Recently we bought a 1974 C-3 BB. It was supposed to be stock but it does have a spread bore Holley double pumper Carb (don;t know size yet but it is big), The other day I was driving home and doing about 35 mpg when it backfired through the carb and started loping as if it was not getting gas. It finally went dead and never restarted. We had it towed home and have been trying to figure out the problem ever since. We have put a new coil and spark plugs in it but it still is not running correctly. The points look new, the distributor cap, rotor button and plug wires look new so we have not changed them yet. The car will crank but we smell a strong gas odor then it runs for a second or two and goes dead. Sometimes after working on it for a while (cranking it several times in a row), it will back fire through the carb and then run perfectly for 5 mins or so. When we pulled the spark plugs, they appeared to be drowned in gas. This has caused us to think the carb is the problem. Does that sound likely? This is the first time I have worked on a C-3 in many years and I am a bit stumped. The car sounds as if it is getting too much fuel and at an uneven flow. Any help will be greatly appreciated.

 

Look down the throat of the carb while someone else cranks it.......is fuel being dumped into the venturis?
Replace the plugs and have a friend attempt to start it again......if it runs for a second or two like you said it would is fuel being dumped into the engine?
An engine that is getting way too much fuel will "lope" like you say......and blow black smoke out of the exhaust.

Jebby

 

If you have a points ignition before you mess with the carb try changing the condensor in the distributor

 

Aftermarket electric fuel pump overrunning the carb?

 

Careful to not position your face over the carb if you're gonna look down the throat of a carb that is likely to backfire.

I know next to nothing bout those Holley carbs, but don't the older ones have a diaphragm that will rupture if it backfires and will need to be replaced to run right again?

 

Check that the timing and dwell haven't changed.
I had a similar experience, but with a Qjet, that I never did find a cause.
While off throttle on a freeway off ramp, the car backfired and caught the paper air filter element on fire. It was a bit exciting for a while. Luckily, I have a metal top cover on my open element, and saved my hood. I ditched the K&N top years ago. Towed the car home, cleaned up the extinguisher residue, and it started up and has run ever since.
My best guess is that your float stuck, and now, after the backfire, your power valve is damaged/leaking.
Also, remember, that just because a part is new, doesn't always mean its working correctly.
I once solved a intermittent miss by changing a dist. cap. The existing cap was good quality, high $$, with brass terminals, that looked just fine. After messing with the car for a year or so, and changing numerous parts, I eventually changed the cap. Missing solved.

 

sounds like you blew out the power valve and its sucking gas straight through the carb into the intake. You may want to pull your dipstick and see if you oil level is now high and your oil is trashed with too much gas,
You may wan tot rebuild the carb, look into install an anit blow out valve which is nothing more than a ball bearing in the power vavle passage, theres a kit for it. Then give it a full tune up checking evrything form plugs , points, condensor, cap , rotor, timing.......

 

You stated that you recently purchased the Vette. So you probably aren't certain what all has been done. So process of elimination.
Make sure spark is good. Ohm the wires move them around while doing so.
U stated the points look good I would check the gap and or replace they r cheap. Rule out any chance of week or bad spark.
Check timing
Why is there an after market carb!!?? And what fue pressure does it require.
Mechanical or electric pump!!??

​​​

 

You stated that you recently purchased the Vette. So you probably aren't certain what all has been done. So process of elimination.
Make sure spark is good. Ohm the wires move them around while doing so.
U stated the points look good I would check the gap and or replace they r cheap. Rule out any chance of week or bad spark.
Check timing
Why is there an after market carb!!?? And what fue pressure does it require.
Mechanical or electric pump!!??

​​​

 

I had a major issue, so I thought. I put a used 750 Proform on and it ran great for a month and a half, then I adjusted fuel bowl level and it wen to crap. Spent 2 months trying to figure it out. Changed everything you can think of regarding fuel and spark. Cleaned carb about 4 times and replaced what I could. Same thing as you eyetearingly rich, fuel in the oil, couldn’t get it to start most times. I jetted down 9 sizes in front and 10 in the rear and it was diving again. Now I bumped fronts up by 3 and it’s better. Still need some tweaking tho.

 

Pull the valve covers and make sure all rockers are in place. And check to be sure all are opening and closing the valves