How does a leaky exhaust cause a bike to run poorly?

[Shandley]

Just curious, how does an exhaust leak lead to poor running. It seems to me when you have more back pressure, you keep the exhaust out of the combustion chamber more. Since the valves are run on a cam, I don't see this back pressure keeping the bike from pulling the intake valve open or anything like that. To me, that's where it ends. Can anyone out there help me out with this?

[DK]

I'm no expert on this, but I do know that there is a period during which both the intake AND the exaust valves are partially open at the same time. It is called valve overlap and it is designed into the cam. The idea is to let the shockwave from the exhaust leaving to create additional vacuum for the intake mixture.

If there is a mechanic out there, can you please weigh in on this?

DK

[landon]

Having an exhaust leak in a two cylinder engine will cause one cylinder to create more power than the other cylinder, this can either make the leaking cylinder create more or less power (depending on a number of factors) but what you want is both cylinders to be balanced or you end up with vibration and other problems.

What exactly is the problem you are having?

[xrayr]

All I know is that when my exhaust gasket failed, I had some bad popping from where it attached to the engine. I didn't ask why, I just hurried to fix that thing!

[landon]

Oh that, heat + air + unburned fuel = bang.

[jpr1968]

It leans out the cylinder that has the leak and that imbalance is not good.

[Shandley]

Originally Posted by jpr1968

It leans out the cylinder that has the leak and that imbalance is not good.

So if it is a single cylinder engine, it wont matter if there is a leak or not?

I guess another question would be, why do we need to re-jet the carb when we put on a higher flow exhaust?

[landon]

To really find out if an exhaust leak is causing a loss of horsepower you would need hook the engine up to diagnostic equipment and find out where the peak torque and horsepower occur then try different widths and lengths of exhaust pipes to find out where the engine is happy and where it is not. It may like more back pressure or it may be unhappy and wishes that you would let it breathe where in that case it may actually run better with an exhaust leak.

[ShadowDave]

Originally Posted by Shandley

So if it is a single cylinder engine, it wont matter if there is a leak or not?

I guess another question would be, why do we need to re-jet the carb when we put on a higher flow exhaust?

Generally, for optimal performance, the air/fuel mixture is often approximately 14.7 times the mass of air to fuel. If your bike is running well and then develops an exhaust leak, it will most likely allow more air flow without necessarily more gas, and it will run lean and get less performance. The following may help explain:

http://en.wikipedia.org/wiki/Air-fuel_ratio

Rejetting a carb involves, generally, increasing the amount of fuel going to mix with the increased amount of air that a more free flowing aftermarket exhaust allows.

[Shandley]

Originally Posted by ShadowDave

Generally, for optimal performance, the air/fuel mixture is often approximately 14.7 times the mass of air to fuel. If your bike is running well and then develops an exhaust leak, it will most likely allow more air flow without necessarily more gas, and it will run lean and get less performance. The following may help explain:

http://en.wikipedia.org/wiki/Air-fuel_ratio

Rejetting a carb involves, generally, increasing the amount of fuel going to mix with the increased amount of air that a more free flowing aftermarket exhaust allows.

If re-jetting makes the mix more rich, why not just turn out the mix knob?