Removing the end cap may add a couple MPH to the top end but thats about it normally, but one thing I do know for a fact...... is that if you will take the stock muffler and enlarge the port on the flange so that it is a closely matched as possible to the exhaust port on the cylinder/jug and then remove any baffling from inside the muffler and drill one or two extra holes in the end cap with a 3/8'' drill bit, you will see the same 1-2 maybe 3 MPH increase in top speed but the big difference you will notice is the hill climbing power, it will be the most noticeable change you will realize, stock exhaust really choke these engines down and kill the torque.
Now all this said, I agree with Tom if you live in the city or what we call live in town around here, you should try to keep a low profile and not run a wide open exhaust, it could get you unwanted attention from the fuzz and create a bad outlook on motorized bikes in your area, and that is bad for all of us....
the things I described doing are only good if you also have your carb jetted correctly, a very rich running carb will hurt performance no matter what else you do and in my opinion stay away from the pooper scooper pipes.... they will not add any performance... go with an expansion exhaust like the SBP Sick Bike Parts pipe if you want a fairly simple bolt on that will keep the noise down and stilll add performance.
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