Cb250rs + Xr500/xl500s

[xwhatsit]

Mate a bit of advice here -unless you WANT to do this kinda project its never gonna be an ecconomic alternative to just selling ya bike and buying a bigger bike

Oh I'm not going to attempt the project now, it was just an option back in November last year when my engine was toast. Now it's all fixed up, so I'm not considering it for now. Also, I do WANT to do these sort of things -- and despite having no money, economics don't really enter into it -- I'm in deep love with my bike and could never swap her for another bike.

ther was a lot of aftermarket stuff for these motors back in the day mostly for the 500 though honda could supply you with ported blueprinted heat with oversize valves a bigger cam higher comp pistons bigger mikuni carbs and differnt headers end cans even a oil cooler
spare barrels arnt to hard to come by weisco piston was bout 200bucks when i did it
six speed came81 xr250r much closer ratios and some xl250s but 5speeds were made in both models at same time xr runs higher comp than xl and suposedly a lighter flywheel

Cheers -- I didn't think about the XR250 side of things, of course there'd be heaps of aftermarket engine goods for that. $200... not much more than a standard piston. I have a spare bore too, I suppose. I can feel the engine is coming up for the first oversize in the next 20,000kms or so, the piston doesn't feel quite so tight, so I'll have to keep this in mind.

So the 1981 XR250R had six speeds? Close ratio? Sounds like my cup of tea. And they just drop straight in? Brilliant, I'll keep an eye out for XR250R gearboxes then. Never opened up that part of my engine, will be interesting to see. Not fussed about the flywheel, it's a roadbike, so I don't need that `snap' that you do for offroad stuff. In fact a slightly heavier flywheel wouldn't be the end of the world, sometimes it can be a bit twitchy (might be the pumper carb more than anything else though).

Thanks for the info.

[tommorth]

xr / xl are six volt xl has charging system etc xr doesnt is cb 6 or 12 volt my little gp is 6volt and i dont think i will ever try and ride it at nite agian with out street lights its just to scary

[xwhatsit]

CB is 12 volt -- adequate lighting, I do at least 75% of my riding at night. However I wouldn't want to swap the whole engine, just the gearbox. Surely the crankcases would be the same and the gearbox is just a drop-in replacement?

[tommorth]

here we go found the specs xl/r 500 has bigger bolts for engine mounts
xr/l500 also has studs holding top end on 250 has bolts so no go there
500 motor has 89x80 mm bore and stroke and bigger 21mm gudgen pin
250 has 74x57.8 bore and stroke 19mm gudgen pin
xl250 comp 9.1:1, xr250 9.6:1 ,xr250r 10:1, xl/r500 8.6:1(kit was avaiable for 540cc from white bros )
have more but i type slowly

[Muzz67]

I got a 6speed box and a complete NewOldStock crank to sell , if anyone want them.Also got 2mm oversize cylinder and piston. Muzz txt 0211320763. 250 cylinders can be resleeved to take a 12mm oversize Yamaha piston giving 338cc fairly easily.

[xwhatsit]

I got a 6speed box and a complete NewOldStock crank to sell , if anyone want them.Also got 2mm oversize cylinder and piston. Muzz txt 0211320763. 250 cylinders can be resleeved to take a 12mm oversize Yamaha piston giving 338cc fairly easily.

That sounds like fun

How much do you want? Probably can't afford, but would be interested to know. I've already got a spare crank, but I'm not sure how worn it is.

Do you remember what Yamaha piston it is? You mean 12mm oversize from standard CB/XR250?

Hehehe I'd already decided I was never going to sell this bike... but I'm learning about all kinds of new options to keep me amused long into the future

[Muzz67]

All my tinkering is with twinshock XR250's and 500's, but I am pretty sure the CB motor is almost exactly the same as the 250 motors.Apart from the camshaft, no 500 topend parts are interchangable with the 250. However a lot of bottomend parts ARE. In extreme cases, 250 gearclusters will suffice in a 500 for a while. The 500 will bolt straight into an XR250 frame, but to use the 500's bolts will require drilling out the 250 frames boltholes, and using only 1 500 topmount. Exhaust and airbox need minor surgery. An XR500 carb will liven up a 250 considerably, and a 250 flywheel will do the same for a 500.
As for the 6speed box, and have found it is a much nicer box, with better ratios and smoother action .I would like $80 for it. Crankshaft is brand new, complete with bearings, conrod etc. At $150 it's still cheaper than buying new bearings alone. The 2mm oversive cylinder and piston has been run-in but not much more--$100. I used an XT350 piston and rings to make my 250 a 338cc. Cost about $300 to have the cylinder re-sleeved.

[tommorth]

thats very interesting do you knw if any of the parts off the xr350 are interchangeable with the 250?

[xwhatsit]

thats very interesting do you knw if any of the parts off the xr350 are interchangeable with the 250?

I've read that the stock jetted XR350 single carb is an excellent drop-in replacement after you fit an aftermarket freer pipe and air filter. Better top end, bottom end stays same as stock or something.

[Muzz67]

Everything I mentioned was regarding the Earlier NON-RFVC engines/boxes.
No RFVC parts interchange with Non RFVC parts.

[xwhatsit]

Everything I mentioned was regarding the Earlier NON-RFVC engines/boxes.
No RFVC parts interchange with Non RFVC parts.

Didn't they make a non-RFVC-headed 350? This was from an American site. At any rate a carburettor is a fairly standard thing.

[Muzz67]

Yes, the '70s model XL's were available in the 350cc, but they will not bolt into your CB RS. A few XR's and FT's here are 400cc. They are 500 motors sleeved down (for the Japanese market). They WILL bolt into your CB with
only minor mods to inlet/exhaust. Another plus is Ascots may also have electric start too.

[vagrant]

I had an XL250RC, not sure of the year. I put a 305 big bore kit in it. The kit had a piston, pin, barrel, a gasket set and carb jets. It was all drop in and bolt up stuff, only took a day to do.
It had instructions for rejetting the XR carb, but nothing for the pumper carb on the XL. It ran fine with any adjustments.