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Discussion Starter · #1 ·
Im just wondering wether someone could give me some advice i have a 4 stroke 125 and ive been trying to figure out whats wrong with it i can get it idling fine but as soon as i give it quick throttle theres no power just sounds like air surging out and also cuts out sometimes but if i slowly increase revs it will rev up to 6-7 k and stop revving im really confused with what to do with it dont have a manual or anything to find out carb settings unfortunately . The bike ia a derbi senda baja sm 125 2009 if anyone can give me any advice it would be much appreciated . Thanks
 

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That describes a very lean engine. Sounds like the jets are partially clogged, or the fuel bowl is not filling properly.
 

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Lack of fuel. Try spraying Starting Fluid (ether) in the intake as you blip the throttle. if the Faux fuel allows it to easily rev, you've confirmed it's simply not getting enough gas.

If you're jets are clean then I'd check your fuel VOLUME flowing from the tank. Sounds like an obstructed line, clogged fuel filter, etc.

Could be something rarer, like a float bowl height incorrectly set, but...seldom seen. (Someone would have had to go in there and misadjust it)

You might also have a vacuum leak (disconnected emissions plumbing?) causing you to run lean.

Taking the air filter off, etc. will reduce the amount of vacuum in the venturi and the amount of fuel sucked IN through the jets. Make sure everything is stock and properly plumbed.

The idle jet provides fuel both at idle AND into transition. If it's too lean it won't go from idle smoothly to high RPM. If it simply won't run at high rpm then your main jet is clogged.
 

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Discussion Starter · #5 ·
Yeah im not too sure about float height adjustment aint really been touched i have played with fuel/air mixture screw so i dont know if that could be it also there is no manual for the bike so i dont know original settings its also on 3rd clip down on needle position my girlfriends brother is more mechanically minded and he said last resort is the cdi is misfiring the spark .
 

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So, how long did it sit? You may have clogged passages leading to or from the jets, or the float valve itself could be clogged. If it sat long enough, the float valve may be sticking near the closed position, too. What I'm getting at is, you may have to remove the carb and give it a good soak.
 
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