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I've changed to an aftermarket freer flowing exhaust system on a previous bike. I was told I might need to rejet. I really didn't notice a difference in performance in that bike at that time.

So my question is: How does changing the exhaust alter the fuel/air mix?

Now changing the air filter the effects are apparent. Same with the jets. I can see restricting the exhaust too much could just plain ol' cause the engine not to run due to back pressure, but more freer flowing?

My 4-2 exhaust (mufflers specifically) is rusted out on my Honda and I want to put some simple aftermarket mufflers on. I don't want a nightmare of having to change the whole damn setup.

All engines use a bit of back pressure to keep the fuel in the combustion chamber. Back pressure is the pressure produced when you try and force 600 cc's of gas out through a small pipe with a blockage in it. Basically it can only force so much out so fast. By reducing the blockage in the pipes you allow the gas to escape faster.

Someone will come along and explain it better for me.