More power for CB250RS needed.

[TygerTung]

G'day. I'm racing a CB250RS in pre'82 at the moment. It goes quite well, I can go around corners at a reasonable pace in it, but it's a bit slow on the straights and as a result I get passed by the 500's etc.

What can I do to this for more power? More capacity? Can I take it out to 350 or 500?

Any help would be appreciated,

Cheers,

Sam

[yod]

throw an SC35 engine in there

[robinm]

Well the CB250RS motor is basicly just a XL/XR250 motor and there are heaps of big bore kits available for them, a common one I recall from those days was the Ballard 280 kit, have a look at the wisco site, see how big you can go? or perhaps a XR350 barrell might go on.

[TygerTung]

throw an SC35 engine in there

What is a SC35 engine? Is it pre'82?

[vifferman]

The XL/XR 500 engine has the same mounting bolts (and case?) at the bottom - it's just a wee bit taller in the top end, necessitating a bit of cut'n'shut on the tank undersides, and cutting the top mount off the rocker cover.
My XBR500RS Mutant was like that. Good for wheelies 'n' stuff.

[yod]

What is a SC35 engine? Is it pre'82?

nah im just joshing mate



CBR1100XX engine

[TygerTung]

Yeah looks like a 500 might be the way to go. Could be quite difficult and/or expensive to get though I think.

[mark247]

Yeah looks like a 500 might be the way to go. Could be quite difficult and/or expensive to get though I think.

Best thing to do would be get a donor bike... probably wouldnt cost toooooo much all up.

[xwhatsit]

XL500/XR500 is good, avoid the FT400 motor though (only one more horsepower than stock RS, shitload of weight). These days the XL/R500s are in short supply, and seem to be being snapped up for silly amounts. I'm yet to see one with an engine that looks even slightly decent go for less than $1100 or so. They're a big, lazy, blunt sort of motor. Not much more horsepower, but all the torque is available immediately.

XR350 is incompatible entirely. That's from a different era motor.

I've long thought about making a monster RS. A man on these forums told me it's possible to sleeve the engine in order to fit some sort of 350cc Yamaha dirt bike piston; combined with the RS short stroke it gives 320cc or thereabouts, but you need to machine the cases apparently. The cam is not particularly wild, I've seen aftermarket cams available in Japan. Port and polish (the cylinder head isn't very good), beefier valve springs to deal with the extra RPM.

At home, just arrived, are an XR250R 6-speed gearbox, and the single gear-driven counterbalancer (and crank) they use instead of the 250RS dual chain-driven ones.

I'm told the balancers sap a lot of power. For a road bike, pulling them out is not a good idea for longevity; on a race bike, who cares? I'd try that.

I'm also (eventually) getting a Keihin CR33 smoothbore carb from Japan. The Keihin PD70 on them stock is better than a CV, but still isn't much chop.

[TygerTung]

I did originally remove the balance shafts, but the vibration was out of control. I have never experienced so much vibration, apart from on my bucket with the 185 piston with 125 crank. The issue is with removing the balance shafts is that the crank is out of balance to account for the balance shafts, so you need to balance the crank.

The vibrations are too much even for a racebike.

[Muzz67]

Check out the thread asking about XT350 pistons, should give you some ideas. Removing the balancers isnt a good idea in a race engine as , without re-balancing, they dont rev much. A 6speed XR box is a damn good box, much better than the old 5speed ones.