I have owned my 79 L-48 for about a year now and have been slowling changing the old stuff out and getting it back on its feet. It had sat for about 5 years so it was dirty and some items need to be changed and others didn't work. One item that has stumped me completely is a pretty serious vibration when I get up to about 3500 RPMs. The whole car shakes/vibrates really bad. What ever gear I am in it does it right around 3000-3500 RPMs. I have had the tranny looked at, filter and fluids changed, changed all the U-joints out, changed the water pump, power steering pump, carb, radiator and fixed the vacuum system so I know that none of those things are the problem (if they could be anyway). Not sure what speed 3500 RPMs is because I think I have to change out my speedometer cable because my gauge does not work.

I have heard about the possibility of bad tires on this site, do you think this could be it? Thanks again.

 

Hey Riley6riley,
Most of the "RPM specific" vibrations I have seen useually are part of your engine rotating parts. I would look at the easy stuff first, Balancer,fan clutch,fan blade (balance or damage) water pump, belts, pulleys.
Good luck.
David

 

I think that "Flying Low" is on the right track. I had a terrible vibration at a specific rpm in a Chevy truck that I had when I lived in Puerto Rico. Turned out to be the fan clutch. Never thought a fan clutch could cause such a shaking vibration. I changed it out on the way from work one night at Pep Boy's, and drove the rest of the way home smooth as can be, and in that traffic, driving on the shoulder, median, or wherever one could, as done over there, it was nice to be running smooth and only have to feel the bumps from the off-road ridingto pass the heavy stand-still traffic. LOL. Seriously, that's how it is there. Anyway...
I also, in early '83, put a nice 350ci and TH350 in my '62 Chevy. When I started it up, running it at about 2.000 rpm to brake in the cam, I had a bad vibration. Now this was a vibration, as opposed to a bad shaking vibration like the fan clutch caused. It turned out to be the flex plate. I bought one at a salvage yard that looked to be in good shape. Turned out to have a very slight wave/bend. Couldn't tell by looking at it, but could see it by placing it on a flat surface and checking carefully. Bought a new one and away I went smooth as can be. Afterward, it's always smooth as can be. LOL.
Good Luck w/ the fix. Start w/ the fan and pulleys, all the external bolt-ons.By the process of elimination, you'll find the problem quickly, I just know.

 

Flying low has it right. Speed sensitive would be trans back (shafts, u-joints, tire balance, bent wheel, etc. In one gear would be trans. At a specific RPM range would be in front of the trans. I usually try to feel or eyeball the problem, and then start with the cheaper and easier parts and work my way up. Belts and pulleys can be diagnosed by removing them and running the engine long enough to check. Vibration dampener and other easy to access pieces can be next. Flex plate and other hard to play with parts come last.

bill.

 

Remove your belts and check each rotating part for any axial play. Especially the harmonic balancer. Also check the outer ring on the balancer as sometimes as the rubber ages, it finally lets go and the ring is loose and worse case can come off. []

As it turned out, my balancer unbeknowst to me, was not a press fit and had a slight undetectable wobble which led to the failure of the crank timing gear (~12K miles)which stopped the cam (@ 5,500 rpm) and bent some valves. During repair I found that the top ring on #8 was broken. Judging from the wear mark at the top of the stroke, I'm guessing it was broke during engine assembly at the factory.

 

I was looking around under the car and saw that there is a piece of rubber that looks like it is hanging out on the harmonic balancer....maybe the problem. Didn't feel any "loose" stuff when I took the belts off but I will tighten the fan and everything up. May have to buy some new stuff it looks like .

 

Sounds like you may need a new balancer. There shouldn't be any rubber hanging out. Did the outer ring feel like it was securely attached to the hub?

 

I didn't check that. Is it hard to replace? I am really getting tired of working under the car all the time but I am slowly replacing everything so it may end sometime in the next few years. I have a Haynes manual or do you have better directions to replace it? Never did it before.

 

Easy to replace. All you need is a harmonic balancer puller which is also used to install the new one. This is because the balancer is pressed on to the end of the crank. You'll probably need to remove the fan shroud to gain access. Recommend replacing the crank seal in the timing chain cover which will require removing the cover which requires water pump removal. Otherwise you'll probably have a little seepage that just becomes real nasty.

Second I recommend getting a Chev. Corvette factory Service Manual. I just looked in my Haynes (which I bought years ago before I knew better) and it basically tells you less than what I said above. The factory manual has fairly detailed instructions and pics.

 

Ok. I just bought the service manual so I should receive it sometime this week and then I will move back under the car and see if I can do it.