Back pressure helps with the lower end of the RPM scale to make the bike pull harder. But on the top end near redline you want as little resistance to flow as possible.
This is what the Yamaha EXUP valve does for the bikes that use it. It's a control valve in the exhaust system that closes it down a lot at idle and for the first few thousand revs then opens progressively as the revs climb until probably about 5000 to 6000.
So technically your leaking system will be making the engine run leaner and at the same time making your pull aways from stops more boggy.
And yes you really should fix the system before you do all that engine work. If you don't then you'll need to fine tune the idle mix and needle height all over again when you finally get around to fixing the exhaust.
This is what the Yamaha EXUP valve does for the bikes that use it. It's a control valve in the exhaust system that closes it down a lot at idle and for the first few thousand revs then opens progressively as the revs climb until probably about 5000 to 6000.
So technically your leaking system will be making the engine run leaner and at the same time making your pull aways from stops more boggy.
And yes you really should fix the system before you do all that engine work. If you don't then you'll need to fine tune the idle mix and needle height all over again when you finally get around to fixing the exhaust.
