Greetings all.

1977 L82. Close ratio 4 speed. 85,000 miles. Previous owners have installed non stock heads and cam (seem like mild upgrades, not sure details but nothing crazy).

Numbers matching block. Carb was rebuilt in recent years.

The car had sat in sealed storage for approximately 3 years and only required a new battery to drive out and onto a trailer.

Really clean, smooth and streetable but powerful. 2.5 inch true duals from stock manifolds with something like 3" power stick mufflers. Open edlebrock air cleaner. Feels like it would outrun my Hemi Magnum.

Took it in to my mechanic and had tuneup. New plugs, wires, dist cap, cleaned up carb. New fuel filter etc. as well as new brakes lines and master cylinder.

Runs super smooth and strong.

The problem is a low idle "roughness" that comes and goes. Did it yesterday but not today for example. Hard to describe but when it feels normal, it idles at about 900 rpm with the normal v8 growl. When it's rough it idles at 650 and for lack of a better term sounds like a v6. This odd sounding exhaust note persists during gentle acceleration until 1000 rpm.

Above that, roars and feels smooth. Doesn't really big down below 1000 but sounds hesitant

Running on 89 octane no ethanol. Going to try 91 and see if maybe it's simply timing.

Thought maybe vacuum or running lean but as I said, it comes and goes. WOT sometimes clears it up.

Also when it does this, I can't tell if it's my imagination but the exhaust seems to pop and rattle more when you let off the gas at 3000+ rpm without depressing the clutch. I know there's a term for that which escapes me. May be imagining that though.

Any ideas?

 

Does the engine have an EGR valve?

 

Make sure the mechanical advance is moving freely and not binding, both springs are there, and the timing is set right. Other than that he just cleaned the carb so that shouldn't be a problem. It seems to be getting harder to find a mechanic that can touch a carb without hurting it. Good luck.

 

Does your Corvette have a Quadrajet?
If yes, check the electrical connection on the idle stop solenoid.

http://www.zip-corvette.com/77-80-ca...-solenoid.html

For the popping when you let off the gas try adjusting the idle mixture screws OUT 1/8 to 1/4 of a turn.

 

And the vacuum advance canister for proper hookup (constant vacuum) and operation.

 

Negative. It has a plate in its place. Pretty sure it's completely free of emissions gear. <br >

Thanks for the direction guys. Update - I drove it quite a bit today and noticed nothing that I described in the OP.

idled perfectly and popping was mostly gone. Seemed smooth as glass which makes me realize that another symptom was some first gear bucking - which again was totally absent today.

I ran gas down to 1/8 and filled up with 91 no eth. Might be nothing but might be the difference? Weird thing though is that it stopped doing that today after it warned up - even before filling up.

Ran it enough yesterday that it wasn't simply cold, but the problem did go away completely today once it warned up.

 

So, what you have described are the poltergiests of inactivity.

I think that your fresh gas and the tune up made a big difference.

When I got my '82, it had been sitting for a couple of months, started often and driven short distances and never warmed up-- It was at a classic car dealer. I was scared I bought a CFI problem.

When I first drove it, the car bucked, popped and had super jerky acceleration--- the gas gauge was on empty.

I filled up with new premium fuel and immediately drove it at 75 for about 10 miles. Making sure that it was WOT on more than several occasions.

Totally different car at that point- smooth and powerful. Must have blasted out the problems.

 

I think this is largely correct. But... the issue has returned. I talked to my mechanic about the possible issues postulated here and when I get some time later this week, I am going to pull the fuel filter and see if it's collected any more junk from the tank. This is what my mechanic thinks is likely. He also thinks if that isn't it, it may ultimately need another carburetor rebuild after sitting for that long. ugh...

So frustrating because when the idle is smooth, it is SMOOOOTH. Now I'll be chasing that until it's back to stay!

 

Besides checking the fuel filter, you might want to also check on the pickup in the fuel tank

 

I will, thanks!

 

Check for water in the fuel tank.

 

I've run 4 or 5 tanks of gas through it since getting it out of storage. Usually running it down to 1/8 of a tank. I figure water and gas separate in a tank, but do you guy guys think that if there were water when I towed it from storage, there might still be some? I would think not, but I honestly don't know.

Thanks for the suggestion!

 

Check all the vacuum lines you have left on the motor. Make sure you have no vacuum leaks at the carb gasket.

 

That is exactly what is needed. If you don't have a vacuum leak, then there is crud in the bowl of the carb running through the low speed jet's at times (Primaries). You might want to try it yourself?

Good luck! Keep us posted.

 

I might! That brake job/tuneup just ate up most of my corvette addiction funds for the spring, so I may give that a try! Hoping it's just a little leftover junk clogging the new fuel filter, so this evening I'll pull that and take a look.

It only has a carb mounted fuel filter. Do you guys see any downside with (assuming a stock fuel pump/carb setup) adding an inline fuel filter?

 

The Forum carb experts recommend using only one filter. IMO the only method of thoroughly cleaning the fuel tank is to remove the sending unit.

 

I had the same symptoms you describe and found cylinders 7 & 8 were miss firing, eventually tracked it down to a vacuum leak that made the engine run lean at low rpm.

 

Was your vacuum leak the carb gasket or a plug/line?

 

It was the line to the wiper door opener.

 

Well... Fuel filter was clean. Maybe a flake or two of stuff. Certainly nothing that would impede flow or suggest a bad tank I think.

Sounds like next thing is to check vacuum lines and carb gasket. If that's not it I'm guessing it's rebuild time.

Definitely believe the problem is best described as a lean misfire at idle.

It no longer comes and goes. Just stays consistently crappy at idle.