Confusion over "back pressure" and Wave harmonics. Look at a 2 stroke pipe for an explanation. No back pressure. Standing pressure wave caused by the tapered section, sends a shock wave back to stop incoming charge escaping the combustion chamber. Now look at those old fashioned tapered mufflers on Brit racing bikes. Same thing. What does matter? The timing is critical. Only works at a narrow RPM range and is dispensed with on a Dragster for the simple fact that it runs at a very limited RPM range and pipe length does that for them. So no back pressure. Why is the idle unstable? Something is variable. Air fuel mix(vacuum leak) or, something is moving when it should not, or holding still when it should. Stuck power piston, and loose throttle shaft come to mind. OK now Panic will hit me where I am wrong.
