plate on my 74 L82 years ago, and had an immediate vibration as you describe.....and off it came to go the the machine shop for balance. It could be an internal balance problem, but I would first suspect the external components. If this test goes well, and it feels smooth at a stop when the engine is accelerated, then I would start checking the driveline components...Craig

[Robert E.]

Re: C3 Vibration Problem

Craig, Thanks for your response. The vibration does in deed occur at a stop as well. When I rev the engine upto the range I described earlier, the vibration is definately there as I just realized this recently. I too suspect the clutch assembly for one other reason, My clutch peddle vibrates at idle and at some speeds and doing so makes a rather loud rattling sound. How would a mechanic know for sure that the vibration is from the clutch? I don't think it would be wise to just start replacing parts to see if it goes away.

Robert

[Richard G.]

Re: C3 Vibration Problem

Robert

Did the car sit for a long time without being driven, based on your tires it sounds like it to me? If this is the case, I believe that the clutch would be the first place I would check. I've had this happen to me before. Have you driven the car much since you bought it?

Rich

[Craig S.]

Re: C3 Vibration Problem

Robert - when you say your clutch pedal rattles and vibrates, I am assuming you mean you can feel the vibration transfering from the clutch asm through the linkage to the pedal and your foot. This is also a good way to detect issues with the throwout bearing or pressure plate. If it were me, I would be removing the clutch, flywheel, friction disk, and pressure plate and they would be going to a compentent machinist that does precision engine balancing. Pay attention to the orientation of the pressure plate when unbolted and mark it compared to the flywheel so you can convey that to your machinist. First look at condition, you don't want to start balancing bad or worn out parts. New parts need the same attention as I mentioned, never trust the balance. If there is evidence that the balance was way off, I would not tear in and suspect internal engine balance was the issue, but it could be. In my case, I had a know factory orignal engine, and smooth performance UNTIL I changed the clutch myself with GM parts, and then had the problem, so I knew what the problem was. When I built up an engine, I ALWAYS spend the extra $180 or so to precision balance the entire reciprocating assembly from harmonic balancer to clutch. You will thank yourself over and over......nothing feels better than a glass smooth engine to redline, and it dramatically reduces stress on the reciprocating components as well......Craig

[John H.]

Re: C3 Vibration Problem

Pop the drive belts off and run it briefly to satisfy yourself that it's not accessory drive-related; only takes a minute. If the vibration is still there, it's most likely clutch-related.

[Robert E.]

Re: C3 Vibration Problem

Thanks to all for your input. The vibration in the clutch peddle is not only felt but is also audible. Since I bought the car during the winter, I didn't get a chance to drive it until this spring. I not only have the space to carry out any repairs, I don't have the mechanical expertise to even attempt it. I live in the Toronto area and wouldn't mind a recommended mechanic to bring it to. I will definetaly print out each response and bring it with me when I get a change to bring it in (the weather has been mostly rainy for the past 2 weeks and it doesn't look any better for the next couple of weeks).

Thanks once again to all, and wish me luck.

Robert

[Jeffrey S.]

Re: C3 Vibration Problem

Robert:
I had a the same problem on my '69 years ago. Do as John suggests but only get the fan and fan clutch disconnected. That was the problem I had at almost exactly those rpm's- the fan clutch was bad. Had it rebuilt by Fred Oliva and it was fine. Hope this helps.
Jeff Salz

[Robert E.]

Re: C3 Vibration Problem

Jeff, that seems like something I could tackle myself. So, what you suggest is to simply remove the fan and clutch from the waterpump without removing any belts or accessories and give it a try?

Robert Eisner

[Jeffrey S.]

Re: C3 Vibration Problem

Robert:
Leave the belts on and remove the fan and clutch from water pump. Be sure to replace the nuts on the mounting studs and tighten properly (assuming that your fan clutch is fastened with studs and nuts and not bolts. If attached with bolts replace the bolts and tighten). Yo can run the engine for a short period of time without overheating or damage without the fan. I hope this is the problem because it would be the easiest to fix. Let us all know what the results are.
Jeff Salz

[Paul O.]

Re: C3 Vibration Problem

Robert also check the throw out bearing and clutch fork. A failing bearing and or missing or damaged fork bearing retainer springs will cause the pedal feeling you are getting along with the sounds you have described. Also a incorrectly mounted throw out bearing will cause this. Also when mounting the bearing pay close attention for proper installation. Paul #18046

[Robert E.]

Re: C3 Vibration Problem

Jeff, well it worth a try but with my luck it wasn't the cure. I removed the fan and clutch, also removed the AIR belt. The vibration was still there. Unless there's something just as simple for me to attempt, I'm forced to bring it in to a professional mechanic.

Thanks again

Robert Eisner

[Robert E.]

Re: C3 Vibration Problem

Thanks Paul for your advise. I neither have the tools or the experise to attempt what you have suggested. I'll copy your response and give it to a professional mechanic and let him decide.

Thanks again,

Robert Eisner #41801

[Craig S.]

Re: C3 Vibration Problem

Good luck Robert - I would go for the clutch first.....let us know what you find.....Craig