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Post Info TOPIC: Part Throttle Misfire


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Part Throttle Misfire
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Hi Folks, looks like my bike has developed a misfire on part throttle, the first part. Starts OK, always has and I’ve not had this before. I usually turn the fuel off and let the float bowl empty after a ride as the bike can stand for a while not being used. Not good, I know ! I had a look at the float bowl drain screw and it’s chewed up so I could not open it to see if there was any muck or any water in there (from the Ethanol). I’m going to take the carb off and have a look inside and apart from the obvious, if there is anything obvious, what else should I look for ? I had the carb ultrasonically cleaned and rebuilt a couple of years ago and it’s run fine. I’ve had a look at drain screw replacement guide, hopefully it will come out without the need for an extractor. I think I’ll replace the coil and plug cap too, can’t hurt.



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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….

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Don't forget to ream the pilot jet with a bit of copper wire to make sure you got all the crud out wink

Brian



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

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Got the rounded off float bowl drain screw out, so that’s one problem out of the way. No spring came out so I can assume it would have been the original drain screw. Followed Brian’s how-to guide and fortunately the extractor did what it was supposed to do. The new one will be going back in with plenty of anti seize stuff and regularly moved. Now I can get back to trying to fix the original problem.



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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!!

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Yes, well pleased thanks. I’ve never used an extractor, or easy-out before either so that was a first. I’ve blown all the jets and passages through and gently poked them with some wire too, as advised. I’ll check the needle too, not something I would have considered. I have a rebuild kit with a new diaphragm so hopefully when I get it all back together that will cure the misfire. I also have a new coil, plug cap and plug, just in case. Sadly the cold beat me today in the garage.

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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…

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If it dies when you put it into gear, check the sidestand switch if you have one - if it's sticking it will kill the engine.
Two stuck clutches on the forum in a week. Nothing for years beforehand. Weird!

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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.

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OK Certainly sounds like stuck plates. You may end up taking the side cover off and prising the plates apart like Wiresharkttr
ttr250.activeboard.com/t68121582/hello-everyone/

Good luck!

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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.

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Good news!
Always good to talk things through - even when noone has any bright ideas it helps get your head round things and shows you're not alone!

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It sure does, always helps !



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