Opinions on a C3
 

I ran across a 1976 model with 86k miles. All stock, no mods.
Paint is good, very minor bump to the nose (scuffs and some hairline spiderwebbing but no breaks or tears). Interior is really nice, needs a good cleaning and detailing. Only real bad spot is where the left speaker is. Perforated area has a chunk missing.
AC is cool but not cold. Windows work. Doors shut well.
Cons are small exhaust leak on passenger side, tach doesn't work, and the issue I am curious about: runs and drives out very well, but when first taking off there is a shudder feeling from the rear end for a few seconds, and when backing up it will make two or three popping sounds. Looks to be an axle seal leaking somewhat on the right side coming out of the differential. Not dripping, but one can tell it has a leak. Fluid level is good in the chunk. No noises or vibrations when driving, only when first pulling off from a dead stop.
Local shop says likely spider gears and he can repair it for around $400/500. States it is a common issue. They have an '80 in now with the same issue. My concern is it may be worse, and I know that gets expensive.
Anyone have any experience with this?

That Posi pop is common. I had to pull the one on my C3 and have it rebuilt as well. I noticed my issue on sharp turns.
That price quote is too cheap if you are dropping the car off. I pulled my diff out and took it to the guy.

May not have posi lube in the rear? They absolutely require it.

Not a tough job to pull axles and do it yourself. If you aren't into doing that is not a bad price, if you are in love with the car or can make coin on it.

I think if you do continuous circles in a empty parking lot for example and keep hearing the noise it may be the lack of posi lube.

The big thing with those cars are the soft bumpers. They deteriorate badly and the repros look like crap with panel lines.

Mechanical stuff is usually cheap or not too bad on Vettes. Cosmetics is where the money comes into play.

C4 - 1 :D

j/k, C3's with the receeding rear window are one of my favorites.

The shudder occurs when taking off straight, not just a circle. Popping two or three times backing up.

Front and rear covers aren't cracked or split anywhere. Some spiderwebbing across the nose in the paint.

You know me. End game here is to drive it a little while and sell it. I can get it pretty reasonable. As long as I don't have to put more than 2k or so in I should still get a couple of grand profit.

Oh, I still have the '94 vert. It ain't going any time soon.

Might be the axle u joints.

from google

"A "clunk" in the rear when putting in gear can be a few things. First would be the front differential mount bushing. It is a rubber/steel mount that holds the front of the diff. to the chassis.
When the rubber deteriorates, it allows the diff. to flip up/down, as you put it in gear, or even sometimes when you first take off, or slow down(on-off the gas quickly). They are replacable fairly easily.


Second, you could have u-joints in the driveshaft, or either/both of the half shafts worn/damaged, that could cause a clunk noise. (There is also a possibility of the half shaft flanges being loose on the stub shafts. This is not a common issue, and doubtfull that it is your noise...just info for you.)

Finally, you could have wear in the ring/pinion gears in the differential. This usually manifests itself as a growling, or whine noise while driving, but can also make a clunking noise, depending on what may actually be worn."

I think if you could put it on a lift or jack stands and rotate the wheels to see if you hear exactly where the sound is coming from before having the shop tear into rear pumpkin

How long ago? More than spider gears involved?
I am going to make a couple of calls tomorrow and get prices from a couple of friends who have shops on a rebuild.

No noise going into forward or reverse. Only when pulling off from a stop. Sort of a shudder and slight popping. I suspect pinion.

My money is on u joints pick it up and wiggle it.

Spider gears my ass, about the last thing to fail. Soft faced axle shafts are probably shot. Anybody quoting you less than about $1500 for a rebuilt diff is lying their ass off and even then most of them are screwing you. Parts for a full rebuild will be about $500 & the rest is labor, unless you want a real shit job. Posi case needs polished and checked for cracks, posi unit needs replaced and tuned, and not with that spring loaded POS.


Now it could be all the other things people have mentioned, but no one goes into the diff for $400 - $500.

We had a local radiator repair shop here for decades. I asked someone one time if the guy was good or not. His reply, 'he sells a lot of new radiators'. He kind of lured people in with a clean, boil out, resolder, etc...for around $75 on his sign (back in the day). But, the news was ALWAYS...can't fix it. You need a new one and i just happen to have one. It was probably true that they just needed to buy a new one. But, he made good labor and parts money luring people in thinking they would be out the door for only $75.

I wouldn't call anyone or say anything until I bought the car and found out for sure where the sound was coming from. Don't put it in their mind you want gears up front.

Shudder *could* be a busted tail shaft on the transmission, or it isn't properly secured to the mount. I had this on my '73, and discovered it when I pulled the engine/trans. Tach was electronic after '75.

Don't own one but friends tell me working on the dash components is worse even than the C2, so be aware. Windshield wiper system iand headlights can be a mother too. If the C3s have a "subber bushing" at the front differential mount it can cause a clunk, otherwise its internal. My issue with the C3 is that I graduated in '69 and they were EVERYwhere. I just got jaded with the look and I still am..

Lights and wipers work fine

They all appear tight, no play on driveshaft or axle shafts.

How much play from the axle to the drive shaft? Can you grease them? Can you tell if they are dry? Have you replaced the reared fluid? There was a camber problem with that year as well.

Stick or auto?

And what did the rubber mount look like that the Ghost told you about?

https://www.corvetteactioncenter.com...ation-708.html

I knew I remembered something about this found it now see if I can post it.

I recently started work on a '76, manual, 76k miles, no air (or heat for that matter-heater core delete when engine replaced).
My observations so far:

Automatic's are quite reasonable in price vs manuals.~$3k min more around here for 4spds).
Harder to sell at higher prices ($8-15k prices for nice drivers. I see the a lot of the same cars still for sale since May this year).

I'm guessing you're looking at a dirt cheap price on this unit to realize your potential profit . You'll be surprised how quick you hit the $5-7K mark in parts and paint (not a trailer queen paint job) alone.
Sounds like major $$ issue in the drive line to me. I saw a Local Marketplace ad for a C3 rear diff. for sale in the $1.5k range.
I purchased a slightly used dual exhaust for $250.00. Spent $50.00 on brackets and such to mount it. Stuff ads up quickly.