How to fit a 500cc into a CB 250 RS:
(My RS is a 1980 kick start only model)

I used the engine of a XL 500 and only bits and pieces of an FT:

The engine of the XL (almost) fits into the CB250RS frame.
You have to cut the cast off the upper half of the cylinder head that usually accomodates the upper engine mount (which you throw away). This is not critical. The upper engine mount was removed often on XL's as well as 250RS as it seems to aggravate the problems with leaking cylinder head splits. Cut nicely with a hacksaw and finish with a file.
Then you have to clip a very small piece of the backbone of the frame above the cylinder head. This is also not critical as you only remove the seam of the box profile between two of the spot welds.
Further you have to drill out the upper rear engine mounts in the XL 500 crankcase to 12.2mm to accomodate the thicker CB250RS mounting bolt (!)

This lets you fit the engine into the frame which is actually the easiest and quickest part of the operation.

I used a second hand OEM Honda FT 500 exhaust due to lack of alternatives. A XL 500 manifold would also be a good basis to start with but the "through frame" routing will not work on the "RS" at all. I also wanted a low slung street bike type can.
The FT ports are larger in diameter than the XL500's and dont't fit into the XL's head. I cut the headpipes and replaced them with a pair custom made from thick walled mild streel tube on a lathe :-)
The rest of the exhaust job was a number of hacksaw cuts and rotated rewelds of the manifold and expansion chamber.
A complete custom made stainless steel manifold & silencer is planned. Sometimes.
The FT 500 carb (one of the features I was most keen to use) was fitted using three of the insulator discs and o-rings that normally sit between the intake rubber and cylinder head. This lets the carb's diaphagm housing slip in between the fork of the frame at the rear end of the backbone as if it's never been anywhere else. Minor cut & weld modifications to the throttle cable attachments.
A huge problem was the airbox:
The original box was nowhere near giving a workable base to something accommodating the FT carb. Open carb or a racing filter was no option due to my determination to "TÜV" the project (I was still living in Munich at that time) which I succeeded to do in October 2000!
I ended up manufacturing an airbox from 0.8mm sheet steel which contained the XL 500 air filter.
The tank has to be raised a few centimetres in the front to allow it to fit over the 500cc cylinder head. The cleaner solution would be to cut out the tank on the right side underneath and tig weld a concave section in to give space to the cylinder head. Left side is fine. Also planned. Luckily I have a second tank in mint condition to use for this project. The restriction of steering angle, which had to be fitted to protect the raised tank can then be removed.
All electrics are standard CB250RS. I use the 250RS alternator (much lighter than both XL 500 and FT 500 flywheel and work fine without tendency to idle stall or cutoff at low revs.) as well as pickups and CDI ignition. Full advance is 45° instead of 38° on the 500's but id does not seem to matter :-)
I fitted FT 500 instruments to take the lower red line of the 500cc engine into account (and to avoid the needle of the speedo getting bend :-)

Further non engine related modifications were:
Front brake: Disc of a 1984 CB400N, LHS twin pod caliper of a VF750S, Spiegler steel braided hose.
560mm Magura M-shape bars mounted upside down (!) Looks better than clip ons originally considered and came for 5€ in mint condition out of the scrap bucket of the motorcycle breaker :-D
Self made rear sets.
Plus lots of small things.

The thing goes, vibrates & sounds nicely.
Of course I have big CB250RS stickers on the side covers :-D

acjeske

Where in SA are you? I also ride a CTX200 and have been on various tours. Maybe we can meet up?

__________________
www.trail-rider.co.za

Work just enough to live a good life - time is the most valuable currancy!

Hi Trailrider!

In the process of getting "small" wheels for the better half. Tested and bought a Suzuki 200cc Djebel yesterday. Amazing to see how much fun these little things are although I have to fold myself a bit :-)

Hi LeCap!

Fancy meeting you here...

__________________
www.trail-rider.co.za

Work just enough to live a good life - time is the most valuable currancy!

Trailrider: Busy fishing the net for valve settings for mango's 250 :-)