Honda XBR 500 motor

[JnJ]

Has anyone ever buggered about with one of these motors or is it just me ? I am building the motor for single cylinder racing. Usual stuff so far done to motor, cam, head skim, valves & springs, speedway ignition etc etc. Obviously a clutch upgrade is on the list. However it is the carb side of things I am pondering at the mo. As it is a strange style intake on the head and single carb I am considering making up a manifold to take twin 36mm carbs or similar. Has anyone done this ?
I am finding it hard to get a manual for this motor ( Haynes ) so I am in the market for one.
Any other likeminded souls on here with XBR's standard or otherwise ?

[Snod Blatter]

The '83 and '84 XR500s had twin 28mm carbs, if you want to stay kind of standard and easy. Not twin 36s I admit but still quite a lot of carb to bolt on. This was the original design for that engine and is why your head seems needlessly complicated.

Otherwise best of luck kidda, you're on your own!
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL

[wristjob]

what do the rules in the class you "race" in say?
usually they are quite strict .

[temeluchus]

You'd be better off just getting a good pumper carb like a mikuni TMX, keihin FCR or possibly a lectron than fucking around trying to jam two carbs on it.

If you are on a poverty budget a VM carb would work well.
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Some shite cruiser. Now with guns and FREEDOM!

[Robby]

If you want to cock around with fuelling in a more scientific way, the Mash 400 uses a sleeved XBR engine with fuel injection - so it may be worth acquiring that fuel injection system and running a power commander (or megasquirt if you want to be really geeky) then setting it up on a dyno.

If you want to stick with a carb, a pumper is the way to do it. Find out what the CB250RS used, and then look for a bigger one.

It's a big single, so pumping losses are a big thing and keeping the right airspeed through the carb will make a difference. Air moving slowly through two carbs is going be less efficient than air moving quickly through one in most cases, so a twin carb setup is likely to give you a narrow power band. Worth looking at how the twin carb setup was done on some of the CCMs, and possibly the 650 freewind. They had a clever linkage that opened one carb at small throttle openings and then opened the second one later on.

Whatever you do, tell us when something works. Your work tuning the XBR500 could turn into a nice blueprint for tuning a Mash 400, and they look (on paper) like they need a 25-40% power hike.

[JnJ]

Thanks for the response chaps. On the carb front I have some pumpers in a box somewhere from a Guzzi Le Mans so I will hunt them down over the weekend. I am sure a can make 1 good pumper out of the 2.