XS250 idle / mixture problems

[Nick Frost]

Hi to everybody. My first posting here. I've recently got an XS250, 1978-ish, I think. It starts OK, idles at 1000-ish rpm for a little while, and then decides to idle at 4,000 or 5,000 rpm! Clearly a carburetion problem (?), but I'm new to motorcycles and am not sure where to start. I have the Haynes manual and the bike has what the book refers to as the "lever choke type" carburettor (fig 2.1, p60). At least I can get as far as confirming that this is not as simple as needing throttle cable adjustment - the throttle control arm is resting against the end-stop.

A friend said that the mixture may be wrong, eg. running too lean, and suggested that I adjust the pilot mixture screws while it's over-revving and see if that makes any difference. It didn't seem to but I did observe that the pilot screw on one side was almost all the way in and the other one was about four turns out - a suggestion that someone has previously tinkered, perhaps erroneously...?

I had a brief tinker with the remote throttle stop screw and that didn't seem to solve the problem either - I could reduce the tickover, but when I briefly opened the throttle, the idle would again go up to 4,000-ish rpm.

But what I DID notice when I was adjusting the throttle stop was quite a lot of suction through the RD carb (I had removed the air filters and intake to get at the throttle stop) but very little through the LH side. And the LH carb intake was damp with fuel while the RH was dry.

I should have mentioned that the bike has not been running for 6 years or so. And that both plugs are somewhat sooty, suggesting over-richness I believe. Since first writing the above, I bit the bullet and started taking things apart. The throttle barrels were both quite grimy and it's perhaps possible that one or other of the butterfly valves was simply not closing fully (when warm..!). Any suggestions on cleaning other than a "cotton bud" dipped in petrol?

Further inspection also found one of the floats had a tiny hole and was floating lower (in a jug of water) than the other. I guess that ain't gonna help neither! Could it be that the differently adjusted pilot jet screws were to compensate for the different fuel level in the two float chambers..?

And then I discovered that the petrol tap was malfunctioning, due to the four-holed rubber gasket inside the tap being all mangled up - meaning that fuel was constantly being supplied under gravity (as if permanently switched to "Prime") instead of via the vacuum diaphragm, I guess that won't help either.

I'll see if Fowlers can supply new floats and tap gasket tomorrow, then put it all back together and see if anything's changed a jot! Any suggestion on how to set the pilot jet screws?

While I'm on. Much less critically, I'm trying to re-fit the indicator units which had all been broken off! the units themselves are fine, but not the mounting. I need new collars, dampers and hollow studs. Fowlers have helped with the collars and dampers but say that the studs are discontinued. Any bright ideas? The studs are M8 x 50mm with a 3mm hole down the middle (for the wire to pass through).

Any advice will be very gratefully received!