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1989 Moto 4 250 not starting.


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Picked up a 89 moto 4 250 for cheap a few years back from a friend that was moving.   It ran and drove then, but wouldn't idle without quitting.  Anyway, I pulled it out recently after it has been sitting to see if it would start with a new battery.  It cranks but won't run.  Fuel is getting to the carb from the fuel tank and I have good spark.  If I pull the plug and pour a little gas in and replace the plug, it will run for a few seconds and quit.  I figured it has to be carb related so I pulled the carb and have been inspecting it.  There is some gunk in the bowl.  The big jet looks ok, but I think the small one is completely clogged.  I am guessing this would be enough for it not to start, correct?  Am I on the right track?  I like to dig into this kind of stuff but don't have much experience with small motors.  Any suggestions would be appreciated.   I did order a carb rebuild kit on ebay with new seals and rings for putting it back together too.

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THe small  jets  being plugged are  most likely the reason it's not running at all. The big jet is the main  one for  large throttle openings . The smaller jets and holes are for idling and part throttle.. You  can  use   welders   torch tip cleaners to  poke out the clogged  jets.  Blow the passages out  with "canned air" like they  use for  dusting out computers .

 

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Yes definitely  clean the tank too.

Get all the old gas and  any sludge that settled out of it  out .. I  have an  ol tiller that  gas was never drained out of  and it sat for years . There was still liquid in the carb  but a sticky  goo had settled out of it ..  The  seat  for the float pin was plastic and  swollen up as well as jets and passages were plugged .. I ended up  replacing the  carb.   I had to  completely fluch the fuel tank because it also  had that sludge in it .

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Ok.  Got the carb cleaned and back on.  Took the fuel tank off and cleaned it.  It will now start and stay running, but it coughs and won't keep a steady rpm.  Some backfires too.  Do I need to adjust something on the carb?

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I  searched and like you didn't find a great video or instructions for adjusting your  carb , but this exploded view parts list will help you identifying the parts. The idle adjustment looks to  be the screw by the fuel  inlet.   The coughing , backfiring and uneven running is most likely caused by running lean. It is either a setting  out of adjustment or there is still a partial  plug  somewhere..

I hope this  helps.

https://www.partzilla.com/catalog/yamaha/atv/1989/moto-4-yfm250w/carburetor

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  • 3 weeks later...

Quick update.  I got it running much better but it is idling high.  The idle screw doesn't seem to have any effect.   I checked for an air leak and can't find any.  Could it be the something with the throttle?  Maybe the slide is getting all the way to the bottom?  Any ideas would be appreciated.

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