Single points conversion for the TX650.

I converted my 74 TX650 to single points a while back, for this job I had to made a new points backing plate, install a dual outlet coil and machine a dual lobe points cam. The dual output coil and points backing plate were simple jobs but the cam proved a little time consuming.

The points cam fits on the long advance rod through the middle of the cam which is connected to the auto advance unit. The rod has a diameter of 7.9mm and a 2.5mm pin slotted through the rod to locate the points cam in the right position.. As the points cam has to be a very tight fit on the rod to alleviate any variations in points gap, I bored the points cam 7.5mm then 8mm. 8mm drills are around 7.92mm, so pilot drilling first gives a hole of a smidgen over 7.92mm resulting in a tight push fit onto the advance rod.

After that I plunge cut a 2.5mm keyway on one end of the points cam to depth of 5.24mm to accomodate the locating pin.

The points cam was assembled on the advance rod, the advance rod centered in the four jaw and the points cam turned down to 18.6mm, then checked for concentricity relative to the advance rod.

The advance rod with points cam was then assembled in the bike's cam, the crank turned around to 15 degrees BTDC and a line scribed on the points cam under the points heel. The motor was then turned around to 70 degrees ATDC and another line scribed on the points cam under the points heel.

Then it was a matter of filing a flat between the two scribed lines to a depth of .8mm and the timing checked with an LED. This was quite time consuming as I had to file minute amounts off the cam, slowly creeping up to the point where the LED lit up at 15 degrees BTDC. When correct, the motor was turned over 360 degrees and the process reated for the second cylinder.

When all correct the points cam was polished to a mirror shine to prevent the points heel wearing, then assembled and the timing rechecked.

All sounds good, but I was unhappy with the points cam, whilst the timing was spot on and worked extremely well, I wanted a shorter dwell period and a more abrupt movement of the points heel. So I set about making another cam.

Basically the same process as before, only now I'm better at it and a lot quicker. This time I lengthened the dwell from 95 degrees to 110 degrees for bettercoil saturation. The shoulder coming onto dwell was kept sharp and more abrupt this time but I retained the rounded shoulder coming off dwell.

First pic is of the original twin points setup.

Now I know electronic ignition is the go these days, but do you really think I could have this much fun installing an electronic ignition. And what of my lathe, mill and files, they would get bored just sitting around watching me ignore them.

 

Nice work! I understand the "because I can" motivation for such a project. I recognize also that I don't have the skills or patience for such a project. But I can appreciate those that do.