Finally happy with 205cc TY175 motor

[David Lahey]

Since the big-bore job in 2005, I have not been fully happy with the way the motor ran. It's a 72.5mm bore TY175 motor and while it always had plenty of stick, which was the main idea, it has been a handful at times when the RPM got very low in first or second gear - it was very hard to control - it would either snap forwards if you gave it a touch too much throttle or stall if not quite enough.

At first I tried Boyesen reeds, but found no difference. Then I tried a WES muffler, which improved the sound immensely but didn't fix the stalling. Then I tried retarding the ignition. This helped to a point with the stalling but the motor became very doughy. Then I bought an electronic ignition that has an advance curve so it could run a bit retarded at low RPM and have normal timing at higher RPM. That helped but it still was not as stall-resistant as I wanted.

The next option was to either lower the compression or add flywheel weight.

Head gaskets being easier to make than a spacer to move the flywheel cover outwards, I made new copper head gaskets of different thickness, ending up with a 1.2 mm gasket in place of the 0.5 mm gasket. The motor became so stall resistant, I raised the gearing from 12/51 to 13/51 to take advantage of the smoothness of the low-RPM response.
While it went very well in the extremely dry conditions of spring here this year, it was hard to tell for sure what it was going to be like when finessing for traction in mud.

This past week we had the first rain since autumn and out with a few others today I gave it a really good test-run on slippery clay, wet rocks and wet mossy logs (in a thunderstorm) and it was just wonderful to ride. I think that bike will be getting a bit of use in 2010.

[Stork955]

Good result David. Tuning can be a real hit-and-miss operation sometimes eh! Did you match the transfer ports to the barrell while it was apart?

Cheers,

Stork

[David Lahey]

My memory is telling me that the transfers in that barrel matched the crankcases quite well from new.
I was concerned at the time the work was being done that the transfer ports may not work as well as standard due to their reduced cross-sectional area and the geometry changes at the outlet ends - both due to the bigger sleeve - but it runs very well.