bjenki7

I have a 1977 vette that I inherited from my uncle. It had been sitting for several years, but I have been working on it, and I have gotten it up and running. It cranks right up and idles fine, but once I press down on the gas it backfires through the carb. Once I put it in gear and go down the road, it will drive fine until I try to accelerate, and then it will backfire through the carb again. It doesn't matter if the engine is warm or cold.

I think that the problem started with my uncle when he did a carb and intake swap on the car. He installed an Edelbrock carb and intake. Is this a timing/fuel issue?? I would appreciate any help. This is driving me crazy!

73, Dark Blue 454

Look for an ignition problem first. Sounds like some cross-firing. Check for cracked distributor cap, moisture inside the cap, bad plug wires, etc.

lars

Before doing any carb work, make sure you check the timing and ignition system. As I state in my tuning seminars, "90% of all carb problems are ignition problems." Get a timing light and set it up per my "How to Set Timing" paper with 36 degrees total advance and initial timing in the 8 - 18 degree range to produce the 36 total. Make sure vacuum advance is hooked up and operational, and make sure there are no vacuum leaks. Make sure wires, cap and rotor are in good shape and that the plugs are fresh.

Once timing/ignition is set and verified, the only thing that will cause backfire up through the carb is a lean condition. This will be noticable on acceleration, when a rich mixture is required. If you're popping up through the carb after you've verified the ignition is good, richen up the carb. You'd be much better off getting a correct Q-Jet for the car than to run that old Carter AFB on there...

Lars

johnt365

I would check the accelerator pump shot. A lean condition can cause a backfire and you are getting the back fireupon initial acceleration.

If the pump is functioning, it may be that the amount of fuel delivered is not enough.

EDIT - what he said ^.

roadblock69

starting point Make sure all the plug wires are routed correctly.

daanbc

Listen to this man. He KNOWS his SH_T!!! He does not steer anyone wrong.

Garys 68

Isten to Lars advise first, if that fails, check for cam exhaust flat lobe.

my 76 ray

Make sure youir firing order is correct. It's quick and simple to do and the wrong firing order will definately cause backfiring upon acceleration.

lars

I was thinking about that, too, but a flat exhaust lobe will make the engine pop up through the carb even at idle. The fact that the issue only occurs on acceleration would, hopefully, indicate that the cam is still in good shape and that it's more of a tuning and setup issue.

Lars

Garys 68

With oil reformulation, I've seen more than a few boats with partially flat lobed cams (high spring pressures). Idle fine, but spit back through the carb under load when the partially open exhaust valve cant clear the cylinder. Even firing voltages across the cylinders was the same at idle.
It's easier to catch a partial flat lobe on a boat since they're either idle or load. I suspect you can drive a car under light load much longer and finish off the lobe and mushroom the lifter though.
Anyway, thought I would throw that out there after checking the ignition and carb/fuel. But if there was a recent cam change, checking lobes would be high on my list.

Naki Boy

Have you checked the plugs? Mine would do that when plugs fouled before rebuild

Hamin79

had the same problem turned out to be the cam, my vette sat for 10 years found 3 bad lifters and 2 bad cam lobes, she runs like a champ now, i ordered the cam from summit i told the sales rep i wanted the largest cam available fo rthe the stock valve springs and valves

327_Stingray

Lars, as always your advice was spot on; was having the same issue with my 68. After seeing this thread I started with inspecting plugs and wires. Sure enough, the wire going into the ignition coil was not properly seated. I corrected this and now it runs great without any backfire. Thanks much!

71VetteLover

There's only two common things that can cause a backfire thru the carburetor and that's excessively early ignition timing and an exhaust valve not opening.

lars

Actually, the most common cause of backfire through the carb is a lean condition caused by a failed accelerator pump. Lack of pump shot upon sudden opening of the throttle blades will cause an immediate backfire through the carb. Failed pumps are common with the modern pump gas. The fact that the car had been sitting for several years is further indication that the accelerator pump has shriveled up and failed. But ignition should always be checked first. But let's hope that he got it figured out over the past 5 years since he posted the question...