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Author Topic: 250 Single - Performance Upgrades  (Read 1490 times)

Offline paddyshepherd

  • Posts: 71
250 Single - Performance Upgrades
« on: Apr 12, 2018, 09:26:18 »
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
 

Offline teazer

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Re: 250 Single - Performance Upgrades
« Reply #1 on: Apr 12, 2018, 11:05:22 »
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
« Last Edit: Apr 12, 2018, 11:11:26 by teazer »

Offline crazypj

  • Posts: 13199
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Re: 250 Single - Performance Upgrades
« Reply #2 on: Apr 12, 2018, 12:13:06 »
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
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline teazer

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Re: 250 Single - Performance Upgrades
« Reply #3 on: Apr 12, 2018, 12:32:56 »
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 

http://www.visordown.com/forum/general/cb250rs for comments on the swap.

Offline paddyshepherd

  • Posts: 71
Re: 250 Single - Performance Upgrades
« Reply #4 on: Apr 12, 2018, 13:40:37 »
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.

Offline crazypj

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Re: 250 Single - Performance Upgrades
« Reply #5 on: Apr 13, 2018, 11:08:12 »
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
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline paddyshepherd

  • Posts: 71
Re: 250 Single - Performance Upgrades
« Reply #6 on: Apr 13, 2018, 12:44:19 »
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.

Offline teazer

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Re: 250 Single - Performance Upgrades
« Reply #7 on: Apr 13, 2018, 13:33:57 »
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.

Offline crazypj

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Re: 250 Single - Performance Upgrades
« Reply #8 on: Apr 13, 2018, 15:13:21 »
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?
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline teazer

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Re: 250 Single - Performance Upgrades
« Reply #9 on: Apr 13, 2018, 17:01:44 »
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.