catraprez (catraprez) wrote,

The XR75 from Hell

Been a while since I've done anything on the XR75. Between skiing, baseball, remodeling the dining room and work there just hasn't been much time to work on it. I've gone through it six ways from Sunday. It fires up and runs then slowly starts to sputter and refuses to idle. It's running very rich and fouls plugs. Eventually the plug gets so fouled that it refuses to fire. Sounds like a carburetor problem, no?

So I pulled the carb, take it completely apart, soak it in carb cleaner and blow out all the passages and jets. No change at all! Pull the valve cover and check valve clearance. It was out by 0.010". Setting it to the correct value did quiet the valve train noise, but the bike still ran like crap. I went through the whole ignition system again. Reset the points, checked static timing and even made sure all ground connections were good. Put in a new plug and checked timing with my timing light. One thing I did notice was the ignition timing was bouncing all over the place. Point float or point bounce is a common problem with breaker-point ignitions, but usually only at high RPM's not just off idle.

A little more investigation reveals the mechanical spark advance mechanism, pictured below, tends to wear out. This causes the ignition timing to move all over the place.

Inside of flywheel showing mechanical spark-advance mechanism.

Not only do the springs weaken over time, but the fit between the flywheel boss and the mechanism cam wears out and becomes sloppy. I've never had a problem with the points in my Sherpa-T, but there is no ignition timing advance mechanism. Rather than monkey around trying to find a working advance mechanism I decided to convert the points over to CDI. Now I could've spent $450 on a PVL unit, but I'm a cheap bastard. I picked up a Mega-Fire II universal ignition module. Wiring it in was fairly simple.

Remove points and condensor from the stator plate.

Solder the stator wire to the ignition wire and cover the union.

Mount the module to the ignition coil, positive terminal spliced into the ignition wire from stator and negative terminal to ground.


Three parts replaced by one module.

The only issue with using this module is that ignition timing is now fixed as there is no advance/retard feature available with these modules. The only way to adjust timing is to rotate the flywheel on the crankshaft, which would require removal of the woodruff key. Luckily, when I put the timing light on the engine, I found ignition timing to be at the advance line on the flywheel. I'll need to use my degree-wheel to find out exactly where this is in relationship to the piston. Being at full-advance may make the bike more difficult to start, but will provide maximum power at WOT. In fact it worked so well I'm thinking of putting one on my Sherpa-T. On that bike I can rotate the stator plate to adjust the timing.

Of course this didn't cure the overrich fuel mixture and plug fouling, although it will fire a plug that has been fouled. The next step is to completely rebuild the carburetor. I purchased a Keyster carburetor kit to install. Hopefully that takes care of the problem.

My last concern is the bike may be using oil. It's difficult to tell since it's running so rich, but there appears to be some blue smoke in the exhaust. Of course the sparkplug is a tiny peanut plug. Neither my compression gage or cylinder leakdown gage have a small enough adapter. So I'm flying blind there.

The P.O. did rebuild the engine, this I do believe. Judging from the quality of work I've seen so far nothing would suprise me. He could've simply slapped new piston rings onto a used piston and stuffed it in a wornout bore. The piston rings could be installed upside down. There could be excessive clearance between the valves and valve guides. The valve seals could be worn, brittle or missing. Then again this thing has been running so rich that it simply glazed over the bore before the piston rings had a chance to seat.

Of course tearing into the motor is something I was planning to do anyway. For now I just need to get it running like a typical Honda. Maybe taking care of the overrich running will fix the oil consumption. Once it is running I can get my son some practice time using a clutch. Perhaps we'll even get to race a vintage MX or two this summer. Next winter I'll do some trick mods to the little XR.

I haven't thrown the towel in yet... Unlike the previous owner.
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