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Carb backfire- with flame.

Posted By: Damned67

Carb backfire- with flame. - 05/24/12 09:08 AM

Right. Been having a few issues with backfiring through the carb, with flame. Not all the time, not every start, but enough to scare me at times.
More info. Pretty much always has a hard time starting when, and only when, hot. Basically, hold loud pedal to floor and crank.

Just checked firing order.
Threw a timing light on it the weekend. Was at 20 BTDC, retimed to 16 BTDC (next start just about set the car on fire).
What's next on the list?


Posted By: 340SHORTY

Re: Carb backfire- with flame. - 05/24/12 10:11 AM

Is this problem on your go cart ??
Posted By: Damned67

Re: Carb backfire- with flame. - 05/24/12 10:15 AM

Engine is a E-headed, 10.4:1 505", 0.587/0.593 287/294 cam, M1 intake and 1050 Dominator.
Posted By: 340SHORTY

Re: Carb backfire- with flame. - 05/24/12 10:50 AM

In my experience backfiring is caused by 2 things.
A lean condition from off idle caused by to lean of a squirt from the acc pump or to small of a squirter.


I had that problem on my old 383 when I put a new 750 Holley on it. I cured it with a little acc pump arm adj and the next size squirters.

A cam with a bad exhaust lobe can cause it also. With the valve not opening all the way the hot charge doesnt get pushed out and when the new fuel/air charge hits it there is a backfire. The backfiring is constant tho ans increases as the RPM increases.

On the hard starting after warmed up Id say you have a heat soak problem. Try a thicker carb to manifold gasket or maybe a thin phenolic resin spacer. .
Posted By: Exit1965

Re: Carb backfire- with flame. - 05/24/12 01:22 PM

I just went through this after replacing my cam.. put my distributor back in "where it was" and first crank, backfire and flames through the carb which lasted for a few seconds while I got my extinguisher out.

Anyways in my case the timing was very retarded. Turned the distributor clockwise to advance, got it started. When I got the light on it, it was still way far retarded. After that I got it to 20 btdc, the idle sped up a lot allowing me to turn down the set screw, and all is well.

Has your engine ever ran right or did this just start? How does it run once started? Crossed wires would be pretty obvious when its running.

If I were you I'd advance the timing more.
Posted By: Damned67

Re: Carb backfire- with flame. - 05/24/12 09:47 PM

No, hasn't always been like this.
Seems to run OK, although I've been running quite rich lately.
If my timing is out, that could effect the A/F...

I did swap balancers 6-12 months ago. The timing shouldn't have changed, but I guess I should verify TDC.
Posted By: terzmo

Re: Carb backfire- with flame. - 05/24/12 10:00 PM

have you looked at float levels...sometimes a float gets a hole and fils with fluid...sounds like carb is flooding..especially when hard to start when hot

once power valves are shot, they will also cause this backfiring
Posted By: Damned67

Re: Carb backfire- with flame. - 05/24/12 10:08 PM

Quote:

have you looked at float levels...sometimes a float gets a hole and fils with fluid...sounds like carb is flooding..especially when hard to start when hot

once power valves are shot, they will also cause this backfiring



I'm actually picking up a rebuild kit today, as I do feel that the carb is flooding (while hot). Float levels have been checked and needle and seats replaced. Also power valve was replaced too.

I just wasn't sure I could get a carb fire just from flooding, as it needs an ignition source, which would presumably come from screwed up timing???
Posted By: maximum entropy

Re: Carb backfire- with flame. - 05/24/12 11:13 PM

verify timing mark accuracy, verify cam timing.
Posted By: Jeremiah

Re: Carb backfire- with flame. - 05/24/12 11:20 PM

Quote:

verify timing mark accuracy, verify cam timing.



Trust no part, especially the new ones.
Posted By: mikemee1331

Re: Carb backfire- with flame. - 05/25/12 12:43 AM

chain condition?
Posted By: CUDA8U

Re: Carb backfire- with flame. - 05/25/12 01:25 AM

mine did this with the new 850 holley, I just advanced the timing a couple degrees and it fired up easily...of course you will have to shoot it with a light after to your full advanced timing them adjust carb
Posted By: Damned67

Re: Carb backfire- with flame. - 05/25/12 01:57 AM

More (worthless?) info:
Engine is now probably about 4 years old, maybe 3? Less than 500 miles.
Assembled by a pro shop that specializes in Mopar and race engines.

Problem is that, since the build, the car sits for 6 months at a time. Although, I've been trying to rectify that, though fighting the issues is getting it close to being mothballed!

The timing was originally set, and unchanged (until the weekend).
The balancer was a budget brand, known to have failures, so I replaced it. Unless the chain slipped while replacing the balancer, the timing should be unchanged. Indeed, the weekend was the first time I put the timing light on the damn thing. It was showing 20 BTDC, so I changed it to 16 BTDC to see if that helped.

If I've got time this weekend, I'm rebuilding the carb and hopefully toss in a piston stop to verify TDC.
Posted By: RapidRobert

Re: Carb backfire- with flame. - 05/25/12 02:38 AM

I'd set the timing marks at 10BTDC on #1 compression then set the dist so the magnet is dead even with the reluctor and the rotor under #1 plug wire & fire it up & time it
Posted By: SixDRacing

Re: Carb backfire- with flame. - 05/25/12 03:53 AM

test fuel condition. six month old fuel will not burn correctly. will cause backfire and rich condition.
Posted By: Exit1965

Re: Carb backfire- with flame. - 05/25/12 01:45 PM

Quote:

I'd set the timing marks at 10BTDC on #1 compression then set the dist so the magnet is dead even with the reluctor and the rotor under #1 plug wire & fire it up & time it



Have a fire extinguisher ready. He said he already tried retarding it once and almost burned it down.

My $$$ is on advancing it.
Posted By: Cab_Burge

Re: Carb backfire- with flame. - 05/25/12 05:01 PM

Quote:

test fuel condition. six month old fuel will not burn correctly. will cause backfire and rich condition.

Drain the tank and buy new gas and go from there Let us know what you find with this
Posted By: dm69charger

Re: Carb backfire- with flame. - 05/25/12 06:28 PM

Quote:

Quote:

test fuel condition. six month old fuel will not burn correctly. will cause backfire and rich condition.

Drain the tank and buy new gas and go from there Let us know what you find with this



Had this very problem. Replaced with fresh gas and not a problem since.
Posted By: Damned67

Re: Carb backfire- with flame. - 05/25/12 10:33 PM

The fuel 'should' be pretty fresh, as I've been putting new fuel in it. I should, however, drop it to see just how much fuel is in the tank to get an idea of how much old crap is in there.

Will old fuel still backfire through the carb with a flame?
Posted By: Cab_Burge

Re: Carb backfire- with flame. - 05/25/12 11:05 PM

Quote:

The fuel 'should' be pretty fresh, as I've been putting new fuel in it. I should, however, drop it to see just how much fuel is in the tank to get an idea of how much old crap is in there.

Will old fuel still backfire through the carb with a flame?

To mimick Sara P, You Betcha , big boy Pump gas has a shelf life of less than 90 days I have a local customer who has a pump gas Hemi motor I built for him, he keeps the car inside all winter, it has a big 34 gallon feul cell that he keeps half full or more all the time. He called me complaing that the motor wouldn't start after sitting all winter, I took a gallon of race gas over and got it started by pouring a little race gas down the carb. several times until it got some heat in the motor, it would set your hair on fire if you where near the top of the carb. when it wanted to die when he tried to rev it up some more Drain the tank and put it in Moms car with some of the local new fuel in her car, or your pickup EFI and modern system will tolerate thet old fuel better than our cars do
Posted By: Damned67

Re: Carb backfire- with flame. - 05/27/12 06:05 AM

Right, dropped and emptied the fuel tank.
Also removed the carb for a refresh. During the process, I because less than confident of one of my floats. So it's all back together, but I've decided to wait until I replace the float during the week before I fire it up.
Posted By: 70HemiGTX

Re: Carb backfire- with flame. - 05/27/12 12:15 PM

The only times I have ever had this problem is if I pulled the distributor out and "put it back in exactly the same position as it was before". Yea right. I always had the timing retarded too far. Usually bump it up and it will be fine. Never had to drain the tank of fuel and put fresh in it. Yes it does burn differently when old, but I never had a problem with it.
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