I HAVE SPENT A LOT OF HRS ON ONE OF THESE SINGLE BARREL MODEL TOROS. IT WONT GO OVER 6500 RPM WITH OR WITHOUT A STOCK MUFFLER. IVE DONE EVERYHTING THAT I CAN THINK OF BUT NO GOOD. THERE NOT ENOUGH MATERIAL OVER THE EXHAUST PORT TO RAISE IT THE .140 TO GET THE TIMING RITE AFTER I LOWER THE JUG .040. I CUT OUT THE FIN OVER THER PORT AND MADE AN ALUMINUM FILLER BLOCK SO THAT WHEN I RAISED THE PORT THERE WOULD BE ENOUGH MATERIAL SO IT DIDINT COME THROUGH. I NOW HAVE .O25 HEADSPACE BETWEEN THE PISTON AND THE HEAD, EX TIMING IS 98 ATDC. I MACHINED THE BOTTOM OF THE PISTON SHIRT TO GET 108 ABDC ON THE INTAKE PORT. I HAVE TRIED 4 DIFFENT CARBS 8MM, 2-9MM, 11MM. ALL HAVE THE SAME PROBLEM. I HAVE TO GET UP HIGH RPM BY HOLDING MY FINGER IN THE VENRURI TO CHOKE IT THROUGH THE LEAN SPOT. TWO OF THESE CARBS ON MY ECHO 23.6 CONVERSION TURN 7600 AND 7800RPM'S. SAME 16-6 PROP ON BOTH ENGINES. IVE CHANGED PLUGS WITH DIFFERENT GAPS, CHANGED MAG COILS. I DONT KNOW IF OF YOU HAVE PUT A TIMING LITE ON ANY OF YOUR ENGINES BUT THESE 2 COILS HAVE TIMING ADVANCE BUILT INTO THEM SOMEHOW. AT LOW SPEED I HAVE 21 DEGREES AND AT HIGH I HAVE 31 DEGREES. I HAVE NO IDEA HOW THIS WORKS IT MUST BE SOME KIND OF MAGIC. IF IVE MISSED THE BOAT HERE SOMEWHERE GIVE ME SOME IDEAS. THIS THING SHOULD BE PERFECT BUT ITS REAL TERD ,[&o]THE SMALLER ECHO THAT I HAVENT DONT THE COMPRESSION OR THE PISTON INTAKE MACHINE WORK YET PUTS IT TO SHAME. THIS TORO TURNED 6500RPM STOCK AND ITS STILL TURNS 6500 WITH ALL THE HI TECH MODS. ONLY THING I HAVENT CHANGED IS THE FLYWHEEL, MAYBE WEAK MAGNETS?? I GOT 800 TO 1000RPM WITH ONLY A CARB CHANGE AND RAISING THE EXAUST PORT .100 ON MY ECHO! THE OTHER TORO THREAD HAS GOTTEN TO LONG SO I THOUGHT IT WAS TIME TO START A NEW ONE