My Yam WR450 has ben ultra-reliable since I bought it a few years ago, despite what Fats25 will tell you. It has regular oil changes and air filter cleans, little bit of maintenance here and there and all's good.

Recently when we went out however, it was running like a pig. Wouldn't idle, running rough, kept stalling. Got it him again and stripped the carb, expecting to fins some muck. Instead, I found that the entire pilot screw assembly, including spring, washer and O ring, was missing. Gone. An ex-pilot screw!

It's a Keihin FCR carb. So I called Allens and ordered a replacement, which I then fitted. Did that, took it out again, and although improved, it's still not right. I've tried adjusting the pilot screw, using the "in until it cuts out, out until it starts to run rough, screw back in halfway between the two points" method, but actually screwing it in hard, the engine still wouldn't cut out.

When I use the bike now, it will still sometimes stall when I stop, but most noticeably, when I have the throttle closed but still moving, eg going down a big hill, the exhaust pops and bangs like a good 'un, which it never used to do. If I put my hand by the exhaust when it's doing this, the pops are red hot, ie presumably unburnt fuel igniting somewhere. And the other thing is, the bike seems to be running really hot - at standstill I can feel the heat off the exhaust on my leg, again which was never the case before.

The air filter is fairly new and fully oiled. I guess I could try putting more oil on it, but my tried and tested method of pouring some filter oil into a sandwich bag, putting the filter in, tying the top and squeezing all the oil around the foam, has always worked in the past, so why not now....

I'm not a carb/fuelling guru, but everything does seem to suggest that it's running lean and hot. BUt any suggestions, advice, etc would be muchly appreciated, ta.

What does the plug look like?

Have you checked your valve clearances?

Thanks all. Valve clearances is next on the list to check. I've heard some odd tale about valves stretching & dropping on these single-cyl motors, so apparently it's always worth changing them if the clearances are out.......?? Plug was fine. Float height hasn't been changed nd presumably it would pi55 fuel out if that was wrong.

So I'm thinking valve clearances and check whether there's a different pilot screw size available. If not - full carb overhaul of all jets, and valve clearances. Next stop new bike carb?

No, the bike gets used a lot, plenty of fresh fuel in use. The odd thing is that it happened so suddenly - which makes me question the valve clearance option, although I do plan to check them.

I would guess on it being the other way and the engine running lean.

You can get pops 'n' bangs from an engine running lean and your exhaust will get hot due to the increased temperature from lack of fuel.

Is the engine running hot?

As others have said is the pilot jet the correct one?

Check float height as if it is set wrong and not letting enough fuel in then it wont be p***ing out of the carb.

Also what colour is the plug? Picture of it? Ideally after some running and not just idling for a while!!

It only pisses out if the float is stuck, just at the wrong level only causes running problems.

That's lean as fook! the white bit should be a biscuity light brown colour.

Being a tight fisted git (hence I moved to Yorkshire ) I'd strip it down & check all the jets etc against the manual to see if I could see why it's running lean. Considering the pilot jet dropped out I'd also be looking if something was loose & allowing an air leak to lean it up like that.

Carbs can and do 'wear out' sometimes - and no amount of fettling and re-building can sort them - as my story below thus:

I had a Yamaha YZF 750 converted to 1,000cc by Pete Beale Racing. Involved fitting a 1,000 exup top end to my original YZF casings and gearbox. He also fitted a good set of used 1,000 exup carbs.

All fine for around 5,000 miles.

Then, it started to play up a little - a bit like what you're getting.

I took it to a specialist, who spent a day overhauling the carbs, fitting a Dynojet Kit, and setting it up on a rolling road.

It was better, but not perfect.

I went off round Spain on it. Halfway through the holiday, it really began to mis-behave, like yours, and at the end of it on the way back to England, became so recalcitrant it was difficult to ride.

When I got home, I ran it back to Pete Beale.

We checked all the usual things first - plugs, compression, valves - all fine. We stripped the carbs - could see no obvious measurable wear or damage. We tried all new jets, emulsion tubes, settings, balancing - all to no avail.

In the end, despite not finding anything in the existing carbs to indicate a problem - we fitted another good set of carbs.

The bike was instantly transformed - perfect running, strong linear power.

We agreed that we couldn't spot anything in the old carbs that would be an obvious problem - and so surmised that they were just 'worn out' - maybe the slides had got too much wear on them - who knows?

But I do know I spent lots of time and money chasing a problem that could not be fixed on one particular set of carbs - that was instantly fixed by fitting another set.

Whether that's your problem too - I don't know - but maybe before spending lots of time and money like I did - it might be worth trying another unit first?

HTH