It's a 1974 Honda XR75. My oldest boy is going to race vintage MX, in the XR75-class, on this bike. I looked at several XR75's and twin-shock XR80's (also legal for the XR75-class). Prices, and condition, were all over the place.
The bike pictured above wasn't the best-looking bike, nor the cheapest. It was, however, complete and not a beat POS. Kids, here's a tip for you; You are not going to be able to sell your thrashed '80 XR80 for $1600 to buy a, probably equally thrashed, '92 KX80 from your buddy for the same price.
The PO had rebuilt the bike for his son to race the XR75-class. Unfortunately his son outgrew the bike before he got a chance to race it. He was also honest about it needing some carburetion and, possibly, ignition timing work. Two things that I have plenty of experience with. So we negotiated and agreed upon a price, met and did the exchange. He also gave me a bunch of the old parts. Just what I need. More junk to clutter my garage.
First thing I noticed was the handlebars were loose, rotating in the clamp. Years of working on bikes I've learned that you never just tighten a bolt. First you loosen, then re-tighten. Unfortunately I discovered two things not uncommon with old motorcycles. The four bolts were a mixture of two metric 6mm x 1.00 bolts and two SAE 1/4-20 bolts. The other problem was one of the 1/4-20 bolts was broken. I removed the clamp, drilled a small hole in the broken bolt, heated the clamp with a propane torch and removed the broken piece with an EZ-out. Normally I don't like mixing metric and SAE fasteners, but in this case I could not drill and tap for an 8mm bolt. So I tapped the other clamp for 1/4-20 threads and put it all back together with grade 3 SAE fasteners. The bars are nice and solid now.
Next order of business was to check the air-filter. A fairly simple procedure. Remove the right side-plate. Remove three 6mm nuts rotate the cover and presto! In fact it was so easy that I had my son help me. In the future he will be doing this work himself. The filter was clean and well-oiled. At least enough to see if this thing will actually run. Before it gets ridden I will clean and re-oil the filter with No-Toil.
This is where I should probably drop the coin for a service/repair manual. Yet on the past six bikes I've owned, I have not had a repair manual. Only three of them had the owners manual and I've been able to do most of the work, including a complete top and bottom end rebuild on my '98 GasGas EC250, by finding the information on the web.
When everything was back together it was time to see if this sucker would light-off. I pulled the drain plug on the carb and opened up the petcock on the tank. Fuel flowed out of the float bowl. It didn't have that "varnish" smell. Nor did the fuel in the tank. I closed the drain plug, flipped the choke lever and kicked... and kicked... and kicked... and... Finally I heard a sput, but more kicks produced nothing. So I moved the choke lever to "OPEN" held the throttle WFO and kicked some more. A few sputs were heard, then nothing. At this point I pulled the spark plug. It was black and wet-fouled. Too much fuel. I had my son kick the bike over while I checked for spark. Nothing! Not even a hint of a flash. I didn't have a similar plug to check so I put a screwdriver in the cap and had him kick it over. Sure enough, there was a nice fat spark.
By now it was late on a Sunday night. Anything still open would, most likely, not have such a small plug in-stock. Then it hit me! The little CRF50 probably has the same type of spark plug. I pulled the spark plug out of the 50 and compared it to the one from the 75. Close enough for government work, so it went in the 75. With the choke "OPEN" and the throttle WFO one kick and the 75 roared to life. While not overly-loud it was definitely nosier that the little 50, but no horrible sounds eminated from the engine. The smell of freshly rebuilt motor soon filled the air. I turned both the air screw and the idle speed screw, but could not get the engine to idle. My worst-fears subsided I shut the bike down. When I pulled the spark plug out, to put it back in the 50, I saw that it was also black and wet. After surfing the internet I found the correct spark plug for the 75 at my local Napa store. It actually takes a non-resistor plug as opposed to the resistor plug that was installed.
The only other work done was to swap out the stock engine kill-switch for a kill-button. Through experience I've learned that kill-switches can get knocked into the off-position during a crash. You're so pumped on adrenaline that, if the engine dies, all you can think about is getting it re-started and getting back in the race. By the time you've exhausted yourself trying to get the bike started, you start thinking clearly again. It's then you notice the switch in the off-position. DOH! You don't have these issues with a kill-button. Luckily I had a Honda-compatible kill-button in my big ol' box-o-motorcycle-stuff. Just had to change the connector on one end. Pull the seat off, slide the tank back un-plug the kill-switch and plug-in the kill-button. Black to black. Green to green.
Below is the list of stuff we'll need to address:
1. Clean carburetor, drain fuel-tank, clean petcock.
2. Check ignition timing.
3. Fix oil leak, either shift shaft of countershaft seal.
4. New throttle cable. PO replaced clutch and brake cables.
5. Replace bent foot-peg/side-stand bracket. PO supplied replacement part.
6. Check/grease/replace suspension and wheel bearings.
7. New clamp-on clutch perch and lever to replace slip-on perch.*
8. Replace fenders. PO installed aftermarket fenders from wrong-year XR75.*
9. New seat foam and seat cover.*
* = Not critical
Finally I need to teach my son how to ride a bike with a clutch.