Backfiring

[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. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe, causing the backfire.

Any 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. 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 700˚F intermittent. Available in auto parts stores.

[weldingshop]

I greatly appreciate your time and thoughts!

[stryder]

Nice post describing "after fire"; a combustion of unburnt fuel "after" the combustion chamber, heard as a "popping" in the cannister. A "backfire" will send the combustion flames "back" through the throttle bodies/carburetor and is a timing/valve train issue, not a fuel metering issue. I have a BMW K75 that can sound like a cannon when it after fires on trailing throttle, (oddly, these bikes will normally run a tad rich) and memories of singed eyebrows trying to set the timing on my '70 T-bird.

[Chest2Tank]

Well now I'm confused. I'm shooting flames out my pipe which I notice at night. This happens when I'm engine braking. Also, I've noticed that the engine is running unevenly - i.e.: the cylinders are firing like a horse gallop, not symmetrically. What's the problem?

[DukeDesmo]

Originally Posted by Chest2Tank

the cylinders are firing like a horse gallop, not symmetrically

A Ducati V-twin does not fire symmetrically, due to the angle of the crankshaft, the V4 Desmosedici has been made in both 'big-bang' and 'screamer' version.

'Screamer' is smoother, may make more power (think Japanese i4) but 'big-bang' gives better drive and traction out of corners by allowing the tyre to 'recover' between the close-together firing pulses.

Flames out the tailpipe look cool (and race bikes do it) but I'm not sure if it's entirely healthy for the engine if it's excessive.

[stryder]

Originally Posted by Chest2Tank

Well now I'm confused. I'm shooting flames out my pipe which I notice at night. This happens when I'm engine braking. Also, I've noticed that the engine is running unevenly - i.e.: the cylinders are firing like a horse gallop, not symmetrically. What's the problem?

I'm no expert, and maybe somebody who knows better will chime in and correct me if I'm wrong, but it sounds like you have excess/unburnt fuel in your exhaust which is being ignited "after" it's gone through the combustion chamber. When the throttle plates are closed as when gearing down, it increases intake velocity so during the "overlap" stage of the valves' stroke, when both are open for a short period of time on intake, more of the fuel charge being "scavenged" will be pushed out the chamber before the exhaust valves are shut and into the exhaust where it is igniting due to contacting a hot spot, or being mixed with colder air from an exhaust leak. Afterfire is often associated with a "lean" condition re a/f mixture, though too rich can also cause "popping." If it was "backfiring" the combustion would be throwing flames "back" up the intake track, into the air box because the ignition is happening with the intake valves open. My guesses are you have a lean condition and need to "fatten" it up; you have and exhaust leak and are sucking in air, maybe at the header, or your exhaust valves are not closing properly due to timing, shim-ing or carbon build up.

[Chest2Tank]

Originally Posted by stryder

I'm no expert, and maybe somebody who knows better will chime in and correct me if I'm wrong, but it sounds like you have excess/unburnt fuel in your exhaust which is being ignited "after" it's gone through the combustion chamber. When the throttle plates are closed as when gearing down, it increases intake velocity so during the "overlap" stage of the valves' stroke, when both are open for a short period of time on intake, more of the fuel charge being "scavenged" will be pushed out the chamber before the exhaust valves are shut and into the exhaust where it is igniting due to contacting a hot spot, or being mixed with colder air from an exhaust leak. Afterfire is often associated with a "lean" condition re a/f mixture, though too rich can also cause "popping." If it was "backfiring" the combustion would be throwing flames "back" up the intake track, into the air box because the ignition is happening with the intake valves open. My guesses are you have a lean condition and need to "fatten" it up; you have and exhaust leak and are sucking in air, maybe at the header, or your exhaust valves are not closing properly due to timing, shim-ing or carbon build up.

Great.... now what do I do? lol

[stryder]

Any number of things, but I wouldn't think you necessarily have a major problem as after firing is common especially if the bike's cannisters have been swapped for aftermarket ones and no fuel metering adjustments have been done, as they are usually needed. Depending on your model, I'm sure there are others who own the same bike who can advise you if there are any clearly prescribed best tuning practises, like a "chip" or a PCIII etc. You may want to start with the least intrusive method of checking your plugs and seeing if they are blistered, as that would be an indication that it's too hot and that's a lean condition. Yank the fairings and do a visual inspection of the exhaust headers from the head back with the bike off, and then with it running to look/feel/hear any leaks. Spray some wd40 around the headers, joints with the bike running, and if the revs change either way, air's getting in. Take it to a dyno tuner experienced with your model and have it dyno tuned with an exhaust gas analyser. Check shim clearances, timing, TPS. Run some combustion chamber cleaner type additive in your fuel. Sea Foam is said to work wonders. Undertake a leak-down test to check for leaking valves. You may never get totally rid of after firing in all situations after a good tune, buy maybe no flames will come out the back.

[raistlin]

I have tried every type of sealant to seal the exhaust but no luck. is there anything i can use to seal it, besides welding them shut

[Shazaam]

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


http://www.permatex.com/products/aut...sket_Maker.htm