XR 250R Engine Rebuild

Honda XR250R in its natural habitat

Engine Reassembly

This bike was listed on Facebook Marketplace for a while, and eventually, it followed me home. The previous owner had fixed it all up with new plastics, a powder-coated frame, all new bearings in the rear suspension, etc. Then, a shaft in the transmission broke; he split the cases, lost interest, and put the engine in pieces into a couple of totes.

I had a lot of fun putting the puzzle back together. It wasn't too bad, actually. He had done a great job of keeping track of all the parts.

Here is the culprit: a broken output shaft. The tip snapped off, and the gears crashed together, sending metal fragments throughout the oil system.

The oil screen at the bottom of the frame had a significant amount of shavings in it. I rinsed the frame out thoroughly, filling it with gasoline and flushing it several times to ensure all "glitter" was gone. The oil pump was also scored up from the debris, so I sourced a used pump in better condition from eBay to ensure the new build has plenty of oil pressure.

The cylinder was scored up a bit. The previous owner had replaced the sleeve with a larger one and bored it to 80mm, bringing the displacement to approximately 300cc. After talking to some folks online and at XRs Only, I decided that 300cc on an engine designed for 250 wasn't ideal for long-term reliability. Parts are also more expensive, so I opted to return to a stock bore size.

This is the internal boss for the kickstarter. After seeing this damage, I found a complete bottom-end half for an XR250 of a similar year on eBay. I ordered that along with a stock cylinder (73mm bore). That cylinder turned out to have some corrosion pits that wouldn't hone out, so I picked up a 73.5mm Wiseco piston and ring set (4466M07350). The machine shop bored the cylinder to match the piston.

Here I have the bottom end going back together. I love seeing how the transmission gears work with the shift drum. It's such a clever and beautiful design - 6 speeds with just an input and output shaft!

Onto the head: the intake valves were heavily cupped/worn - a common issue with these XRs. Notice the head has extra fins welded onto the casting. This is a specialized mod by XRs Only to increase surface area and help the air-cooled engine run cooler. I'm definitely happy to have that. I picked up a new Kibblewhite stainless valve set and had the machine shop cut the seats and fit them. So much for this being a cheap rebuild! To make matters worse, after I received the kibblewhite valves and took them to the machine shop, we realized that someone had put larger seats in for the intake valves. Stock intake valve diameter is 28.5 but the old ones were 30mm. And after looking around extensively, no one had the oversized valves available anymore.

Shumann Motorsports is able to make custom valves, I found out about him from some FB posts. I sent him one of the old intake valves and he made two very nice new ones that match.

Here is the finished head. The machine shop blasted it and cleaned it up very nicely. I'm happy how it turned out.

Here's the head before the valve cover went on. It's interesting how they do the valves on these - the rockers are in the valve cover.

Here's the right side just before the cover went on. It was fun learning how the kickstart mechanism works.

Someone along the way also fitted an XR400R oil cooler to the bike. Hopefully with this plus the oversized head, I'll have an unstoppably reliable, trail riding machine!

Putting the engine back in the bike was pretty straightforward and didn't need much extra consideration, with the exception of the exhaust. Someone along the way had cut the header shorter and adapted this DG slip on pipe using a sleeve thing. It didn't seal well at all given that the diameters of the header and the slip on are a couple mm different. I realized that the DG pipe was exactly 1.5" ID, so I got a short piece of 1.5" tubing and welded it into the header. I tacked it with everything in place on the bike so it lines up very nice. The pipe will slip on now but it's quite tight and seems to seal fine.

Carburetor

Somewhere along the way, someone changed the stock Keihin carb out for a flat slide Mikuni t34 02e. It had a 360 main jet and a 25 (I think, going off memory on that one) pilot jet. After getting it back together and riding it around a bit, it was definitely running rich. I swapped in a 280 main jet, and that was better but it was still burbling and choking a bit at full power under load.

Then I went to a 240 and the burble at full power was gone but it felt a little less strong in the pull.

So I went to a 260 and that seems pretty good. I'm running with that for now!

I tuned the low speed by adjusting the air screw to get peak RPM, and then counting the turns open. It's currently at 1.5 turns which I think is about right. Too much more or less than that is a sign you need to adjust the pilot jet.

After riding it on a rocky section, I found it was getting too much fuel again. There was some running out the overflow as well. I got a carb kit and replaced the fuel needle, and set the float level at 18mm (measured from carb body where the gasket goes, to the float arm). It would still overflow a little when I shook the bike side to side so I lowered it to 19-20mm. This seems to be working well now.

Starting

I had never had a kick start bike before. I started off like you typically do any small engine - get it up on compression, and then pull (kick in this case). That does work - but this engine has a LOT of compression and it pretty much takes your full body weight to do that kick. And as soon as the compression stroke completes, the kick lever pretty much falls out from under you which is hard on both the kick lever and the legs!

Then I watched a couple videos on the subject and I understand now what they're all getting at. You get it up on compression, but rather than trying to push through it, use the decompression lever to easily get past the compression stroke. Now you can give it a kick pretty easily. The lever is geared such that at the bottom of that kick, you'll push it over the next compression stroke but now with the engine's momentum helping you. The stock XR had an auto decompression tab which probably made kicking simpler. That has been removed on this bike and the manual decompression lever is the only thing in play.