Step by Step

Get your motor in the frame

First thing to to is to strip the bike of all the things you don't need. That includes all the YSR electronics. I took pictures of how everything was wired and mounted for future reference. Note that you can click on many of these pictures for a larger view.

You'll need to cut off the mounting tabs for the coil to get the motor in.

The motor is a really tight fit and is hard to get in the frame by yourself, but it's possible. I bolted the custom made front mount to the motor and left the rear mount in the frame after spreading out the mount about 1/8" to fit the XR motor. You can just grab it with a pair of plyers and bend it out slightly. I'm sure theres better ways to go about it, but this worked for me. I had this front mount made by Dennie, a fellow YXR racer. He already had a template made and everything, so there isn't much i can tell you about the fabrication other than you want the motor as far foward as you can get it for weight reasons. The front and rear mounts are pictured here. the rear mount is shot from above the bike with the tank and everything removed.

Fabricate your exhaust

Next step is creating your exhaust pipe. pictured above is the stock xr100 pipe i got off ebay for 50 bucks. you should be able to get one cheaper than that, but i was in a hurry. Below is the same pipe after modification. I marked the stock one to give you a rough idea of where we cut it and how we put the new pipe together. note that we shortend the can a couple of inches, and that # 1, 4, 6 and 8 got left on the garage floor. We also cut everything out of the exaust tip, so it flows straight through the tip instead of going through all the spark arrester stuff.

Here we're starting to cut the pipe using the band saw. After cutting we used the grinder to get the angles we needed between the pieces.

now we start taching (sp?) it together on the bike. having the pipe properly supported is VERY important. It doesn't matter how good your welds are. The vibration from the motor will crack your pipe and leave it on the track if you don't support the pipe right. DO NOT attach the pipe to the frame. bolt it to the motor only. We had a case of lucky engineering and were able to bolt the header to our motor mount using one of the brackets already on the exhaust. i also put a rubber groumet here about 1/4" thick to cut down on some of the vibration. Later you'll see how we used the stock bracket on the silencer to bolt to the bottom of the motor.

Be sure to clean up the metal before you weld it. We used a Miller Mig welder i believe.

We made this bracket to support the pipe from under the motor. There are 4 holes under the motor that are already threaded for #8 bolts. I'm not sure if its for a skid plate or what, but they work perfect. I didn't leave enough space in this bracket for some good, thick rubber groumets, so we had to redo it later. we got some thick ones from a muffler shop to use. they're about 1/2" thick.

Here is the new bracket with the thick rubber washers that we got from the muffler shop.

New Exhaust!

I picked up this undertail exhuast from a fellow racer for 100 bucks. i think it's 1 3/8" in diameter. doesn't sound like much over the stock 1" pipe, but it sure looks bigger. i think its made by ACE. i've never heard of that company and don't know if they still make these or not. it sounds almost the same as the one i made before... loud. i had the loudest bike on the track before, and i think i still do. it doesn't sound bad, its just loud. i'm a little afraid of the tire hitting the can while turning hard and compressing the suspension. we'll see what happens. I wraped some heat wrap around the pipe where it passes the air filter and again up in the front because it hits the frame a bit there. I'm hoping the wrap will buffer it enough to keep anything from getting damaged. i also had to cut up the "mud flap" to make way for the exhaust.

Clutch

The XR100 has a clutch cable bracket to hold the cable housing in place while the cable itself pulls the clutch actuator on the motor. I have a theory that the stock xr100 bracket will bolt onto the ysr rear motor mount and work perfectly, but i haven't bought one yet to check it out. I went to Home Depot and bought some metal stock (dunno what it was... about 1/8" thick and 1" wide maybe). Cut it about 1 1/2 or 2" long and drilled 2 holes. drill one hole close to the edge and cut an opening with a hack saw for your cable to go though. now bend a 45† angle in it and put it on the motor mount as shown. I bought a XR100 clutch cable to use, but it maybe possible to use your YSR clutch cable. i didn't try it. The first pict is of my first attempt. This one wasn't long enough. the second picture is of the one on the bike now. it works pretty well, but still needs to be longer for better range of adjustability with the clutch. i'm also told that eventulally you'll want one stronger than this when you upgrade your clutch springs.

Everything else

Once you have your motor mounted and exhaust done, the rest is fairly easy. If you have a stock Intake manifold then you'll have to get a foam Uni air filter to mount to it, or run no filter at all. I'm scared, so i run a filter. Its a pain with the stock intake, but it works. The first picts here are of the stock intake set up. I've got this custom one made by Kelly at AW Tooling. This allows me to run the filter with no problems. As for jetting, right now i have a 38 pilot and 97.5 main with the needle in the middle position. my plug looked pretty lean when i checked it at the track last weekend, but it ran a lot better than with the 100 main.

You'll need to run some tubing for your crankcase vent back to a catch bottle. I did this at about 1am the night before my race. I was exhausted, so its not pretty.

Originally I zip tied my CDI and Coil to the frame above the head. Others put it where the stock YSR electrics go, on the "mud flap" plastic piece near the rear shock. Eventually i'm going to relocate mine to the back as well. Don't forget to ground your motor! It took me forever and ruined a race weekend trying to figure out why my motor wouldn't run right, and why i was getting shocked through the clutch and brake levers. Turns out the XR100 frame has metal motor mounts, and the YSR has RUBBER. To properly ground the motor I ran some wire from the ignition cover to the frame. No more shocking and the bike runs much better. The picture to the right is of my ground wire on my case, and new pictures with my cdi mounted on the "mud flap".

Put on your xr thorttle assembly. I had some problems with my stock throttle. i had to regrip it to get full throttle. if i forgot that i had regripped and went into a corner without letting the throttle close all the way, it would stay on the throttle through the corner and push me wide. I picked up a quarter turn throttle from the local shop and it seems pretty good now.

I picked up a kill switch from the local motorcycle parts store for about 7 bucks. you can also use one off the XR if its available to you. attach to bars and wire it up. my dad wired it up and i was off doing something else, so i'm not sure how he did it. you can basically just keep trying wires until it works right.

I would love to give you tips or guidance on building your XR motor, but I didn't build mine. John Cartwright of Cartwright Conversions was nice enough to put my motor together for me. He's also where i bought the motor. He had the motor dissasembled already when i bought it because he was stealing a shift fork out of it for another project, so he needed to assemble it anyways. I just bought a gasket set and all the new parts (piston, rings, wrist pin, timing chain, cam sproket, timing chain tensioner) and gave it to him. I did take my cylinder and piston to Elmer of Northside Cycles in Corinth, TX, for a bore job (2 over or .5mm).