04-16-2003, 01:01 PM
Erm, okay I didn't mean to post that...
To continue, I have had an rs250 for a couple of years now, and during the winter I had a big end bearing go - this sprayed shards of metal into the cylinder... I rebuilt the engine, with a different top end, and took the opportunity to do some flowing/minor porting. Now summer has come I have found the bike will not rev-out in top gear. I can think of several things that might be causing it, and would like a second opinion. Firstly could it be the porting? Nothing too radical here, I ground out the casting marks in the crank casing, and smoothed the area between the reeds and the transfer ports, (roughly as in the RGV kit guide) - I have heard that the volume ratio of the crank to cylinder is critical, could removing this material be causing the problem. I also smoothed the transfer ports where they enter the cylinder (there was a small lip) without altering their angle or timings. The second major mod was to re angle the fixed portion of the power valves so that when fully open there is a smooth exit from the cylinder - the original valve vas a lip about 1.5mm from the top of the port - with this removed it would effectively raise the exhaust port by about 1.5mm, although gas could escape around the side in the original. The last thing is that the exhaust from the lower cylinder seems to have blown a hole in the packing in the expansion chamber - presumably the shards from the big end have made a hole - is this packing critical to the back pressure in the exhaust?
So any opinions on what could be causing it to not rev out? It seems to be okay in first/second gear, and quite happy to pick the front wheel up if starting with a clutch slip in the power band...
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