Hi All,
The Background Story:
When I first got the IT400C it was running a reasonably fresh 0.25 Yamaha and rings. The bike seemed to be running well apart from the motor sounding a bit mechanically noisy.
After pulling it apart, I discovered that the barrel was within tolerances for 1st oversize. At the upper end of tolerance, but still within tolerance. The rings were sealing properly, with no blow past, and correct end gap, but the Yamaha piston that was marked 0.25 was actually undersized and out of round??
As CD6 was fast approaching, and I had no time to source a new Yamaha piston kit, (and not wanting to ruin what was still a good 0.25 barrel), I took my spare worn out barrel down to have re-bore to a new 1.00 Wiseco piston.
The Question/Problem:
With the original barrel/piston combo, the jetting seemed spot on, with no hesitancy anywhere in the rev range. It pulled well, and revved well.
With the new barrel/re-bore/1.00 Wiseco piston, it pulls well down low, but doesn't want to rev out quite as well as before. And it seems to have a hesitancy in the upper mid-range? It doesn't feel right at 1/2 to 3/4 throttle. It's not so bad when you start from low down and pull all the way through the rev range in one gear. But it is especially noticeable when you go down one gear more than usual into a corner. When it's running a constant 1/2 throttle it feels very hesitant.
I can, and will eventually get round to fiddling with the jetting, but my question is this. Has anyone come across this sort of thing before? What causes it? I suppose I'm trying to understand why a seemingly well jetted bike now seems poorly jetted...
Thanks,
Tony