Backfiring with new carb.

[Steve Mormile]

I am restoring a 1967 Coupe (327/350hp). The Carburetor on the car when I purchased it was the most "economical" Holley available. The cheap Holley had been rebuilt and began to flood fuel frok the butterflies so, I purchased the stock 4 Barrel Holley Carb from Corvette Central and itwas supposed to be tuned and ready to bolt on. Now I am noticing very noticeable backfiring while down-shifting or slowing down. My question is; is it more likely that the new Carb needs to be tweaked or is it possible that the old flooded carb could have gummed up the spark plugs or other aspect of the ignition?

Any advice would be greatly appreciated,

Steve

[Douglas L.]

Re: Backfiring with new carb.

Steve,

First, I assume the backfiring is through the exhaust.

Sound like your running a little "fat" with new carb. Take a reading on the plugs. Could be a idle mixture adjustment or possibly a float adjustment.

[John D.]

Re: Backfiring with new carb.

Steve, I get a lot of phone calls with guys that have FI problems. If you car had an FI and acted this way I may tell the fellows that it is possible the nozzles are plugged up. Possible that the unit is running too lean. Not calibrated properly.
Now the same theory may apply with your carburetor. It may be set up too lean.As the other post said though looking at the spark plugs and the exhaust pies should tell you something for sure.
Dirt and plugged up jets,etc. can cause backfiring as the carb or FI is starving the engine for fuel.
Learned that when I was a kid playing with Model A's. The old replacement cheep die cast carbs from Sears and Tilletson (sp) used to run lean and when you were really into it and let off the gas to double clutch and shift you often heard backfiring and popping. But put the original cast iron type carb on and the problem went away. John

[Duke W.]

Re: Backfiring with new carb.

It sounds to me like you are talking about "afterfiring" - popping in the exhaust when you lift off the throttle at speed or downshift.

This is usually caused by an excessively lean idle mixture. The mixture is so lean that it won't fire in the cylinder, but ignites when it passes by the hot exhaust vavle causing popping noises from the exhaust system. On Nextel Cup engines you can see visible flames at the exhaust exit.

Follow the idle speed/mixture adjustment procedure in your '67 CSM. Prior to this you should check that the ignition system is in good working order.

Duke

[Jim T.]

Re: Backfiring with new carb.

Steve it is possible that you have a ruptured power valve. All it takes is a backfire while starting the car or a stumble with a cold engine to send pressure the wrong way and rupture a power valve. Quick check, remove the air cleaner lid, start the car, and put a finger over the vertical vent for the front fuel bowl. If you 67 dies almost as soon as you put your finger over the vertical vent, the power valve requires replacement. I know whenever my 68 327/350 with owner installed Holley has the after firing in the exhaust that my power valve has ruptured, the finger test just verifies it for me. A good power valve will let the engine run for awhile with the fuel bowl vent blocked with your finger, but blocking the vent will eventually stop the engine from running. Later model Holley's have a check valve installed to help prevent the ruptured power valves.

[Frank C.]

Re: Backfiring with new carb.

Jim.....do you feel that the power valve is really neccesary? IIRC, we eliminated them in our drag car with a block from Holley....Frank

[Doug Flaten]

Backfiring thru carb vs Afterfiring

What causes backfiring through the carb? I was under the impression that it was retarded timing or lean mixture. I was under the impression that afterfiring in the exhaust was due to a rich mixture being ignited in the exhuast as a result of high rpm's on deceleration drawing in extra fuel. It sounds like I was off base on the cause of after-firing. However, I have never been real concerned with the after-firing as it does not seem to be a drivability problem. Whereas carb backfiring seems to produce stumbles etc that really affect drivability. So can someone clear up my confusion or have I got it right?
Thank you,
Doug

[Duke W.]

Re: Backfiring thru carb vs Afterfiring

"Backfiring" (through the carburetor) can be caused by a lean transient mixture - such as when you quickly open the throttle or a timing/ignition problem. It is sometimes accompanied by a nice orange fireball, so it's a good idea not to be looking down the carb when you are messing with this or keep the air cleaner installed. (Ask me how I know!)

An excessively rich mixture on overrun can also cause afterfiring, but it is usually due to too lean a mixture.

Back when I hot lapped my SWC at Kent (with open exhaust) I would often get "popping" (afterfiring) on downshifts. Richening the idle mixture about a quarter turn on the AFB eliminated it.

As you say, afterfiring does not cause driveability problems. It's more of a nuissance, but can cause some damage. I remember as teenagers a popular "trick" was to shut off the ignition for a few seconds, then turn it back on. This was usually accompanied by a loud "explosion", and I recall one friend who split open the muffler on his parents car doing that stunt.

Duke

[Kevin M.]

Re: Backfiring thru carb vs Afterfiring

A.I.R can also cause afterfire. Failed valves or just plain the way it works. It was a complaint when they were new.

Kevin

[Duke W.]

Re: Backfiring thru carb vs Afterfiring

Yes, AIR sytems have diverter valves, which shut off the flow at very high manifold vacuum - greater than about 20" Hg. Afterfiring on overrun is a sign that the diverter valve may have failed.

Duke

[Doug Flaten]

Re: Backfiring thru carb vs Afterfiring

I'm familiar with the destructive afterfire. In college @ A&M I openned the Vette up past the Dixie Chicken and under the train-tracks overpass. When I let off the gas, It blew both mufflers apart. Engine died, and a big black cloud of smoke followed. I thought I had killed the car. Turns out, both mufflers were getting rusty.

Currently I have a '58 Harley that is backfiring thru the carb. I added some heavier springs in the distributor and I have developed a backfire. I wanted to make sure I was on the right track before I work on that today.
Thanks,
Doug

[John H.]

Re: Backfiring with new carb.

Frank -

The power valve is absolutely necessary on a street-driven car; without it, you have to jet the carb WAY up, and it becomes almost useless trying to tune it for part-throttle operation; it's there for a reason (part-throttle enrichment).

[Steve Mormile]

Re: Backfiring with new carb.

Thanks for all the answers guys. I guess truly I would describe it as after-firing as opposed to backfiring. I know for sure the popping is occuring after release the throttle and the car is slowing or when I downshift. Also the sound is undoubtedly coming from the side pipes, which sounds like too rich or lean a mixture coming through the hot pipes. I don't really mind the sound so much, (kind of cool if you ask me) just so long as I'm not hindering performance or possibly damaging the car. Again, I appreciate your very in depth answers as this is really my first classic car I'm restoring my. I'm 24 years old, not a mechanic, and my Father has a 1961 that is in excellent condition (not NCRS certified, no frame off, but the best driver he could hope for). I'll try and get some pics linked on here when I get a chance, and I'll check the plugs and the ignition, fuel intake, and the actually tuning of the carburetor when I get a chance.