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440 round slide 34mm mikumi carbs, screw question


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#1 Swallow

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  • Sled:Ski-doo 1998 mxz 440 rave

Posted 03 January 2020 - 03:27 PM

A friend bought a sled this fall and has asked me to do some service on it problem is I have no idea what model Mikumi carbs are on this machine the machine is a 98 mxz that came out with a 670 rotax rave and was converted to a 440 rotax, each of the carbs have the normal idle screw , a brass slotted screw and another screw that had a white plastic knob, the knobs have been broken off and |I have replaced them by epoxying on new knobs, problem is I have no idea what these screws do, they are 34mm round slide carbs but no model numbers, any help would be appreciated . The sled runs but could do with some fine tuning as it's a complete pig on fuel. 



#2 Dougsnash

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Posted 22 January 2020 - 10:24 AM

I won't claim to be an expert on tuning Mikunis but I have run across some carbs where the mixture adjustment is an air bleed as opposed to a fuel bleed.  In other words, opening the mixture control more actually leans the engine instead of enriching it. 

 

Likely though, the problem you have is that the carbs from the original 670 have been re-used on the 440 with no internal changes and you are just over-jetted for the lower cc engine.  An engine is just a glorified air pump and the 440 is ~33% smaller than the 670 so therefore it needs ~33% less fuel.

 

Doug M






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