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Carb backfire- with flame.
#1238717
05/24/12 05:08 AM
05/24/12 05:08 AM
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Joined: Jan 2003
Posts: 2,309
Tomorrow.
Damned67
OP
top fuel
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OP
top fuel
Joined: Jan 2003
Posts: 2,309
Tomorrow.
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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? 
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238718
05/24/12 06:11 AM
05/24/12 06:11 AM
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Joined: Aug 2003
Posts: 12,291
Kent, Wa
340SHORTY
Truck Nut
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Truck Nut
Joined: Aug 2003
Posts: 12,291
Kent, Wa
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Is this problem on your go cart ??
I am truckless..
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238720
05/24/12 06:50 AM
05/24/12 06:50 AM
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Joined: Aug 2003
Posts: 12,291
Kent, Wa
340SHORTY
Truck Nut
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Truck Nut
Joined: Aug 2003
Posts: 12,291
Kent, Wa
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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. .
I am truckless..
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238723
05/24/12 06:00 PM
05/24/12 06:00 PM
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Joined: Mar 2005
Posts: 2,060
Pendleton NY
terzmo
top fuel
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top fuel
Joined: Mar 2005
Posts: 2,060
Pendleton NY
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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
Last edited by terzmo; 05/24/12 06:01 PM.
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Re: Carb backfire- with flame.
[Re: terzmo]
#1238724
05/24/12 06:08 PM
05/24/12 06:08 PM
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Joined: Jan 2003
Posts: 2,309
Tomorrow.
Damned67
OP
top fuel
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OP
top fuel
Joined: Jan 2003
Posts: 2,309
Tomorrow.
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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???
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238725
05/24/12 07:13 PM
05/24/12 07:13 PM
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Joined: Feb 2007
Posts: 5,080
organ
maximum entropy
master
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master
Joined: Feb 2007
Posts: 5,080
organ
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verify timing mark accuracy, verify cam timing.
for what is the good life if not doing things thoughtfully?
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Re: Carb backfire- with flame.
[Re: maximum entropy]
#1238729
05/24/12 09:57 PM
05/24/12 09:57 PM
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Joined: Jan 2003
Posts: 2,309
Tomorrow.
Damned67
OP
top fuel
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OP
top fuel
Joined: Jan 2003
Posts: 2,309
Tomorrow.
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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.
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238730
05/24/12 10:38 PM
05/24/12 10:38 PM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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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
live every 24 hour block of time like it's your last day on earth
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Re: Carb backfire- with flame.
[Re: RapidRobert]
#1238732
05/25/12 09:45 AM
05/25/12 09:45 AM
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Joined: Jul 2003
Posts: 3,378
Rancho Cordova, CA
Exit1965
master
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master
Joined: Jul 2003
Posts: 3,378
Rancho Cordova, CA
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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.
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Re: Carb backfire- with flame.
[Re: Cab_Burge]
#1238734
05/25/12 02:28 PM
05/25/12 02:28 PM
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Joined: Dec 2006
Posts: 388
Southington, Ohio
dm69charger
enthusiast
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enthusiast
Joined: Dec 2006
Posts: 388
Southington, Ohio
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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. 
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Re: Carb backfire- with flame.
[Re: Damned67]
#1238736
05/25/12 07:05 PM
05/25/12 07:05 PM
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Joined: Aug 2003
Posts: 43,228
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,228
Bend,OR USA
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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
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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