A forum for owners of Yamaha TTR250 trail and enduro bikes!

Took the carb off and had a look inside the float bowl, no muck or Ethanol goo, looked pretty clean in fact. If I was being really picky the pilot jet may have had a tiny bit of restriction but hard to tell. The diaphragm looks a bit second hand so guess it’s time for a new one. The float bowl and diaphragm cover screws are all past their best so will get changed. The float bowl drain screw is well rounded off, looks very much like the one in the “how to replace” guide !! Drilling and extracting needed….

Thanks Brian, I will and I’ll make sure everything else is blown through thoroughly before it goes back together.

Good work! That's a good job done.
Your misfire certainly sounds like fuel starvation. As Bri suggested, ream the jets with copper wire (soft enough not to damage the brass) I wonder if the needle has come loose allowing it to drop too far. Also check the pumper is pumping (little squirt of fuel from the brass tube in the inlet duct) when twisting the throttle - could be a blockage, or as you suggest, diaphragm porous. Unfortunately not cheap, but if it's gone it's gone!
Worth also checking the little filter behind the float valve, but for goodness sake be careful with the float pin!!

Everything back on the bike today and it started pretty much straight away, sounded OK and off choke idled a lot better than previously. Checked the fuel level in the float bowl with a bit of fuel hose and the level looked like the diagram in the manual, but I didn’t actually measure it. I’m not 100% certain the misfire has completely gone as I couldn't ride the bike as I think the clutch plates are stuck !! It stalled when I put it in first gear and pulling the clutch lever fully in did not free things off. I put a set of new clutch plates in November 20 with a new cable and set the clutch arm up so hopefully they are just stuck due to the cold. Clutch worked fine last time I rode it towards the end of November. Sounds like you have to warm the bike until it gets really hot, stick it in gear and work the clutch lever while pushing the bike. Guessing I’m paying for not using the bike regularly…

Definitely will check that. Just been out in the garage to see if the clutch has freed off. In gear, clutch in, back wheel locks and slides across the garage floor. The clutch actuating arm, at rest, lines up with the raised mark on the crankcase which I think is correct. Will warm up the engine and have another go tomorrow.

Clutch came unstuck fairly easily today, fortunately. Warmed the bike up with the clutch lever pulled in and let it get quite hot. Once it was hot, switched it off then pushed it about in gear and it soon freed off. Can’t technically ride it on the road at the moment as the MoT is out but a quick run up and down the drive confirmed the clutch was free and working properly. It definitely idles better, far more steady but does increase as it gets hotter. Still not convinced I’ve cured the misfire so a bit more looking at is needed and then I’ll fit the new coil, lead and a new plug and see what that does. Pleased though and thanks for all the advice it really helps.