Backfire in muffler after fuel stop

[Uthud]

I’ve had several issues in the last couple of years that I’ve posted about.
Thankfully, most of the major ones are solved, at least for now. Fingers crossed that the improvements are permanent.

So this is the last issue remaining:
When restarting a warm engine (like after refilling with fuel), I often get a backfire in the exhaust. Sometimes it is really loud.

Have any of you had this problem, and were you able to fix it?


The bike is a ‘97 FLHTCU, carbureted Electra-Glide Ultra Classic. 136,xxx mi. I am original owner. I have had a butt-ton of stuff done in the last few years. Most recently, I took it to a local tuner, well respected, in part to try to make the backfires stop, as well as to verify carb jetting, etc. The ignition timing is spot-on, leak-down test was 8%, carb was rebuilt, intake and exhaust seals and gaskets replaced and verified no leaks, makes 65 hp on dyno, runs and idles great, doesn’t appear to be running rich or lean.

Things I’m thinking might be contributing:
Supertrapp slip-on mufflers, Arlen Ness Big Sucker air intake and filter. Because of the way the Ness filter handles the vented oily air from the crankcase/heads, I wonder if oil vapors are collecting in the exhaust/ mufflers.
I am also wondering if I need to run closed baffle plates in the SuperTrapps vs. the open end plates that I’ve used for 125,000 miles without a problem (seems unlikely, doesn’t it?) But just maybe.

Any thoughts? Or similar experiences?

 

[Jeff Klarich]

This issue has come up in the past and I believe the general consensus was unburned gas or gasses remaining in the exhaust which ignite at start up.

 

[joel]

I don't know how fuel vapor in the tank is controlled so do some research about that, if there is a high amount of fuel vapor that gets drawn into the intake causing a rich condition that could be a factor in the unburned fuel in the exhaust.
How do to shut down the engine?
Come to a stop and let it idle for a few (5) seconds?
Shut it off while still rolling to a stop?
Come to a stop, give the throttle a little twist then shut the engine off?
A few riders I've been with throttle the engine a bit as they shut it off, some of those experience the same condition.
It the tune is right and you don't have a carb with a high float setting. I would suggest you let the engine idle a few seconds to see if it helps.

 

[Uthud]

Thanks guys. I’m in agreement with y’all that the explosion in the pipe(s)/muffler(s) is residual fuel lingering in there.
I have tried letting the engine idle down before shutting off, and I’ve tried shutting petcock off, then idle before shutting off engine. Neither scenario works all the time.
I’ve even tried slowing the idle speed down at the carb screw.
The tank vents through the gas cap which seems fine. I did have a tuner replace the diaphragm in the petcock, thinking maybe there was extra fuel/vapors drawing in through the vacuum line.
I was hoping that going to single-fire ignition, with new parts (plugs, coil, module, wires, etc) would help. But not; although not a complete waste because it does run smoother.
Why and how does that unburned fuel collect in the exhaust? I dunno.
I’m thinking of just waiting it out for a while.
Weird that it kinda started out of the blue.
The good news is that the machine is running really well.

 

[Breeze3at]

The fuel could be fuel that is expelled from the cylinders in the 1st couple of revolutions of the starter. Plugs don't always fire on the 1st couple of revolutions of the crank, then the first exhaust after cranking that hits the pipe is rich and still ablaze. My carbed Dyna has gotten the attention of bystanders a few times at startup.

 

[Uthud]

@Breeze3at I do think that may be it. I was hoping that the new Dyna ignition and related parts would help get fire sooner.

I also wondered if some unburned fuel occasionally gets exhausted into the pipes when shutting off the engine. Turning off the engine just kills the spark, but the flywheels keep the pistons pumping and the valves operating. So, I could see that maybe an unburned fuel charge could sometimes be pumped out into the pipes. That’s why I was hoping that lowering the idle speed might let the engine stop rotating after the spark is turned off.
It may just be that the high mileage has loosened-up the engine so it doesn’t stop turning like it did.
I am going to listen and try to kill the spark right after power stroke?
I am starting to think that I will just have to live with it. POW!
Thanks for your input.

 

[biscuit]

Could you try shutting the fuel tap and let the engine run dry?
Use ALL the fuel available to it.
Then turn the tap back on when you want to move out.

 

[Uthud]

Could you try shutting the fuel tap and let the engine run dry?
Use ALL the fuel available to it.
Then turn the tap back on when you want to move out.

I’m thinking that would take too long.
But thanks for your input.
G’day, mate.

 

[dolt]

Backfire in the muffler? I am thinking the backfire is in the combustion chamber and presents in the exhaust. Is there a "kick back" associated with the backfire? Spitballing here, since the OP has addressed all other possible sources. At 136K miles, if the head breathers have not been vented to the atmosphere, there is probably a fair amount of carbon build up on the tops of the pistons. A bit of fuel could be ignited by a carbon "hot spot" which is causing the backfire? Maybe running some Seafoam through the motor to "decarbon" the piston tops would eliminate the "hot spot"? Like I said, spitballing.

 

[Uthud]

Backfire in the muffler? I am thinking the backfire is in the combustion chamber and presents in the exhaust. Is there a "kick back" associated with the backfire? Spitballing here, since the OP has addressed all other possible sources. At 136K miles, if the head breathers have not been vented to the atmosphere, there is probably a fair amount of carbon build up on the tops of the pistons. A bit of fuel could be ignited by a carbon "hot spot" which is causing the backfire? Maybe running some Seafoam through the motor to "decarbon" the piston tops would eliminate the "hot spot"? Like I said, spitballing.

Nothing wrong with spitballing. Thanks for your thoughts. Seafoam, you say? It’s worth a shot. I’ve never used it before.

So, one of the other problems I was having, and am hoping is resolved, was that something was causing my jackshaft pinions to bust, taking out the clutch basket ring gear with it, even destroying my starter once. The dude at the starter repair shop said that kickback , bad kickback, may have caused the destruction. Now you have me wondering.