Making a cam for twin to single points converlsion.

toglhot

Active Member
Twin to single points conversion.
The TX650 came stock with a two point ignition system, same as the Brit twins. If there is a reason for two sets, it escapes, me, after all there are plenty of bikes with one set of points, and cars with one set of points feeding 4,6 and 8 cylinder engines. The only benefit I can see for the TX is if a single carb conversion has been done. The manifolds for these conversions have different length plenums, so adjusting the timing differently for each cylinder may be of some benefit.

The backing plate is about a simple as it gets to make: Just 62mm round 3mm plate, drilled and tapped to take points and cutouts filed for adjustment.

For the cam I started with a short piece of stainless. I started off by facing both ends, then drilled through 7.5mm followed by 8mm, this gives a nice fit on the advance/points rod which is imperative. The cam was then transferred to the mill and 2.5mm keyway plunge cut to a depth of 5mm. The advance/points rod was then mounted on the lathe in a four jaw chuck and centred. The cam blank was then mounted on the rod and snugged up tight and turned down to 18.05mm.

The rod and cam blank were then assembled on the bike and timed at 15 degrees BTDC, the firing point. Points were gapped to .35mm and a line scribed on the cam by running a sharp blade under the points heel. The motor then turned over 75 degree after the firing point and another line scribed on the cam under the points heel, that gives a 90 degree dwell. After that it’s a matter of using a file to file the area flat between the two scribed lines, a micrometer used to keep the filing under .50mm.

The rod and points were then assembled again on the bike. Once again with the timing set at 15 degrees BTDC, a line was scribed on the cam above and below the points heel, then I filed very carefully between the scribed lines and using a micrometer then filed down to around .30mm and the apex between first and second flats rounded over. The rod and cam were again assembled and the timing checked with a light connected between points and earth. I did this at least a dozen times until the light illuminated a few degrees before 15 degrees BTDC. Then used a nylon fibre wheel to smooth over the file marks and gradually lower the apex until the LED light s up at 15 degrees BTDC. I then rounded over the dwell apex until the light extinguished at 75 degrees ATDC. The engine then turned over 180 degrees and the process repeated for the second lobe. Then the cam was polished.

In retrospect, I should have retarded the points backing plate a little to account for cam chain stretch as the bike ran up some miles. That’ll be the next job – one day!
 

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Finished job.
 

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