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Discussion Starter · #1 ·
I’ve recently bought a used trail boss 250cc and we’re having trouble even getting it started sometimes it’ll turn on and rev but soon as you let go it won’t turn on we’ve tried changing the carb we’ve cleaned the fuel line we’ve tried starting it with a battery and from the pull cord and it won’t start up at all any ideas?
 

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Discussion Starter · #2 ·
Also noting we have changed the recoil spring with a new one it’s definitely catching a fire as we get smoke from the carb and she’s offing we’ve put in new spark plugs right etc can’t seem to find a issue anywhere
 

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2004 Sportsman 700, 2004 Scrambler 500, Various Others.
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Is it shutting down electrical power or just bogging out and dying?
 

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2004 Sportsman 700, 2004 Scrambler 500, Various Others.
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Got it. When you say it “turned off” it seems like it just loses power/spark. Anyway... check the etc (electronic throttle control) under the cover on the box above throttle lever. There are two contacts under there that are supposed to cut spark if your throttle gets stuck open. If your throttle cable is misadjusted enough they will always be touching when throttle is released and cause a no idle but will run on throttle condition. There should be I believe an 1/8” of throttle play from idle to when it starts moving the cable and it feels like there’s resistance. Also if you have the original mikuni carb, ether rebuild it or have it professionally rebuilt and put it back on. I don’t trust the knock off Chinese carbs with any American made machine especially 250cc or above.
 

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Discussion Starter · #7 ·
Got it. When you say it “turned off” it seems like it just loses power/spark. Anyway... check the etc (electronic throttle control) under the cover on the box above throttle lever. There are two contacts under there that are supposed to cut spark if your throttle gets stuck open. If your throttle cable is misadjusted enough they will always be touching when throttle is released and cause a no idle but will run on throttle condition. There should be I believe an 1/8” of throttle play from idle to when it starts moving the cable and it feels like there’s resistance. Also if you have the original mikuni carb, ether rebuild it or have it professionally rebuilt and put it back on. I don’t trust the knock off Chinese carbs with any American made machine especially 250cc or above.
She don’t run so I don’t know if this will help at all she the motor isn’t blown since it’s giving compression and sparks up etc and she’s getting the fuck compression for sure
 

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Discussion Starter · #8 ·
She don’t run so I don’t know if this will help at all she the motor isn’t blown since it’s giving compression and sparks up etc and she’s getting the fuck compression for sure
And it’s the stock carb and has been cleaned out
 

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Ok you said originally that it ran with throttle input and also that you replaced the carb, not cleaned it. Put some starting fluid down the throat of the carb with wot and see if it fires. If it does, and it should if you have compression and spark, then you have a fuel delivery/carburetor problem.
 

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Discussion Starter · #10 ·
Ok you said originally that it ran with throttle input and also that you replaced the carb, not cleaned it. Put some starting fluid down the throat of the carb with wot and see if it fires. If it does, and it should if you have compression and spark, then you have a fuel delivery/carburetor problem.
It starts up long enough to not even rev her once she just dies out then you have to keep pulling the cord and waiting and waiting for it to offer up. We get spark and we got the compression for sure cause she does back fire and the spark plug was brand new as well
 

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So it’s a fuel issue then. Back out the screw on the bottom of the bowl for the overflow drain and see if fuel comes out. If so, you have a clogged pilot jet, if not, you have an air locked fuel pump, a stuck float, a misadjusted float, clogged petcock, or some other issue that’s not letting fuel in.
 

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Discussion Starter · #12 ·
So it’s a fuel issue then. Back out the screw on the bottom of the bowl for the overflow drain and see if fuel comes out. If so, you have a clogged pilot jet, if not, you have an air locked fuel pump, a stuck float, a misadjusted float, clogged petcock, or some other issue that’s not letting fuel in.
No it gets a spark even backfired at once and we even shoved a little hose and filled inside of the carb and gas line with fuel and still nothing
 

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No it gets a spark even backfired at once and we even shoved a little hose and filled inside of the carb and gas line with fuel and still nothing
If you don’t have enough air moving through the carb you wont get fuel pulled up into the Venturi. Bad crank seals will cause a loss of vacuum in the carb because it’s easier to pull air through bad seals. Could also be reed valves and it’s pushing air/fuel back out the intake. The engine also needs a place to send all the air it pulls in, so if your exhaust is plugged it will be like losing suction on a shop vac when you put your hand over the exhaust. Suck bang blow
 

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If you don’t have enough air moving through the carb you wont get fuel pulled up into the Venturi. Bad crank seals will cause a loss of vacuum in the carb because it’s easier to pull air through bad seals. Could also be reed valves and it’s pushing air/fuel back out the intake. The engine also needs a place to send all the air it pulls in, so if your exhaust is plugged it will be like losing suction on a shop vac when you put your hand over the exhaust. Suck bang blow
there are no reed valves, ignore that
 
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