On fuel injected bikes, the backfiring is caused by what is called "fuel cut". Above a certain RPM when you let off the throttle fuel is completely cut to the engine. This is done for EPA reasons. When the exhaust leaves the cylinder, it is still burning air and fuel as it passes through the exhaust. When fuel is cut, it causes the exhaust burn to lean out and pop or backfire to to the leaner conditions.
Most people end up with some sort of fuel control or FMS. These however don't stop the fuel cut. All a FMS does is tell the fuel injector to stay open longer than the ECU tells it too, this in turns enriches the air fuel mixture so that it is a richer exhaust burn so when fuel cut occurs it is not so lean, but it is not perfect and you will have the same backfire and popping issues just not as often.
Turning fuel cut off in the ECU is the only way to stop this. And the only way to do that is have the ECU re-flashed, and one guy does this and it has other big benefits.
Ivan's Performance Products
Here is a write up I did a while back on ECU and FMS units.
Differences between ECU and Tuners.
The ECU is the brain of the engine, it controls fuel, ignition, and monitors and has diagnostics. For now, let’s look at fuel and ignition.
Fuel maps are developed for all throttle positions, RPM, engine load, mixture setting requirements, air intake, etc. These maps are setup based on how the engine was intended to operate.
Ignition Timing is also controlled based on engine load RPM, throttle position and other variable.
The ECU is programmed based on requirements set buy the manufacture. With today’s EPA standards, the mapping is setup for emissions, and economy. Since motorcycles don’t have full blown emissions systems like cars. This makes the performance and readability undesirable. Also to make things worse, motorcycle ECU’s are limited on the real time adjustments they can make, so they cannot adjust adequately for modified intake and exhaust systems. This is where the tuners come in.
Tuners: Tuners were developed for 2 reasons, to try and smooth out the readability issues with stock ECU programming, and to enrichen the A/F ratio when adding air and exhaust. The simple tuners like Cobra and EJK (no maps) simply add fuel. The ECU normally sends a voltage to the injector to open it. This is typically called the injector pulse. Tuners are placed between the ECU ad injectors. When the ECU commands a “pulse” the Tuner tells the injector to stay open longer thus enriching the mixture. The tuner is simply increasing the injector pulse longer based on the ECU ratio. This helps immensely, but is not optimal because the tuners is simply following the ECU mapping but with a richer mixture. One big misconception: ECU units do not see the tuners or know they are even there. That is because with the fuel tuners, with O2 connections, the O2 connection on the tuner Lies to ECU into thinking that the lean mixture setting is still present. This prevents the ECU from trying to constantly lean the bike out when the tuners add fuel. That is why these tuners work better with O2 connections, to keep the ECU from trying to fix the richer setting.
Jacks O2 mod: Jacks O2 mod simply lies to the ECU and tells it that the mixture is too lean, the ECU responds and very slightly enriches the mixture, it helps but is far from optimal. But works for most who are on a budget. But this only works in the 500 to 4000 RPM range, which is where V Twins pretty much operate. But even though it is richer, it is still a lean mixture.
Ignition Timing directly effects power. Ignition timing refers to when the spark plugs are ignited. In engine 101 class we learn that the spark plug ignites the A/F mixture at the top of the compression stroke (TDC) top dead center. At very low RPMs it is acceptable to fire the spark plug fairly close to TDC. As RPM increases the piston actually is moving fast enough that it is running away from the combustion of the A/F mixture. This means when max cylinder pressure is achieved, the piston has already moved to the bottom of the travel therefore has nowhere else to go but up. This results in a loss of power. So as RPM is increased ignition timing is increased so the spark plug fires before TDC igniting the A/F mixture prior to TDC, of course this means the piston is working against the A/F combustion, but the fuel burn is not instant, it actually takes time. This means the piston still has a way to go down when max cylinder pressure is reached taking advantage of the pressure so that it is still pushing the piston down.
If the timing is advanced too much, this means max pressure will occur when the piston is fairly still high in travel meaning the area for combustion is small. This results in intense heat and then leads to the A/F mixture exploding (detonation), not burning. This results in high cylinder head and piston temperatures which results in engine damage. So there is a range of timing settings that are acceptable.
On our stock bikes the timing does advance as commanded by the ECU, but it is not advanced enough for power, instead it is a compromise to be delayed enough that the A/F mixture is still burning during the exhaust stroke, this is intended to keep the exhaust temps high to keep the catalyst converter hot so it works effectively.
So on our stock bikes, the A/F ratios, fuel mapping and ignition timing are not at all optimized for performance and ride-ability. Instead they are optimized for emissions.
So Ivan’s ECU flash corrects all the parameters that regulate fuel mapping and ignition timing. But since our ECU units are limited on sensor data, they cannot recognize open air or exhaust, so that is why we still need a fuel tuner if we want to maximize performance. But since the tuners follow the ECU, a optimized ECU makes all the difference.
