No harm at all, it is useful. This is wide used on two strokes machines, where gas is contaminated with oil, lowering is combusting properties. Of course, with todays high tension coils, electronic ignition systems and platinum point spark plugs there is no need of it. So if you use a Boyer Bransden or so, forget about it....if you have an original Lucas magneto it will help for sure....
Now...
Back to the topic.
Yesterday I made my final test..
I reduced the end of the exhaust pipe...the result was a killer DB with a kind of whistling sound..SO, I started testing lengh from let say 30 inches of overall lenght until I get the rear axle and BUM....there is a point where the rapping is very loud and deep...
My conclusion..
Depending on your compresion, and cam and carb...there is a moment where unburned gasoline is trapped between the head and the first part of the exhaust pipe...the correct lenght of the pipe will make a return source of fresh oxigen reach this "dust of gas" that is actually very hot...So combustible+oxidizing+temperature BOOM, an explotion happens in the pipes and is amplified by the pipe itself.
So I am very happy with the result. Now I can ride smiling with my BSA fellow!!!
Thank you so much for your support.
All the best
P.D Triton Trasher, you nailed it!!! Thank you so much