AD7ZJ Wiki - Recent changes [en]

XR 250R Engine Rebuild

Elijah — Mon, 03 Aug 2026 21:47:53 GMT

← Older revision		Revision as of 21:47, 3 August 2026
(2 intermediate revisions by the same user not shown)
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			[[File:Xr250r_running_muddy.jpg\|200px\|thumb\|right\|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.		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.

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	[[File:Xr250_intake_valves_worn.jpg\|600px]]		[[File:Xr250_intake_valves_worn.jpg\|600px]]

	Onto the head: the intake valves were heavily cupped/worn - a common issue with these XRs. 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!		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.


			[[File:Xr250r_new_intake_valves.jpg\|600px]]

	~~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.		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.


	[[File:~~Xr400_oil_cooler~~.jpg\|600px]]		[[File:Xr250r_head_finished.jpg\|600px]]

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


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


			[[File:Xr400_oil_cooler.jpg\|600px]]

			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!


			[[File:Xr250r_exhaust_header.jpg\|600px]]

			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.


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	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.		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.		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.

			[https://www.vintagebikebuilder.com/setting-mikuni-float-levels.html Good info on measuring float level for these carbs]

	==Starting==		==Starting==
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	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.		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.

			==Lighting==
			The stock headlight is Ok but could definitely be brighter. A high and low beam would be great too.

			Stock tail light is a running light, it doesn't have an individual brake light. That's technically not street legal so I'm adding a brake circuit to the rear brake. The rear master cylinder uses an M10x1.25 banjo style connection which you can get aftermarket switches for. It triggers off the hydraulic pressure when you step on the brake. The tail light is a standard two filament bulb, and the holder even has the second filament wired up. So, I will just connect the brake switch across the existing running light and the unused 2nd filament to get a brake light function.

File:Xr250r running muddy.jpg

Elijah — Mon, 03 Aug 2026 07:26:39 GMT

Elijah uploaded File:Xr250r running muddy.jpg

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File:Xr250r head finished.jpg

Elijah — Mon, 03 Aug 2026 07:21:09 GMT

Elijah uploaded File:Xr250r head finished.jpg

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File:Xr250r new intake valves.jpg

Elijah — Mon, 03 Aug 2026 07:17:39 GMT

Elijah uploaded File:Xr250r new intake valves.jpg

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File:Xr250r complete.jpg

Elijah — Mon, 03 Aug 2026 07:06:36 GMT

Elijah uploaded File:Xr250r complete.jpg

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File:Xr250r exhaust header.jpg

Elijah — Mon, 03 Aug 2026 07:01:00 GMT

Elijah uploaded File:Xr250r exhaust header.jpg

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XR 250R Engine Rebuild

Elijah — Mon, 03 Aug 2026 06:58:14 GMT

← Older revision		Revision as of 06:58, 3 August 2026
(One intermediate revision by the same user not shown)
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	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!		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!


			[[File:Xr250r_engine_valves.jpg\|600px]]

			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.


			[[File:Xr250r_engine_right_side.jpg\|600px]]

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


			==Carburetor==

			[[File:Xr250r_carb.jpg\|600px]]

			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.

File:Xr250r carb.jpg

Elijah — Fri, 24 Jul 2026 06:16:17 GMT

Elijah uploaded File:Xr250r carb.jpg

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File:Xr250r engine valves.jpg

Elijah — Fri, 24 Jul 2026 06:15:06 GMT

Elijah uploaded File:Xr250r engine valves.jpg

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File:Xr250r engine right side.jpg

Elijah — Fri, 24 Jul 2026 06:13:46 GMT

Elijah uploaded File:Xr250r engine right side.jpg

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