hey all, I just completed putting a small 2 stroke on my old bicycle, yes majorly geared down, Im actually rather pleased with it. I used an internally geared 3 speed hub on the rear wheel so i could shift back down to 1st gear after being in 3rd and coasting to a stop (you dont have to have the wheel moving to shift gears, unlike derailers) and basicly welded a 20in wheel rim to the small sprocket off of the hub attached to my 24 in wheel, giving me the reduction i needed. anyway, having completed the drivetrain, i would like to now move into getting the most out of my 23cc 2 stroke engine. after all my reading, I realized that the biggest problem they have is the fact that they waste some fuel during each cycle out of the exhaust port. Racing dirtbike engines use what they call expansion chambers in the exhaust pipe to reflect sound waves back at the engine, and timed right, they push the unused fuel/air mixture back into the engine. this also helps the compression ratio. take a look at the bottom of this linked page if you want a visual http://www.vf750fd.com/blurbs/stroke.html I had 2 ideas about how to tune these engines in a better way. the first is an adjustment to the expansion chamber design. on this page http://www-staff.lboro.ac.uk/~elvpc/bikes.html?http://www-staff.lboro.ac.uk/~elvpc/progs/expch.html you can see that all the pipe bits diameters and lengths are all dependant on the RPM that the engine is wanted to run best at (giving a better bottom, middle or top end power band). my idea is to create the different sections of piping of the expansion chamber to be variable in both their lengths and diameters, and have it vary depending on the rpm of the engine, thus giving you the best timed reflection back at the engine. this could be accomplished much in the same way afterburner nozzles change their diameter, and as far as length goes, have the pipes overlap one inside the other and depending on what the equations give you for a particular section of the chamber, have a spring of a certain spring constant attached between the 2 sections (smaller constants for a length that should change more relative to the other lengths, etc). my second idea may be more of a question, so let me ask it. Why isnt there just a valve on the exhaust port that shuts just before the fresh unburned fuel mixture starts to flow through it? it would prevent the bad gas "mileage" and help control pollution and increase compression ratios as well, and it wouldnt even have to deal with the pressure of the combustion like in a normal 4 stroke engine, so it could be pretty small and light. perhaps a disk valve? anyway please let me get your feedback on these things, thanks.