|
Ports and cams.
Ports are easy to modify. But they are not easy to modify correctly. Ideally, the port area should open up from the carburetor exit to the valve pocket area at around a 2 degree angle. Because we want to maintain a cohesive gas flow right on through to the valve seat, and beyond. I think this is important because as a dual sport, we want the broadest possible power band. A short hand version of this is we want higher BMEP right from 2500 rpm to 7750, or so rpm. An engine that comes on the cams at 7000 rpm, and signs off at 8000 rpm might work on a road racer, but is hard to ride off road. A 28 mm carb (1 3/32 inch) is mucho carb for a 250, but it should do the job ok with a very carefull porting job. May need a square slide carb to get enough action coming off a closed throttle. A round slide would not be as "sudden", so that's the way I would go. Needless to say, the exhaust side has to be opened up to get the best possible "blow down" in order to get even better cylinder filling. It is easy to over cam an engine to where all the power is at the top end, and low end is not as good as stock. And if the low end is soggy, too much clutch fanning, which overheats not just the clutch, but the oil in this type of engine, will be required. Like certain over-tuned two strokes in days of yore. Unlike a Bultaco Matador, the sweetest running two stroke I ever rode, off road....ARH
|