Backfire from 360

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rtee007

rtee007

Deplorable Texan
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My 360 (390hp) MP crate (4K all new parts) just developed a backfire out of the pipes on deceleration. R/S only and it is inconsistent. Sounds like a dammed pistol going off sometimes. Usually only on higher rpm decell. What would you look at first? Has MSD ignition system, headers, Eddie dual feed carb.....and plenty of voltage.....engine was running just fine. Haven't changed anything.
 
It's probably in the timing.

Did it backfire out of the carb or exhaust?

If it backfires out the exhaust, your timing is too far advanced.

If it backfires out the carb, your timing is too far retarded.
 
Bad plug, plug wire or cracked distributor cap.
 
krazykuda said:
It's probably in the timing.

Did it backfire out of the carb or exhaust?

If it backfires out the exhaust, your timing is too far advanced.

If it backfires out the carb, your timing is too far retarded.
Click to expand...

I always thought it would be the other way around. Too far advanced and it ignites while the intake valve is still open, too far retarded and combustion happens after the exhause valve has opened.

I was never taught this though, just sort of how I always figured it.

Why is it the other way around?
 
Check your plugs. Mine was hesitating and back firing. My rotor to cap contact was shot and all of my plugs fowled out.
 
Will check it all out, motor only has 4K and all parts are new as well. Backfire is periodic. Out of the r/s exhaust only.
 
My educated guess would be timing chain. Throw a timing chain tensioner on er
 
After you go through your ignition, check how the carb is running. On decel, the heavy vacuum draw runs the engine rich. The extra fuel exscaping could be ignited from the other cylinders exhaust in the exhaust tube.
 
It might be the header gasket on that side. I just went through this about a year ago...back firing on deceleration. I replaced the header gaskets, problem solved.

Edit: Got tree'ed by Daves69 by a few seconds, lol!
 
Header gasket, you know it does sound to me like the r/s is different than the left side.
Listen and tell me what you think.......
[ame]http://vid659.photobucket.com/albums/uu317/ubinsniped/MVI_1079_zpsc47029e4.mp4[/ame]
 
It does seem to have a different tone to it.

Get a piece of fuel line/hose about 3 feet long and hold one end to your ear and the other end move around the suspect side (think stethoscope). I would also listen to the other side for comparison.

Side note: NICE car!
 
Thank you for liking the car Bad Sport. I get lots of looks and plenty of thumbs up.

OK, here is what I have been able to do so far.......

Header gaskets...Good, but the r/s collector gasket does have a leak and I will replace that.

Timing chain....I would put that last on the list for now since the motor is an MP crate and has under 5K on it and it has been driven pretty gentle so far. I believe it is the 390 HP design.

Spark plug wire(s). MSD 8mm. I did find one that was loose. Had to crimp the clip on the distributor end a little to get it to snap on. Still have backfire periodically. All others were tight, cap and spark plug ends.

Distributor cap. Looks good, also brand new MSD part. Don't see any cracks.

Timing. I think it could use a little more timing, but I have no way to measure it here at home. I have done a little reading and found that around 38 degrees seems to be a good spot for this engine. I have no idea what mine it timed at currently.

Carburetor, it is an Edelbrock 650(?) it has dual feeds. I am not much on how to adjust or diagnose fuel induction problems. I do not have a way to measure vacuum either. Willing to try and learn through.

Racked my brain for something that has changed, the only thing I have done was install a MP air cleaner set up, removed the Edelbrock air cleaner, it was the short 14" x 2"(?) filter.

Header gaskets are good, but the collector gasket does have a leak and I will replace that.
Bad Sport said:
It does seem to have a different tone to it.


Get a piece of fuel line/hose about 3 feet long and hold one end to your ear and the other end move around the suspect side (think stethoscope). I would also listen to the other side for comparison.

Side note: NICE car!
Click to expand...
 
I've always gone to the intake gasket first. Close your throttle plates at 5k and it'll suck air in through any and all leaks be it the intake or carb gasket. Then it ignites once the air fuel ratio
gets to its combustible range. Usually in the muffler.
 
I was the connection at the collector, I have shorty headers and two of the three bolts were missing. Replaced the entire exhaust system and ran it all the way out the back, work like a MoPar should now, Thanks all for the help!

Leadfoot said:
I've always gone to the intake gasket first. Close your throttle plates at 5k and it'll suck air in through any and all leaks be it the intake or carb gasket. Then it ignites once the air fuel ratio gets to its combustible range. Usually in the muffler.
Click to expand...
 
My Mazda has a hard line that goes right into the exhaust manifold, and it opens to fresh air on deceleration; controlled by an "anti-backfire valve" so it burns off in manifold (air injection same thing) and not explodes in muffler. This valve goes back to the 60's in AMC cars. A leaking collector/muffler will allow oxygen into exhaust far enough away from manifold and will light off fuel by muffler, just as explained.
 
Leadfoot said:
I've always gone to the intake gasket first. Close your throttle plates at 5k and it'll suck air in through any and all leaks be it the intake or carb gasket. Then it ignites once the air fuel ratio
gets to its combustible range. Usually in the muffler.
Click to expand...
Kudos, done this myself. Loose collector gaskets,backing off a tuning run.
 
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