XT350 stumbles at 4,000 RPM

[scottieh]

3500-4000rpm is right where the cdi will max out and convert to steady advance , me thinks there is a connection if it will then rev past it and run normal . Or doesn't it run good up top either ?

It does run okay above that rev. It's been a few weeks since i rode it and i was going to fiddle wih the carb today, but I'll ride it and give a better report in a few hours

I did my test ride and i think this is a carburetion issue. From a rolling stop in third gear i held the throttle at 1/4 turn and let the bike slowly accelerate as it stumbled from the onset at 2,000 rpm all the way to about 5,000 rpm. To me the fact that the bike hesitates and stumbles from a slow roll at 1/4 throttle indicates not an electrical problem but fuel starvation. I will try lifting the primary needle with a few washers and ride again.

[xtzrick]

I will try lifting the primary needle with a few washers and ride again.

Add 1 washer at a time Scottie, adding too many would make it too rich which would confuse your results.

Also, having thought about it, is the secondary carb starting to open when the stumble starts? If it is, add the 1st washer to the secondary carb.

Did you check the shaft in the secondary carb for play?

Rick

[xtrider]

I'd think it would be carb issue as well with that description. It is roughly that 1/4 to 1/3 throttle that the secondary is opening , make sure it is sync'ed up correctly . If it's out of sync it'll do this weird stuff , maybe that's all that's needed instead of the washers under the needle. The 600's need 5/16" or 8mm of the primary slide open when the secondary begins to move , I usually went a little sooner , more like 1/4" , as long as both slide hit full open at the same time or very close.





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[scottieh]

I added .04" shim to the primary and .02" to the secondary. I had to disassemble the two carbs to get into the primary needle to add the shim and when i reassembled i checked the sync of the secondary carb. Do you suggest a special tool or method to check the slide height? I have a nice caliper but it's hard to get a good reading when you can't stick the caliper directly against the slide inside the carb body.

Another suggestion that came up was to advance and retard the timing by offsetting the alignment mark at TDC by a half a tooth for each cam. A half tooth towards the headers for the exhaust and a half tooth towards the carb for the intake. As a matter of fact i think that the intake cam was set a half tooth towards the carb when i disassembled. I remember seeing it wasn't straight up and thinking that to be odd. Both cams are currently aligned according to the factory spec.

[scottieh]

Took the bike out for a test ride today and the results were disappointing. The bike stumbles just as bad as before. I need to get some new plugs and see how they read when held at the point of stumble. I also ordered larger main and secondary jets and a larger idle jet. Did anyone say carb tuning was easy? Anybody?

[ActionManMelb]

Do you suggest a special tool or method to check the slide height?

I use a drill bit of the specified diameter. Just slip it in and it will hold the slide at the correct position while you adjust the sync.

[xtrider]

Yup , what he said .


Now with those changes , it should have changed , good or bad , moved up/down the rpm ect.. it really should've changed . If it's pretty much exactly the same , I'd be more inclined to it being electrical . It's always a possibility a passage is partially blocked though and the secondary isn't doing it's job or something with the slide. The timing light is a good idea too see if when it stumbles the timing goes spotty .

As far as the cam gear timing. For the most part up to 1 tooth forward or back doesn't generally cause total misfiring or stumbling on any that I've tested . You get poorer performance , better performance down low or up top , slow/faster reving, ect... but they usually don't outright stumble or misfire . More advance/retarding it does get bad depending on motor.

Each motor and cam design changes whether advancing/retarding works and where in the powerband . Some motors respond well to just moving the intake, some the exhaust , other both move one way or the other . Hard to tell till the time is taken to do each combo and test it out. Single cam motors are stuck with what they got.



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[scottieh]

Yup , what he said .


Now with those changes , it should have changed , good or bad , moved up/down the rpm ect.. it really should've changed . If it's pretty much exactly the same , I'd be more inclined to it being electrical . It's always a possibility a passage is partially blocked though and the secondary isn't doing it's job or something with the slide. The timing light is a good idea too see if when it stumbles the timing goes spotty .
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It seemed to get worse. I need to reset the secondary sync more accurately and try again. Also, even though the float needle was replaced, I'm getting gas coming out of the overflow tube when the engine isn't running. If it isn't one thing it's another. I'm waiting for some jets to come in the mail before taking the carb off the bike again.

[xtrider]

Might have to check that needle seat if the o-ring under it is leaking , common issue these days with the ethanol in everything. I go to Viton o-ring wherever I can . Otherwise the float too high , may have something to do with the running issue since the raising needles made it worse. The 600 carbs are pretty fussy with float height , more so than other carbs , I would think the 350's be the same then.





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[scottieh]

Might have to check that needle seat if the o-ring under it is leaking , common issue these days with the ethanol in everything. I go to Viton o-ring wherever I can . Otherwise the float too high , may have something to do with the running issue since the raising needles made it worse. The 600 carbs are pretty fussy with float height , more so than other carbs , I would think the 350's be the same then.
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I replaced the seat and o-ring with the valve so that is all brand new. I've heard the float height is really important too, but with too much gas in the bowl, would that not be causing a rich environment instead of a lean one. My plugs are white. One upside of dismantling the carb is that upon reassembly the idle is terrific. It holds idle very well with no choke. It would sputter and die without choke before this last test.