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Hi,

I've been riding for a few years now. I have a 05 ZX6R at the moment.

I'm hoping someone can explain WHY bikes backfire (usually on roll-off) after you've modded the exhaust (added a slip-on \\ full system \\ race air filter etc..).

Here's what I think I know at the moment (correct in line as appropriate):

1. When you mod you bike with a race exhaust (more free flowing than stock), there is less resistance to the exhaust gasses, so the gasses exit the engine faster.

2. For some reason (help me here), the faster flowing race exhaust causes (more?) unburnt fuel to be pulled into the exhaust, and this ignites causing the backfire.

Depending on who you speak to, adding a race exhaust causes the bike to run either more lean, or more rich. By this, I think (?) what people mean is that the engine is running leaner, but the exhaust richer, because fuel that should be in the piston cylinders, is now being pulled into the exhaust and burnt there instead (hence the backfire).

Is this correct?

My Questions:

A) What I don't understand is why adding a freer flowing exhaust causes more fuel to be pulled into the exhaust. Engine timing is still the same. Valve timing (&overlap) is still the same?....

B) Also, in order to 'cure' backfiring, almost everyone (on an FI bike) adds a power commander and sets it to run richer (to cure the lean engine mixture). The same problem that's causing fuel to be pulled into the exhaust will still be there, so making the mixture richer should make the backfire problem worse?

Yours,

Confused.
 

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Exhaust leaks

The other ingredient that adds to the backfire is a leaking exhaust, air drawn into those leaks can cause the backfire when mixed with the unburnt fuel. This would be caused either by badly fitted or reused exhaust port gaskets (which would also be indicated by a ticking from exhaust gases blowing out) or the system joined together without the use of an exhaust sealing paste. Make sure these are right before you go looking for a more complex/expensive solution :squint
 

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Hi

My 98 zx6r g1 has just started to back fire when i drop off the power! is this doing long term damage? how do i fix it? + It seams to be running a bit better top end. all part are standard.
 

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The base cause of backfire in the exhaust is lean, too lean to burn and the raw gas fumes build up in the exhaust. The raw gas accumulates until there is enough to burn and bang. An air leak will also do it because it will draw air in when the pressure drops negative and cause an artificial lean as well as provide air to burn after the fumes build up. The FI puts a little gas in to keep the motor running at part throttle and this can get messed up with a different exhaust. Most fuel injection these days will cut off the gas altogether when the throttle is shut at speed to prevent issues with unburned gas but after the deceleration is over, gas comes back on and if it's too lean, backfire will occur.
 

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Most bikes have very primitive fuel injection. The maps are not that defined compared to most cars and trucks.

There are "holes" in the maps during some conditions, like throttle position, manifold pressure, engine rpm, etc.

as stated above also lean conditions. most new bikes are tuned so lean for emission compliance it is almost dangerous to the engine.
 

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Having the bike jetted or remapped can sometimes cure backfiring, sometimes not completely, though. The sonic tuning (pipe length, diameter, changes in volume) of an aftermarket exhaust cause different effects on the motor as compared to stock. From the factory, stock machines are set up to run as lean as possible and still produce smooth power. You can't just change one thing, such as the pipe, and expect the machine to still run perfectly. Most of the time, other things have to be changed as well, such as fueling, ignition timing, cam lift duration and timing, port and valve sizes and compression.
 

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Hi,

I've been riding for a few years now. I have a 05 ZX6R at the moment.

I'm hoping someone can explain WHY bikes backfire (usually on roll-off) after you've modded the exhaust (added a slip-on \\ full system \\ race air filter etc..).

Yours,

Confused.
Frankly all of the exhaust notes we hear when aftermarket systems are installed were present with the stock exhaust, yes its true. the exhaust tones become amplified to the next level so to speak with less restrictive, free flowing pipes & canisters. This is not to say a change in the A/F ratio isn't part of the equation when a slip-on or full system is installed because it often times is but it isn't the root of the end result.
 
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