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Should I rejet the carburetor?

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Arggg! 96' 250 Trail Boss carb issues, jet sizes?

1142 Views 6 Replies 4 Participants Last post by  DBAP79'
96' trailboss 250. I am at the end with this machine, please help! I have gone through the carb repeatedly, all passages are clean. Checked timing, ETC, tank vents and oil pump function. Put a new top end on, bottom end still seemed tight and moved smoothly. Replaced fuel filter, fuel shutoff, air filter, spark plug. Exhaust pipe is clean. Adjusted needle settings to extremes and settled back on factory settings again. Some things have helped (some a lot) along the way but ultimately it does the same thing, boggs down after a short distance on the throttle. If you give it a few seconds to catch up it will often pick up again for a seemingly random amount of time, then bogg again. When it is making power it is great and throttle response is a lot better than when we first got it. But the bogging is just destroying the feeling of any progress or joy in riding it. I am leaning towards rejetting the carb at this point but not sure what size range to get in a kit for 60/40% - 4000/8000' in altitude (rocky mountains)? If you've got any info on jets or suggestions on other things to check - PLEASE! - Let me know.
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Sounds to me that the carb is running low on fuel and when it "catches up" (fills again) then it will run until it gets low again.

Assure the float bowl vents are not obstructed and the fuel tank is venting properly. If that checks out, then check for an obstructed or kinked fuel line, an obstructed petcock or clogged fuel filter.

It may be an over heating issue and it may ultimately end up being an ignition issue.

I had a KX125 Kawasaki that was completely rebuilt (engine, suspension and all bearings) - it ran great most of the time, but it was temperamental. Some days it was hard to start, other days started easily. Some times it fouled the spark plug repeatedly and then it would run for weeks with no fouling (it was pre-mix so I know the oil to fuel ratio was not changing, I always use the same gasoline, oil and ride at the same appx. above sea level) - in a final attempt to solve the issues, I replace the one part that was still original - the stator. Immediately the bike started better, had more power and no longer fouled spark plugs. The stator solved the temperamental issue.

Let us know what you figure out, but I think you can quit fooling with the carb.
"Sounds to me that the carb is running low on fuel and when it "catches up" (fills again) then it will run until it gets low again."
That was exactly my thought and why I spent so much time on fuel supply and carb. I have given serious thought to replacing the stator as the bogging is so random. One run up a hill it goes 1/4 way boggs, next is 3/4 way, then right off the start. But when I do an open spark test the spark seems very strong. When I first checked the timing it was 100% (max adjustment) advanced. So I think someone who didn't have much mechanical experience may have worked on it previously so who knows what I'll find once I get the flywheel off. Never had this much problem on a machine before. I've been a mechanic in one form or another (automotive, construction, forklift, marine) my whole life. Can't believe such a simple little machine is giving me such a headache.
Electrical problems can manifest in so many ways that it can make you think it's something else and with spark advance built into the CDI box, it can advance properly one time and a little heat or cold can make it advance incorrectly or not at all a little later.
I went ahead and replaced the stator and cdi. I got to thinking about it and the best it ran was during an ice fishing trip this winter. I fully charged the battery before that trip. It ran great day 1 (which was a very cold /windy day) then it started bogging again day 2 (warmer no wind). I'm not sure if the full battery was covering for a struggling stator & battery was run down by day 2 or if the cold was helping out a hot cdi. Either way, my charging voltage is a little higher now (was 14.3 v now 14.8), my timing is set back to the factory marks (was fully advanced when I got it, but bogging was worse when I set to factory) and the bogging is gone. My boy finally got to climb hills on it all afternoon without issues. Fun day!
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