250 Single - Performance Upgrades

paddyshepherd

Active Member
So I've stripped down the motor in my CB250RS.

I now have 3 options:

a) Freshen it up with new seals etc and bolt it back up, run it how it is (it's in good condition, we initially only stripped it due to it having sat for a long time. It did actually run)

b) Try and add a few ponies while I'm in there.

c) Swap it out for the 500 motor - this would be great, if I can find one......

I've rebuilt engines often enough in the past, especially ones as simple as this old thing. I've not however, really delved into the tuning aspect of it, so I'm after some advice!

It'll be getting a completely custom exhaust system and I'm considering upgrading to a bigger carb too.

First of all, I'm kind of struggling to think of a carb to use that will be suitable.
Secondly, I can't find anywhere doing aftermarket cams, so if I want to go that route I'll have to get a regrind done.

I'm unsure on what to with the displacement side of things, though. Unfortunately nowhere seems to still do big-bore kits for these things. The motor is pretty much the same as the XL250 and XR250 of the same era, but IIRC a better tuned version of it (did those bikes use the 4 valve head? I'm assuming they did). I'm able to find slightly bigger pistons from Mitaka (up to +1.5mm) but that's about it. No "high comp" pistons that I've seen either. Will an XR or XL big bore kit work? I'm of the impression they were lower compression motors so not sure how much benefit it would be.


I'm in the UK and these kinds of parts are difficult to find - I do however have friends in both US and Aus that could ship parts across if I find any there.

Cheers
 
Assuming that the motor is the same as an XR of similar vintage, it should be possible to get a cam from Megacycle in the US or someone like JOY or Kent in the UK. Years ago I had a piston that came in a pile of other parts. It was supposedly a 350 conversion for a 250 single, but I can't swear if it was an older XL or later 4 valve model.

The GoTo place for that sort of stuff used to be Powroll in Oregon. Not sure if they are still in business. http://www.xlintperformance.com do XL parts. They may have something suitable. The cams they list are megacycle.

some ideas on an XL http://www.xlintperformance.com/BillBellsBajaBulletComplete.htm

Would this fit: https://www.ebay.com/itm/HONDA-XR-250R-WISECO-PISTON-KIT-10-5-1-COMPRESSION-78MM-86-04/182610747534?fits=Make%3AHonda%7CModel%3AXR250R&epid=171182752&hash=item2a8472e08e:g:ZNQAAOSwxvxW8Wrz

http://www.nichecycle.com/ncs/categories/engine/pistons/127918-honda-xr250-piston-kit-270cc-76mm-10-5-1.html

http://www.wiseco.com/ProductDetail.aspx?ItemID=4466M07700&ModelID=428&ModelYear=1995&AppID=4866

http://thumperracing.net/index.php?route=product/product&product_id=70

https://procycle.us/bikepages/xr250-xr400.html
 
Pretty sure none of the RFVC or XR250R stuff fits? (been a long long time since I messed with them)
A flat top piston with increased dia is worthwhile, particularly if you can find one with 2~3mm higher crown and a 'low' top ring placement (turbo pistons) Then you can machine top to combustion chamber profile
 
Correct PJ. It's an earlier XL design, but a later XR would probably drop in. And when you start looking at web sites for one model, often you find parts for earlier or later derivatives, so the search may lead to a suitable solution. We had an early XL250 that was sold before I tore into it and my son had a later XR. It's been a long time since I messed with them too.

Here's one with an older XL500 motor https://www.youtube.com/watch?v=oaJMmllW5eM

http://www.visordown.com/forum/general/cb250rs for comments on the swap.
 
Thanks for the info guys.
Teazer, there's some great links there, I'll have a trawl through them when I get this uni work out the way.

I've seen that video - unfortunately engines like that are pretty damn rare over here now.
 
IIRC, it's same engine as XL with 23" front wheel?
Later (and earlier) XL/XR motors are 'shorter' with or without balancer shafts.
Bigger carb, decent exhaust and bore will make it 100 mph bike.
It can get real expensive if your swapping liners and boring cases to get ~400cc
 
crazypj said:
IIRC, it's same engine as XL with 23" front wheel?
Later (and earlier) XL/XR motors are 'shorter' with or without balancer shafts.
Bigger carb, decent exhaust and bore will make it 100 mph bike.
It can get real expensive if your swapping liners and boring cases to get ~400cc

It's the same engine but different states of tune etc so I need to work out what parts can be swapped over etc.
I'm about to send of an e-mail to XLint and see what they can do for me. 100mph would make me a happy man. As long as I have a little bit of grunt with good response, I'll be happy. I just don't want to put it all together and find I'm underwhelmed by the power.
 
What about a later model XR motor or CRF or whatever the latest models are called? That assumes there's one available from a wrecked bike and I suspect there are more blown motors than wrecked chassis, but who knows. There was also an XL350 that's likely to be worn out by now, but might be a better starting point.

Your motor looks remarkably clean inside, so I'd be searching for a slightly larger piston and cam and spring kit and come up with a moderate cost Moto3 bike. With a short stroke it will never be a stump puller, but you could make it go pretty well.
 
The smaller XR/CRF is based on the old (very old, 1971) CB125.
As you say, dirt bikes rarely get the maintenance they need, lot more rolling chassis than good engines about
As there isn't anything wrong with 250 motor it makes more sense to me to just 'fix' it?
 
For sure, and the cam bearings looked remarkably good in the photo in the other thread.

Stage 1 Megacycle cam to let it breath a bit, maybe clean up the ports and give it a valve job and slip in a 1mm to 2mm larger piston to freshen it up would all work together to give it a little more get up and go.

Alternatively, save some cash now and make the motor pretty and leave it stock, then focus on making the chassis light and the suspension work. I'd probably leave the front forks for now and save the Maxton dollars for later upgrades. I was brought up up to believe that the way to better performance is to add lightness AND add power. Make it reliable and get the most out as few dollars as possible.
 
Cheers Gents.

You're right Teazer - it is immaculate inside. I guess I'll just do oil seals and gaskets for now then and consider more power internally at a later date. I might give megacycle a look though as the cam does have a little bit of pitting on it. In that case, I'll just be looking at carbs and exhausts for now.

The stock carb is a Keihin PD70 - 33mm carb with a straight outlet (i.e no venturi). How do I go about deciding what would be a worthy upgrade?
 
Flatslide off a modern MX 250? (probably a bit big?)
Should be easy to find as maintainance was low hours on early models (RMZ 250 was only 30 hrs on crank, CRF dropped valves pretty regular, etc)
Maybe one out of a 600/750 inline 4 set, pre-2005~6? (before they all went fuel injected)
 
For a carb, I'd either look at a VM32 Mikuni, or if you wanted to stay with a CV, then the stock 35mm off an FT500 Ascot would work. The stock carb off an XL500s is a 32mm slide needle Keihin. The 250 motor and the 500 motor are almost interchangeable. I think I heard the 500 is a little wider in the rear for the motor mounts and won't quite bolt into the 250 frame without some slight mods. None of the RFVC single parts interchange with the earlier XL or RS motors and none of those parts interchange with the first generation XL250 or 350 motors. 3 pretty distinct families of engines. I have looked at it, but not measured anything out, but I wonder if a 500 crank and top end would work with 250 cases?
 
VM32 would be OK if you could get it with even close to correct parts. By the time you change just about everything except body and float, it's cheaper and easier to use something more modern. Using CV you can use a much larger carb but not have to learn to ride all over again, using a mechanical slide carb means you need to be a lot more careful not opening throttle too far too fast at lowish rpm, slide and jets need to be a lot more carefully matched to engine
 
ex119x said:
For a carb, I'd either look at a VM32 Mikuni, or if you wanted to stay with a CV, then the stock 35mm off an FT500 Ascot would work. The stock carb off an XL500s is a 32mm slide needle Keihin. The 250 motor and the 500 motor are almost interchangeable. I think I heard the 500 is a little wider in the rear for the motor mounts and won't quite bolt into the 250 frame without some slight mods. None of the RFVC single parts interchange with the earlier XL or RS motors and none of those parts interchange with the first generation XL250 or 350 motors. 3 pretty distinct families of engines. I have looked at it, but not measured anything out, but I wonder if a 500 crank and top end would work with 250 cases?

The 500 swap is very easy on the CB250RS, slight modification to top engine mount and frame apertures need opening slightly to fit larger bolts. The hard part over here is finding a complete motor in the first place. I did wonder myself if the top end of the 500 would fit on the 250 cases - the bolt pattern looks correct but I guess it depends on oilways and clearences etc.

Will have a look at the carb suggestions by both you and CrazyPJ.

Not got a lot of time at the moment due to university commitments (exam time!) but fitting it in as and when I can.
 
There must be a few 500 motors around, one was fitted to the CJ250 stolen from me a few years ago (you are in Britain?)
 
The FT500 Ascot, the XL500S and the XR500 all have the same basic motor. Th RFVC stuff is next generation and all different.
 
Not a single engine from any of those bikes coming up on eBay at the moment - since not many of the parts are available its better money to split the engines and sell each part separately.

Best I can find at the moment is an XR500 casing set with crank, gearbox and cylinder barrel. Would have to piece it up from there.
 
When CJ was stolen/recovered around 2010 (not worth returning to Britain for at the time, I only paid £30.00 in 1986)
I started looking around for a FT/XL/XR 500 motor. There were none available in USA at reasonable price. I take a look every so often, they are not common on eBay USA so will be far less common in UK. I don't think Americans realise the UK market has less 'restrictions' so a much greater diversity, there is 'a bike for everyone' of some capacity, manufacturer, etc but only so many people to buy them.
In 1980 I bought a 197? Yamaha DT400MX. Turns out, only 90 were ever imported into Britain. The market for 'large' trail bikes has always been limited, the conditions favour lighter bikes (much easier to haul out of a bog ;D )
 
So... I've been kinda sat on this for a while pondering over my options, I haven't really touched the bike itself much recently.

I think for now I'll stick with the 250 unless a good deal comes up for a 500 motor or complete bike.

I'll probably fit a high comp piston (I've found a 10.5:1 in standard bore, stock is 9.3:1) for now until the engine actually needs a rebore.
I'll get the cam reground with a slightly more aggressive grind (more of what we'd class as "fast road" set up)
The exhaust will be completely custom, either stainless or Ti. Will probably run open "shotgun" pipes out one side with an internal baffle.

It will never be a flying machine, but I think with some good tyres and a bit of a diet, the above should help it out somewhat.

I've found a good deal on a Keihin CR33 Carb to import from Japan. Comes with adapters and whatnot to fit the 250RS as they were much more popular over there. Also comes prejetted. Will still need a bit of a play around, but should be a good base point in my mind. I have the option of the standard kit or the Bito R&D Tuned version - I'm currently trying to find out the difference, but as all the info is in Japanese, it's proving a little difficult. I've e-mailed Bito to ask them and currently awaiting a reply.

What do we think? Depending on well how the carb works I may fit a quick action throttle too.

I recently rode a stock RS and it did bother me how unresponsive the stock carb felt.
 
Back
Top Bottom