Popping/backfire on deceleration?

[Ldogg29]

Hello All-

I have a Coleman ct200u with the stock 6.5 honda clone. I just added a filter/intake, weiner pipe/muffler from OldMiniBikes and rejetted the carb with a 36 or 37. I bought both and not sure which one I put in.

Anyhow, it runs great but it is popping or backfiring on deceleration. I know its fairly common and some people will say as long as its running fine you can leave alone but I am not digging it too much. Assuming I do not have any cracks in the pipe, should I rejet lower or higher to try and get rid of or minimize it? Thought about trying a 35 or even a 38 to see what works. I am at 2700 ft elevation if that will help.

Appreciate the help!

 

[ole4]

Most often caused by a air leak in the exhaust.

 

[Ldogg29]

Most often caused by a air leak in the exhaust.

Hmm. The exhaust is brand new and I do not notice any cracks or leaks. Assuming I do not have a leak, wouldnt running to lean or rich create this problem as well?

 

[ole4]

It well may, I just have always found the culprit to be exhaust air leaks but anytime fuel air can collect in exhaust as soon as something lights it off it will backfire.

 

[control4userguy]

Early Honda Civics (CVCC) could blow the muffler apart on command. Shut the ignition off at 45mph and pump the wee-wee out of the throttle then hit the key. Ka-boom!

 

[ole4]

Crank engine over till it is just few past tdc on compression stroke. Now exhaust valve is closed and try blowing inot end of exhaust pipi. You should not be able to blow in it. That will eliminate the exhaust as the culprit.

 

[DeadPixel]

I had this problem on a brigs once as a kid. This was over 20 years ago. My father had rigged a cord to bypass the governor and when you would let off it would pop and bow a flame out the muffler. I did not ride the go kart much as a kid and only had it for a short time but it was new when I got it and only had about 3 hours on it when we hooked up the cord. The only thing I could think was that quickly shutting the throttle at what I would guess was around 5K RPMs created such high vacuum spike that it pulled an excessive amount of fuel through the jet and caused a rich condition. This created one loud POP when I shut the throttle. The motor was totally stock so it could have been a combination of things or like ole4 said we could have had a leak from over revving the motor and blowing a gasket or damaging my muffler. If you see turbo race cars though that blow flames when they shut the throttle like when they go in to corners it's due to a rich condition when they shut the throttle. I assumed this was the same reason my brigs did it. If you have extra jets I would try jetting up and down and see how this effects it.

 

[2SlickNick]

Just out of curiosity are you running the muffler or the open pipe? I have a wiener pipe as well, it does occasionally backfire, and definitely sounds better( as far as less backfire) when I run the muffler with it.

 

[creia]

Early Honda Civics (CVCC) could blow the muffler apart on command. Shut the ignition off at 45mph and pump the wee-wee out of the throttle then hit the key. Ka-boom!

Back in the early 1970'sI used to do that in my dad's 66 Ford Falcon- It made one hell of a bang.
Michael

 

[Ldogg29]

Just out of curiosity are you running the muffler or the open pipe? I have a wiener pipe as well, it does occasionally backfire, and definitely sounds better( as far as less backfire) when I run the muffler with it.

I run it with the muffler and agree on the sound. I will also double check for an air leak but fairly confident ( hopeful ) that isnt the issue since its brand new. It sounds like a may need to experiment with the jets as well.

 

[Raskin]

Back in the early 1970'sI used to do that in my dad's 66 Ford Falcon- It made one hell of a bang.
Michael

I did that alot ford 250 v/8 work van .. had a 4 barrel..don't ask why my employer bought that service van it had 2 gas tanks..i wonder why..lol$$$$ it would make people on the golf course jump..that thing would power brake great cough* :biggrin: i would have been nicer if my employer was good to his help..he was a dog

 

[trinik7597]

What header are you running ?

 

[Raskin]

I always thought was rich condition on decelerate putting unburned fuel in hot exhaust..:shrug:

 

[buckeye]

Back in the early 1970'sI used to do that in my dad's 66 Ford Falcon- It made one hell of a bang.
Michael

I had a old Toyota in High school.
I did the same however, it blew the muffler wide open.
Also shook that car like someone hit me.

 

[DeadPixel]

I always thought was rich condition on decelerate putting unburned fuel in hot exhaust..:shrug:

That't my thought too. Thought your pulling a lot of fuel through jets and shutting throttle either caused a spike in fuel due to the sudden spike in vacuum for a split sec or the fuel that is being pulled had the air cut off before jet could react and mixture went super rich for split sec. Either way I am thinking it is a sudden rich spike. Maybe my thoughts on how it gets rich are wrong but I am leaning on rich mixture.

 

[DeadPixel]

O crap could not be spike in vacuum that't a dumb thought hahaha. The jet is in front of the throttle plate.

 

[Raskin]

Did you replace the emulsion tube also? I think it has less holes than stock

 

[control4userguy]

I did that alot ford 250 v/8 work fan .. had a 4 barrel..don't ask why my employer bought that service van it had 2 gas tanks..i wonder why..lol$$$$ it would make people on the golf course jump..that thing would power brake great cough* :biggrin: i would have been nicer if my employer was good to his help..he was a dog

What a coincidence, I owned a '85 F250 Supercab Triton with dual tanks. Floor it, let off quick on the gas and wait for the explosion.

 

[Ldogg29]

Trinik-I'm running the OldMiniBikes wiener pipe and muffler.

Raskin- I did not replace the emulsion tube but perhaps I should have. I thought the upgraded emulsion would have more holes but apparently not?

Dead Pixel-I was thinking the same thing. I guess I should be replacing the 37 in there now with a 35 jet and upgraded (less holes) emulsion tube.

I enjoy the sound and all but damn is it poppin on decel and I dont want to piss off the neighbors anymore than I already have since I got the bike.

 

[DeadPixel]

I don't know if it will help or not but you could try getting an advanced key for the flywheel. Maybe firing the fuel a little sooner will help lessen the popping. Try putting a 4* advance key in it. You will need the key and the tool to pull the flywheel I believe. These motors have some pretty slow timing stock. I think they only are advanced to 20* and I know a lot of people run around 28* on pump gas. You still have the governor in so I would try a 4* and see if it helps at all. You probably don't need 28* at low RPM.