Trochę inna wersja wypowiedzi:
"On the original 1986 Jawa 350-638 press release and sales leaflet it stated that the engine produced 34 HP @ 6,500 RPM. A friend of mine has two 1986 638 bikes in addition to the later 639 and 640 models and he will tell and demonstrate that the 1986 models are a lot faster than those produced after 1988. He has managed 100 MPH / 159 KMH on one in stock tune.The differences in the motors are as follows:Piston skirt openings are 25mm. wider and completely open the 4 transfer ports when the piston is at the top of its stroke (TDC). After 1988 the 25 HP motors only open 2 of the ports cleanly.Exhaust system:The 1986 silencers were taken from the 1985 Jawa 350-634.5-05 final model are longer and have a larger expansion area.The baffle does not have the exhaust gas choker collar restrictor (the 3rd collar without a gasket near the baffle's tail end) allowing the motor to rev higher and harder under load.After 1988 the Exhaust system was shortened, a restrictor collar was added to the baffle so that even when in no gear, with the throttle wide open the motor will never rev higher than 5,500 RPM even with no load.Engine power was restricted to 25 HP (and 23 HP for Holland).The 1986 - 1987 rev counter clock was also altered so that instead of the powerband range being marked on the dial with a yellow line between 5,000 and 6,500 RPM with the redline at 7,000 RPM, the new dial had a green "Economical" zone marked between 3,000 and 4,000 RPM.The early 1986 - 1987 34 HP motors sound different as well and resemble Yamaha RD twins. Later 23 and 25 HP motors sound like all the Jawa 350 motors since 1950.I have converted many later 638, 639 and 640 bike engines to 1986 34 HP rating by either fitting 1986 - 1987 early pistons (hard to find now) or by removing the restricted pistons and widening the skirt openings so that all 4 ports are cleared at TDC.Removing the restrictor choker collar from the Baffle on any 632, 638, 639 and 640 will liberate another 5 - 6 HP. It is only held on by 2 spot welds and seems like Jawa wanted it to be easily removed by owners.The other thing is that the 1986 to 1987 models ran on 33:1 fuel oil ratio which made for a smoother running engine and less centre bearing failures."