Gyrobob
Serious Thumper
Offline
Posers ain't motorcyclists
Posts: 2571
Newnan, GA
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You have an apples and oranges thing going here comparing 650s and 1500s and 125s with and without mufflers.
There are a few basic rules that apply to performance and mileage on motorcycle motors.
The stock setup usually works pretty well. The manufacturers usually spend a lot on getting everything in that motor balanced properly -- cam timing, compression ratio, heat ranges, carb jetting, intake and exhaust resonances, back pressure, air filter flow rate, ignition timing curve, etc. So,.. it you mess with any of it, there is bound to be some ripple effect that will "unbalance" the system.
If you reduce the back pressure by changing to a freer flowing exhaust, it'll make more noise and will run worse,....
unless,...
you richen the mixture just the right amount, and, if you have an older bike with adjustable ignition, you can advance the spark a tiny bit.
(Also, some of the modern computer-operated fuel-injection/ignition system can actually adjust for these kinds of changes, so some of these comments don't apply.)
You'll get more power, and for the same amount of acceleration you used to use, it will require less gas.
Problem is,.. most guys get a big kick out of having a few more horsepower and hearing the tough-guy exhaust sound, so they accelerate harder and lower their mileage.
Oh,.. about your burned valves comment. The valves don't usually burn up if you lower the back pressure, but, unless you richen the mixture, they might run hotter (and burn up) because of the lean mixtures.
To summarize, if you change to a less-restrictive muffler, you'll have less back pressure, more noise,.... and, if you rejet the carb, more performance,.... and, if you use the same gentleness on the throttle as you did before, a little better mileage.
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