Backfiring when lean, not rich?

[Lukey]

Okay, what's going on with my bike? It smells of gasoline when running and backfires regularly with 8"-long flames.  At a recent service, I was told the plugs look like it's running lean.  I thought the backfiring and gas smell indicate running RICH?

FYI: Cat converter is gone; it runs Arrow exhausts. Never had a dyno tune.

[extra330]

You could also have an exhaust leak causing the backfire issue.

[Shazaam!]

Backfiring is the sound of fuel being burned in the muffler. To happen, you need unburned fuel, an ignition source (hot metal, exhaust gases) and air. When you quickly chop the throttle and decellerate you send the greatest amount of unburned fuel to the exhaust and this is when the condition is the worst.

An unburned fuel condition is usually caused by a lean condition. What happens is that a lean mixture will fail to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust pipes. This is why you smell gas in the exhaust. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe, causing the backfire.

Any ECU, EPROM chip or Power Commander fuel map that is not well-matched to the exhaust system can produce an air/fuel mixture that results in more unburned fuel being sent through the exhaust pipe and muffler. So a chip change alone can cause backfiring, especially during closed-throttle deceleration.

The best approach is to put the exhaust cans on and have your dealer re-adjust the CO level to compensate for any increased airflow.  Om earlier bikes there's a CO trimmer screw adjustment on some ECUs that provides for limited changes in fuel mixture at idle (with lesser effects across the RPM range). Go easy, a half-turn on the screw is usually all that’s needed. Any more may decrease drivability.

Always a good measure of fuel mixture is to check the color of the inside of the tailpipe. After a few hundred miles it should be medium-to-dark gray, not black or sooty.

Slip-ons as a rule won’t change air flow very much. The most common situation is that a newly-installed exhaust pipe or slip-on is not fitting properly. Aftermarket pipes are not a precision fit and often they let in combustion air that, when mixed with unburned fuel, results in the fuel being burned in the muffler. If this is the case, use a high temperature silicon sealant at the joint to the slip-on to keep the air out.

Full length, larger diameter exhaust systems are designed to flow better so more air through the valves without added fuel will usually give you a lean condition. You need to check the air/fuel ratio on a dyno to be sure.

I recommend Permatex Ultra-Copper high temp RTV silicone gasket maker #101BR for the aftermarket slip-on system joints. Good to 700ËšF intermittent. Available in auto parts stores.




http://www.permatex.com/products/automotive/automotive_gasketing/gasket_makers/auto_Permatex_Ultra_Copper_Maximum_Temperature_RTV_Silicone_Gasket_Maker.htm

Full length, larger diameter exhaust systems are designed to flow better so more air through the valves without added fuel will usually give you a lean condition. You need to check the air/fuel ratio on a dyno to be sure.

[clubhousemotorsports]

+1
if you opened up your airbox as well expect the bike to be even leaner and it will backfire in the intake as well.

If you have no exhaust leaks and have an open airbox close it up and try it. You have a ECU that can be richened with the proper software, it is an across the board adjustment so sometimes it does not fuel 100% everywhere but for some it works very well.
Monstermash on this board can re-program your ECU to a performance map you might contact him if you need this done.

I would start out with a basic fuel injection set-up.
ega check the fuel
synchronise the throttle bodies
set the air bypass adjusters
set the fuel trim (CO)

I bet you will be fine

something else to think about if this started all of a sudden the ECU will trim full rich if it has a failure , I have changed plenty of these ECU's.

good luck

[Lukey]

 

Man, you guys are helpful! I never knew half that stuff! And here I thought it was simple...

This has been going on for a long time, and I do have a fitment issue with my exhaust, so I will try the high-temp sealant.  My only concern is that the poor fit is at the spot where the manifold collectors fit into the midpipe...won't the temperature be too high there?

[clubhousemotorsports]

Some of the sealants will hold up for a while, At this point it makes for a good experiment at least. seal it up and see if it fixes the problem.  It never hurts to fix known issues.

good luck