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Looking for help with an engine issue. Car is a 1975 Corvette with L-48 engine and 4-speed manual.
The car starts up quickly and easily and idles just fine, the problem is when you go to drive it, it bucks and sputters and is completely un-drivable.
I had a car do this many years ago and the distributor was in 180 degrees backwards but I've checked this car, the distributor is correct and the timing is correct.
I've plugged all the vacuum lines and removed/plugged the EGR. It has new plugs, wires, cap, rotor and exhaust system with no cats.
I've also replaced the carb twice. None of the three carbs made any difference.
What could cause this?
What is your timing set to? The factory spec of 4 degrees at idle is what made the car pass emissions when it was new, but is far out of line for what make the engine runs best.
Additionally what carburetor have you replaced twice. Is it the factory Quadrajet or an aftermarket deal?
What’s your timing at? Is advance working?
what is your vacuum at idle and at higher rpm?
what is your fuel pressure?
the good thing is that these engines are simple, so once you gather enough data, you’ll be able to find the problem.
10 deg BTDC at idle.
Yes, It's 3 Quadrajets in total. The one that came on the car, a used one from eBay that was pulled off a running car and a rebuilt one from a carb rebuilder. The car behaved exactly the same with all three carbs.
What’s your timing at? Is advance working?
what is your vacuum at idle and at higher rpm?
what is your fuel pressure?
the good thing is that these engines are simple, so once you gather enough data, you’ll be able to find the problem.
I don't know if the advance is working, or the vacuum or the fuel pressure.
The timing is 10 deg BTDC at idle.
I'm told a non-working advance will just reduce performance and MPG...not cause bucking and misfiring/backfiring that gets worse the harder you press the throttle.
I would bump your initial timing up to 14 with the vacuum advance unplugged. Plug it back in, then if you’re getting a higher number that means your vacuum advance is working. Another method of testing it is removing the cap and rotor and seeing if the arm on the vacuum advance moves and holds when you apply vacuum to it.
I read your other thread from 3 years ago about your carb fiasco. It sounds like if it was running well then but doesn’t now you might have issues with components in the carb that can’t handle the ethanol in modern gasoline. A bad accelerator pump would be the biggest indicator.
The correct carb number for a 1975 L48 manual is 7045223. I would check your carb to see if the one you bought matches this. Otherwise there are a number of carbs on EBay or other places that are perfectly good for a rebuild and would function wonderfully if set up with high quality, ethanol resistant components.
My car is a copy of yours and the first year HEI. Couple things to check..... Do you still run the original HEI? Maybe the ignition module? Some have had issues with the distributor. I am currently installing EFI and while dropping the tank to install the fuel pump the rubber fuel line connections were not to be trusted including the vapor canister vent lines. Fuel starvation from lack of pressure?
I read your other thread from 3 years ago about your carb fiasco. It sounds like if it was running well then but doesn’t now you might have issues with components in the carb that can’t handle the ethanol in modern gasoline. A bad accelerator pump would be the biggest indicator.
I was a bit over zealous in declaring that issue fixed...this is the same issue. It was never fixed. I've determined that whatever the issue is, I'm fairly certain that it is not the carb.
From: At my Bar drinking and wrenching in Lafayette Colorado
Before checking the timing, verify TDC to assure that the balancer has not slipped and is therefore giving a faulty timing indication. Start with the basics: Verify TDC. Set & verify timing. Do a leakdown check. Set valves and observe valvetrain operation to assure no flat cam lobes. E-mail me if you need the "How to check TDC" tech paper.
...and it's not possible for an engine to operate at all with the distributor installed 180 degrees out...
Can you start the engine cold the do a quick drive right away. Does it first seem to rev ok but as time goes on it gets worse. Once the engine is hot and back home does it try to miss when you quickly rev it?
I've seen many an HEI module go into fail mode once the distributor housing gets up to engine temp. You can still drive but engine miss fires
I had a 85 that drove me nuts same thing, started with a tap idled normally you could send the tach to the moon. The minute you let up the clutch put a little load on it. It chugged and popped around 2000
Turns out the coil would go spastic, swapped out the coil all good.
Is the engine, camshaft, exhaust, mufflers, convertor all basically stock? Or list the mods.
Do you see a good solid accelerator pump squirt with engine off as you look in the primaries? Or does it sputter? Or is non existant?
Looking for help with an engine issue. Car is a 1975 Corvette with L-48 engine and 4-speed manual.
The car starts up quickly and easily and idles just fine, the problem is when you go to drive it, it bucks and sputters and is completely un-drivable.
I had a car do this many years ago and the distributor was in 180 degrees backwards but I've checked this car, the distributor is correct and the timing is correct.
I've plugged all the vacuum lines and removed/plugged the EGR. It has new plugs, wires, cap, rotor and exhaust system with no cats.
I've also replaced the carb twice. None of the three carbs made any difference.
What could cause this?
"Bucks and sputters" yet "idles just fine"........bucks and sputters needs more description. How bad is it?
"bucks, sputters and completely undrivable" makes me wonder about the valve train. Flat lifter cams display this behavior when you lose a lobe. That's on the tougher end of things for sure a bent push rod or broken rocker are on the less problematic end of valve train problems in need of a correction.
Then again I've also seen weak ignition handle things "just fine" at idle but as soon as driving "load" is put into the equation it reveals itself as not up to the task.
L-48 sputters terribly
