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Carburetor help! 1975 Yamaha DT250B

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It's a Mikuni VM28SS.  It was running great, but it has been sitting for a month while I did some work on the tank.  Put it all back together with new fuel line, a new fuel filter, and fresh gas, went for a (short) ride and discovered a problem.  It bogs down at anything around 1/2 throttle or more.  Starts, idles and putters around perfectly, but when I give the throttle a twist, it's almost as if it's not getting fuel.  I let off the throttle and all is well again.  I'm hoping I don't have to tear into the carb as those things have always scared me.  Magical devices that have never responded positively to my meddling!  Any advice would be appreciated.

 

Jeff

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You have a plugged main-jet 3p:

 

Easy fix.  On the bottom of the carb is a hex-plug and just inside the hex-plug is the main-jet.   Number 27 and 36 on the first diagram of this manual:

 

http://www.mikuni.com/pdf/vmmanual.pdf

 

Often you can just remove the hex-plug (shut the gas OFF, and let the float-bowl drain into a rag)  and  just blow the jet clean with a pressure can nozzle of carb-cleaner but if that won't work you can loosen the carb clamps and rotate the carb to work on the bottom.  Take off the hex-plug ...just inside is the main jet. Unscrew it with a short flat-blade driver and clean it with a broom-straw or something soft.   When it's clean, screw it back in place gently.....

 

Worst case scenario is you have to remove the float-bowl to get at the main-jet but it's an easy job and a common problem after a storage period.

 

good-luck

 

patuca

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update:  Not the carburetor.  Fuel delivery is fine, no plugged main jet, no plugged anything.  Had a friend far more knowledgeable than I take a look today and we're leaning toward an electrical gremlin now...

 

Jeff

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