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 earlier bike's 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. A Mathesis is needed to set up a 749 CO.

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.

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/pro...tem_no =81878

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.

Whoah! Im glad we have Shazaam! on board!
What is a Mathesis? Am I risking potential serious damage by continuing to ride the bike in its current state? The backfiring isnt terrible, its only on closed throttle deceleration at very low speeds, but its the kind of attention I could do without!
Is adjusting the CO on the 749 a dealer only job?

Forgot to say....the Termis sound unbelievable! :lol:

My approach would to check for air leaks at fittings first. I'd also take the Power Commander out of the loop and see if things improve. The fueling won't be that far off just using the stock ECU.

[Edited on 1-13-2005 by Shazaam!]

Air leak, almost certainly.

I'll check the seals in the morning, thanks for all the advice lads.

Is it Ok to ride it though? Im not gonna do any damage am I? :o

Only to your ears :P

Needs to be trimed in on a dyno for the best results.
I use Italian motorcycles, Crediton, devon tho they have just changed name to ADH services.
They have a dyno and do a good job, they do three crosses beneli race bikes :cool:

They have just made a hon hon hon cb 1300 fire like a twin by making and fitting new cams etc sounds bloody awsome :cool:

Cool. Before I check in the morning, how many bungs are there in the pipes?