brianPA

I was driving my '75 on short trips all day without issues. Went out, it sat parked for an hour, then as I cranked it up to drive home it immediately backfired through the carb with a huge bang, and smoked some. The air cleaner cover was hot to the touch, almost burned me taking it off. The primaries were blackened. I couldn't find anything obviously wrong and carefully started it back up. It had to crank for a few seconds but then started right up and I drove 15min home. The whole ride home it was hesitating slightly but made it fine.

This is the first time I've had backfire like this but I have had a few issues with this carb. It is a holley 4165, 650cfm direct qjet replacement with manual secondaries. Even before this backfire issue whenever I slow to a stop the idle drops and it wants to stall, and I get black smoke out of the exhaust tips when I start up cold. I have the Holley techbook and have tried messing with the idle and mixture adjustments without much success. I think the floats need to be adjusted but on this carb it seems to require disassembly. I had just cheated and turned the idle up to avoid this.

This carb is at least 22yrs old as the previous owner said it was on there when he got it. Does this backfire sound like a carb problem or something else? I am planning to just order a new carb and rebuild the old one as a learning exercise. I have been holding out because I was planning on doing a turnkey crate engine soon and I don't want to end up with three carbs. Is there a carb you guys can recommend that could be appropriate for both an l48 and a zz4? That way maybe no turnkey but at least I'm not wasting $400.

lars

If the car was running fine before, and you suddenly had backfire with a very hot air cleaner lid, chances are good that something else has happened not related to the carb: This is usually related to cam, cam timing and valvetrain.

A backfire up through the carb is usually indicative of a lean condition, but this does not occur out of the blue - it will be an ongoing and consistent problem. If the carb was running fine before, there are very few issues that will suddenly cause a carb lean condition for no reason.

Rather, if you are suddenly getting backfire and a hot air cleaner lid, you need to take a look at exhaust valves and valve train: Exhaust valve(s) not opening will cause backfire through the carb and a very hot air cleaner. This can be caused by a flat cam lobe, loose exhaust rocker arm, pulled-out rocker arm stud, or other valvetrain issues.

Lars

74modified

After this many years it is due a renew, that said you should check timing, advance and ignition components first. A basic renew kit for the carb would be $40.00 or $50.00 and is not too hard to repair. But, if you don't have the skills to check the the electrical first, you probably shouldn't take on the carb build.

brianPA

Yeah it was running fine all day, and to be clear it only backfired once on startup and not again. Also, the air cleaner lid wasn't hot when I got home. Still, the hesitation the whole way home is definitely not good. Hope it's just the carb but I will investigate.

BHRIG

You might check for a broken valve spring....

brianPA

I had sprayed some seafoam in the primary when I got home earlier, engine still running. I just now took it for a spin and it ran fine with no hesitation and a nice cool air cleaner lid afterwards. I'll pop the valve covers tomorrow and check things out in there.

MotorHead

This is a fact, I left a vacuum hose disconneted and boom flames were shooting up through the carb, I happened to have a wet towel and fire extinguisher on hand and the carb was open as this was the first fireup of a brand new motor, and fireup it was

7T1vette

I suspect that your carb is leaking down when you shut it off and the raw fuel is pooling in the intake manifold. When you start the car, that fuel will ignite and make one he!! of a bang.

You may have some leaking block-off plugs on the bottom of the carb or there may be some other reason for that leakage. To verify, you will need to remove the carb from the engine. I suggest that you start the engine up cold and run for just 10-15 seconds...enough time to be sure the float bowl is full of fuel. Then, disconnect the battery (prevent sparking) and remove the carb. If you don't have an inlet filter on your carb with a check valve, you will have to figure a way to fill and retain fuel in the carb float bowl so you can see if it leaks down.

Lastly, I would tell you that a carb that leaks raw fuel into the intake can be pretty dangerous. If left long enough, that fuel will evaporate and you won't have any problem. But, if left long enough to drain down but not long enough to dissipate can be the cause of your backfire.

toddalin

Or just maybe your right foot pushed the gas pedal and put some extra fuel in the engine that puddled on bottom of the intake that subsequently back-fired.

With no choke tower, the only way to get my engine to start and warm is with lots of gas. This gas does puddle in the intake and I get backfires through the carb on a regular basis. But the the Holley has never blown out the power valve and always runs fine once warm, so NBD.

Just don't start the car with no air cleaner and the hood closed.

brianPA

This is what I suspect, also.

Yes, I do step on the gas as I start it up, and let off as soon as it fires. Maybe combination of the two, plus hot weather and car already warmed up from driving? I drove it a few times today and it ran fine, like this never happened. I popped open the valve covers today just to be sure and everything looked alright to me.

While I know my carb can be rebuilt I think I am just going to order a new one so I can swap it in and rebuild the old one at my leisure, as I've never taken a carb apart.

So, is there a carb that would be appropriate for my l-48 and also for a zz4 if I make the upgrade soon? Not sure if they even have the same intake design. I know the zz4 turnkey motors ship with a 770cfm carb and they recommend a single plane intake. Thing is, I don't want to change my hood so I'll likely end up with a setup that flows less than zz4's potential and it may not be carb limited. I do have 1 5/8" headers, true dual 2 1/2" w/ magnaflows. I don't really intend to keep adding power just want a bulletproof engine that has more horsepower than my motorcycle

(ignore the air cleaner I just set it on there to keep leaves from falling into the carb)

MotorHead

Yes just disregard my post, flooding is the problem although I never had a engine backfire with pooling in the the intake or flooding, it just won't start.

However if I ever did have a backfire because of this it was through the tailpipes not back up through the carb.

loup68

Gentlemen, I went through hell two years ago with my ZZ-4. I had the HEI dist. with mechanical tach from D.U.I. and did not know where to get the keyed hot to the dist. I disregarded their directions about a full 12 volts and used the normal resister,key wire for about 2 years. I finally found out to use the IGN. on the fuse block on my 68.
After about 10 months, I shut the car off after a drive and a had a small backfire out of one of the exhaust pipes.
The car would not start the next day , but would backfire through the carb. I ended up having it towed to a garage and it took them quite a few days to figure it out. No one had seen a car act like this. I had fried the HEI module and also the cap coil with the resister wire.
Your car , a 75, has the HEI distributer and if it acts up and continues to backfire or will not start, I recommend changing the coil and the module. Lou.

MotorHead

So that's the problem then, he just goes out and buys an new ignition system so it's not pooling in the intake or a lean condition, problem fixed

74modified

I agree, but couldn't he put a timing light on it to rule out several problems? It could have jumped time (timing chain), distributor could be off (moved). It could be your flooding problem, but the flooding could be a result of poor ignition from a faulty HEI module.

toddalin

My theory related to my car:

When the engine is cold, the fuel puddles in the bottom of the manifold. The car experiences a lean condition condition because, as we know, gasoline does not burn in liquid form but only when vaporized.

So the gas (only takes a little bit) sits in the bottom of the manifold getting warmer and vaporizing and the engine actually experiences a lean condition creating the backfire. The backfire instantantly vaporizes what's in the manifold and it too burns creating a nice flame.