Nice build up. I love xr250's most under rated single track bike ever. I don't know how that m12 will behave on the little thumper, but on bigger bike's it has a tendency to step out sideways under throttle instead of driving forward. To me the s12 is the ultimate rear tire.so predictable, so consistent. paired with either a s12 or m12 front, depending on terrain.
Awsome project! I had an XR250 in '96, and I loved it. Half of the fun is the simplicity. I raced it a bit (this was before the current crop of higher performance four strokes) and beat a lot of bigger, "faster" bikes, gettign top three finishes in the four stroke B class. People don't like getting passed by a little trail bike. Definitely get the suspension properly set up. I found out the stupid way that these bikes can go a lot faster than the suspension will tolerate. And watch your toes! Ten inches of travel leaves them in harm's way quite often.
I had an '01 with 285 kit, hot cams, full exhaust system, ( i forget what brand) and ohlins forks, The bore kit was the single best mod, if i had it to do over again I would probably not do the hot cams, i don't think they really did that much for me, although to be fair I did not upgrade carb or stiff valve springs. There are weld on adapters that widen the footpegs that helped me keep my toes up.
I didn't do anything to the suspension. The last time I raced the bike I broke my hip, going too fast for the suspension. I was stupid for not knowing better. After healing up I switched back to two strokes. I would expect you could get good results with the stock components (with revalving and springs), but upgrading the suspension would be a good idea. That is a whole different project in itself.
One of the disadvantages of older 4 strokes is poor throttle response. Newer bikes have light flywheels, and carbs with accelerator pumps, and the extra snap helps immensely. I was thinking about a Keihin FCR 35 and Sudco listed a kit for the XR. So I ordered it up. Side by side with the stocker: More.... BP
First thing to do was to test fit the carb to see how it fit, how cables routed, etc. It was a bit of a hassle to get into place, but once in, fit fine: More.... BP
A few additional parts were needed to fit it. First, the throttle cables come off the stock carb on the left side, on the FCR they are on the right. Additionally, they are really close to the upper motor mount: I called Motion Pro, and they sent me cables with 45 degree elbows. I had to be really careful so they did not contact the cylinder head and melt the housing. Additionally, the bracket is threaded, so I had to spin the cables into the bracket before putting the carb on, otherwise.....not enough room to spin the cables into the threaded bracket. I ended up carefully drilling out the threads on this bracket: Once I got this done, it was much easier to fit, position, and adjust the cables.
The airbox side OD is bigger, so an XR400 boot is needed: One thing that is different, and still unresolved, is the crankcase venting. My bike is a California model, and had a more extensive crankcase recirc system. The 250 boot has a boss to attach the hoses to, that the 400 boot does not have. Also, I noticed that the stock intake manifold had hardened considerably, so I took no chances and replaced it.
The biggest hassle of the carb install was figuring out how to time the accelerator pump. You have to time the squirt to start a little late, and 2. limit the duration of he squirt to 1.5 secs, to not bog the motor. The newer FCR-MX pumpers have a leak jet, which can be changed out to control the squirt. A larger jet bleeds fuel out of the circuit instead of pumping it into the motor. It is quantitative and works well. But, the older FCR slant body carbs like the 35 do not have leak jets. There is an accessory from Boyesen that duplicates the bleed-off, so I got one to try: and mounted it up: I started testing the squirt off the bike, with odd results. The pump would hesitate, then squirt for what seemed like forever, definitely not 1.5 seconds. No amount of adjusting was helping, and I was getting confused. By now I had burned a load of time and money ordering various diaphragms and add ons, and was getting kinda frustrated. I finally decided to just send the carb to Eddie Sisneros ("Burned" on Thumpertalk) and have him set it up. After a few days , Eddie discovered that the squirt passageway WAS NOT FULLY OR PROPERLY DRILLED FROM KEIHIN. "Yeah, the thing would hesitate and spit, and then squirt forever. I drilled the hole, wired the AC pump linkage, and now it is fine." Good news, I would have never spotted this, and would have chased the squirt timing forever. More.... BP
Verbage, more verbage, some more friggin verbage, pics, more pics, blah blah blah. When in the hell do I get to flog the shit out of that thing, break something, so you can work on it some more.
Any updates. I am in a similiar position myself. I have a dr650 that I modded with rmz forks, cogent shock, fcr carb, gsxr exhaust and a long list of farkles. It does great for dual sport rides where we may cover half the state in one day on dirt roads, railroad beds and trails. The tougher atv trails in this area tend to be mud, rocks and roots, not as much fun on a big bike. I keep thinking about adding a smaller bike for the tight stuff.
Yes, some updates. Did two rides. Power was very good, some jetting circuits are way off. I started with the following jetting specs: Slow air screw 1.25 turns Pilot 42 Fuel Screw 2.5 Needle CEMT #2 position from top Main 140 Boyesen 1.5 turns out I did some short runs around the neighborhood, and these settings were close. So, I went for a shakedown ride First, the bike was really hard to start. Once it got warm, it would run well when it got up onto the main jet. But the pilot circuit, needle, AP pump, are all still off a bit. Power was better than I thought. It is not fast, but it makes consistent, steady power. It is a little sloppy - I had to slip the clutch out of turns to get past a flat spot. Once I get it dialed it will be better. BP
Way too much technical jargon bullshit here. Just ride it somewhere cool and give us friggin pictures. We don't need all the other bullshit! :fyyff Call me when you want me to school that thing with the KDX!
Did you know they make a CRF 250X? You could probably pick one up for about the same price as half your mods JK I spent way too much on my 125 so I know what it's like
I'll dissent. Vey useful information here as far as XR setup is concerned. Keep up the good work! I've got an older XR250L that I just unexpectedly picked up due to a very low price, and this thread, while not directly relevant due to generational differences between our XRs, has been a great source of information. Thanks again - looking forward to reading more about this bike in the future.
