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350 SBC missing and stall at full-throttle

5K views 8 replies 7 participants last post by  williamC-3 
#1 ·
I have an '81 corvette with a 350 and a Q-Jet carb. Under slow accelleration and at a steady speed I can hear the engine missing. If I travel a distance at a steady speed (hwy driving) I at times get a backfire. Under heavy accelleration it stumbles and I have to let off the pedal. Also at times the engine dies while idling.

History of the car: The car was sitting up for approximately 2 years. The engine had been rebuilt and about 5K miles were put on it before it was garaged.

When I got the car I changed the oil, oil filter, and coolant. After noticing the problems stated above I put on a new dist. cap and rotor and had the carb checked out to see if it needed rebuilding. I was told the carb did not need rebuilding. In order to pass emissions (in Texas) we had to change the timing from the factory settings. In trying to get it to pass emmissions I have checked all of the AIR system to make sure it is working properly, which it is. The only thing I have not done is change the plugs and wires, although they appear to be in good condition. I have also tried a higher octane gas as well with no resolution.

Any suggestions or ideas on what to try or what the problem may be?

Thanks,
William
 
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#4 ·
For as cheap as it can be done change the plugs to be sure. Go around the cap while its running and pull off the wires one at a time (replacing one after it checks okay) and see which cylinder doesn't cause the engine to drop in rpm. Does it seem to be missing at idle?

What did you change the timing to? What do the plugs look like?
 
#5 ·
I`m not sure being it`s a 81 model if it has a computer as some early 80`s chevy`s did, if it doesn`t, and uses a HEI with mechanical and vacuum advance, yank the cap and check the mechanical advance, it could be stuck in the closed position and not advancing at all, with a low timing setting it could miss and backfire due to the timing not advancing with the increased RPM`s. I also agree with the above posts, if the car sat 2 years, it`s a known fact the gas turns into crud, all kinds of trash deposits form in the carb and it`ll never run right, I went through this same thing with a 79 Z28 that sat for several years, the gas tank was full of trash from the stale gas, I rebuilt the carb and then kept having to clean it many times over to get it to run, it would spit, back fire, skip, not idle correctly or at all at times, the trash deposits are so small the filter can catch them all and the carb collects them and the result is the car runs like crap. the factory fuel filter inside the front of the Q-jet, remove the filter and replace it, it`s a possibility from the car sitting it could have rotted when the car was awoken from it`s long nap.
Changing the plugs and wires is a very wise step, use spiral core plug wires you can get from summit or the like, they`ll last longer, provide a hotter spark and have half the resistance than the cheapy parts store solid core wires. good luck.
 
#6 · (Edited)
80 Vette

Doc Here:pimp:

I have the exact same problem with my 80 Vette. This occurs when Plugs #4 and #3 get fouled out...and the rest are slightly past changing time...

Change the wires and plugs (easiest done on your back from the bottom, the other way will take ya forever!) and you'll notice the
difference right off..

When changing the wires, USE the wire channels Corvette provides! just about anything else will melt the plug wires over time! at best you'll get crossfires.

My 80 came out of hiatus in storage when I bought it, Some of the things I had to do to get it back to snuff were, Rebuild the Q~jet... It was rich and making poor mileage at best adjustment, but ran OK, the rebuild made a 100% improvement, in gas and drivability..

Clean and Check the Vacuum Filter (located at the rear of the Q~jet on the manifold port) Check all your vacuum check valves, and Vacuum lines to be sure they all work and don't leak. If your Headlight doors don't operate , open them, and disconnect the canister input line and the Vacuum solenoid (s) control lines and plug them until you can fix the doors. This will put a big leak into the induction system even in bypass, if the check valves are stuck or missing causing popping.

Change The Fuel and Air Filter! I got rid of the stocker air filter and removed the front scoops, and went with a Chrome Eldebrock cleaner, It breathes better and the engine runs cooler by about 10 degrees with the Air intake scoop removed, I think it blocks air flow over the front of the engine to some degree. But anyway I noticed right off the temp dropped.

If your advance is too far retarded (and I mean just a few degrees) or the Vacuum unit is not working (if equipped) this will cause the popping...The Static timing should be between 6 and 12 BTDC (depending on your model 350 , look at the hood tag for spec) I moved my advance unit from the Metered port on the front left of the Q~jet to the rear port and tee'd in where the secondary pull-off goes in.. and removed the spark delay valve, gives full vac, and runs strong there.(since you already passed smog..)

Also check you Cat to see that it isn't plugged.

OTHER ODDITIES WORTH LOOKING AT:

The Computer is mounted in the Battery Box behind the driver's seat on the 80, (probably is in the 81 also) It is mounted on a frame in mid air and sways in the breeze..

I had to pull it disassemble it down to the board (Don't do it unless your familiar with digital CMOS / HMOS technology) Clean the Corrosion

(due the close proximity of the battery, FINE JOB, Corvette Engineers!! why not mount it on the freakin" Radiator!)

Re mount it back in the old hole using a stouter <--- is that a real word?? Frame to keep it from vibrating back and fourth.

Clean and burnish ALL the harness and Board Pins , this was causing a pulsing "Check Engine light" with each lope of the engine and causing All the Gauges to sway like a metronome at a stoplight..

lastly added a hard ground wire from the computer frame directly to the battery ground. This stopped all the little annoying things that go "Flash in the night"

Key to all older Corvettes is Corrosion...everywhere you have electrical, your gonna get corrosion... so that's something you always have to look at, I think it's due to the lack of metal, and high heat inside the car because of no insulation.

Your Computer system should be CCCC and I think should store codes, mine is the CCC and does not, so if yours is, code retrieval is a helpful thing for troubleshoot.

Hope it helps!
Doc :pimp:
 
#8 ·
80 Corvette miss

I had that problem on a 74 Camarro before I knew much about cars. My mechanic friend said change plugs ; fixed. When I put a new engine in my 76 Corvette it acted that way ; I tried everything so I took it to the Chevy dealer; about an hour later said it was fixed . I had a couple wires crossed; the thing is on the Corvette , the plug wires go through a crome cover so when they come back out they can be crossed .
 
#9 ·
All, thanks for the suggestions and comments. I found out what the problem was. There were three spark plug wires that were arching. With initial inspection of the wires I found that the #2 wire had some breakage in the wire itself. So this led to me replacing the wires and plugs. As it turned out two other wires were bad as well. After the replacement, it ran like it never had before.

Thanks again for your comments and suggestions.
 
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