Thread: Test ride, 74 TX650.

Thank you for sharing your journey , it was brilliant. You should be very proud of your achievement.
pker

Tony; I've been following the build since day one and have really enjoyed it. Well done for for making what I can now only describe as my Number 3 dream bike; after a Black Shadow and '75 T160.


30+ years ago as an apprentice with no money but plenty of enthusiasm and ingenuity I cafe'd up a little 1976 Yamaha RS100. I monoshocked it; built rear sets, converted it to 12V, lowered the front end by about 2" and gave it a single seat. If it wasn't a necessary component it got cut off! Everything was done using just using hand tools and copious amounts of swearing. I finished it on a Monday evening; had it roadworthy'd etc on the Wednesday and on the Friday I rode it the hundred miles or to the Kent Custom Bike Show. When I get home next week I'll find some pics and post them on here for you to laugh at!

Well, it ain't finished and never will be, such are custom bikes, always something to change, something to modify, something to redesign. I'm currently working on a new stainless single points cam. The original I made was mild steel, stainless work hardens, so it should wear better, not that the original mild steel one wore, but you know how it is!

The bonus with making a second, is you learn from your original mistakes and work out better, faster ways of doing things. Here's how I did it:

First a description of the XS650 points ignition system. The engine is a 360 degree twin with a single overhead camshaft. A timing rod runs through the centre of the camshaft, riding on bronze bushes pressed into each end of the camshaft. The right side operates the advance weights and the left has a single ramp cam mounted on it to operate two sets of points. Basically exactly the same as the old Brit twins. I grew up on these bikes and hated setting the points and timing a real PITA, hence this conversion to a dual ramp points cam operating one set of points. I could have gone down the electronic ignition system as there are several available from XS650 outlets, but, I much prefer mechanical things and of course anyone can buy and bolt parts together. If I ever get sick of setting the single set of points, I have all the electronics in my workshop to make a hall effect system.

I’m using 304 stainless for the points cam this time around. Stainless work hardens so, whilst the mild steel cam has worked OK, stainless should be better. As stainless hardens quite quickly when turning it, I drilled the hole to fit over the timing rod first. The hole has to be exact, the size of the timing rod is 7.97mm so reaming it will result in an 8mm hole, giving more play than I'd be happy with. If you turn the stainless down first, drilling then becomes troublesome as the stainless has hardened a little and you risk burning drills and ending up with oversized and irregular holes. Sizing is extremely important in this mod as it will affect the points gap. The gap is .13mm, so if you are out by .01 or .02mm that means whilst one lobe will give you a gap of .13mm, the other lobe will put the points gap out by the corresponding amount.

Ok, the hole in the centre of the stock is critical as that slides over the advance rod, you can’t afford any discrepancy in sizing here. So, I mounted some stock in the three jaw, centre drilled it, then drilled through with a 5/16” drill bit, then followed up with an 8mm. The 5/16” bit is around 7.86mm actual bit size, giving a hole around 7.9mm, I then followed up with an 8mm (7.93mm actual) which gave me a hole of 7.97mm which just happens to be the size of the advance rod the cam slides over. No good using an 8mm reamer as that will give you an 8mm hole – too big!

Next I turned down the stock to 18.5mm and parted off so the blank is roughly the same length as the stock cam. Then faced both ends.

The cam blank was then mounted on the mill and using a 2.5 endmilI I cut a slot 6mm deep, then widened the slot to 2.8mm so the 2.8mm timing rod locating pin/key was a neat fit.

The advance shaft was then mounted in the lathe four jaw and centred by running the dial indicator on the bearing surface which runs on the bronze bushings in the camshaft, this had to be exact because that will affect the points gap and the timing. I then mounted the cam blank on the rod and tightened with the securing nut. Then I began shaving the cam blank down to 18mm taking fine cuts to ensure I didn't put the concentricity off, checking with the dial indicator every so often.

Once turned down to 18mm, the blank was then checked for concentricity. It came out perfect the dial gauge not budging off zero.

That was the easy part, now came the hard bit.

I assembled the advance rod inside the camshaft and mounted the points cam on the rod and tightened the nut. Then it was a matter of setting the points backing plate (which I made earlier) in the central position and gapped the points to .13mm, then turned the engine over 360 degrees and checked the points gap on the other side of the points cam, spot on.

I then turned the alternator rotor around until the timing pointer was about 2-3mm before the timing mark on the case and using a sharp blade, ran it across the cam blank just under the points heel. the rotor was then turned so the rotor timing pointer was at 90 degrees to the timing mark and again scribed a line under the points heel, this gives the correct dwell angle of degrees for the XS650. That's one cylinder done, so, I turned the motor over one revolution and repeated for the other cylinder.

The points cam blank was then placed in the mill vice and a flat milled between both marks on both sides of the points cam. Once done it was a matter of filing the edge where the flat meets the round section of the points cam which is exactly where the heel of the points sit. For this operation I made up an LED timing light connected to a 9 volt battery via a resistor. The cam had to be checked with the timing light, removed and filed and then replaced to check the timing light a number of times as I slowly crept up on the right size to get the timing right lit up. Once done, I moved on to the other side of the points cam and repeated the exercise. That left just rounding off all the edges.
The rotor pointer was then turned around to the timing mark and the fire and dwell points polished with a nylon fibre wheel until the LED lit up. Job done