MIKUNI carb question and problem

[fyredude]

Recently acquired a 99 Polaris Sportsman 335.  Spark plugs foul within 45 minutes of operation.  Suspect carburation as the problem.  Keep reading that it is most likely a plugged pilot jet and needs to be cleaned or adjusted.  I have a Mikuni with ID numbers of 33F02A and underneath those there is E812.  The problem:   there is no pilot adjustment screw.  Nothing but a brass plug with a tiny hole where I would have suspected the screw to be.  There is no other adjustment other than the idle adjustment screw.  How does a person adjust air/fuel mixture on this carb?  Thanks for you help.

[Mech]

You need to drill a small hole in the brass plug(sounds like it's already been done) and screw a self tapping screw into it and use that to pull the plug out, then there is an adjusting screw under there.

It would be unusual though for the idle mixture to foul a plug after 45 minutes operation. Does it idle ? Does the choke work, and turn off properly ?Are you sure it's fouling with petrol ? Does it smoke at all ?

Edited August 4, 2022 by Mech

[fyredude]

Thanks for your response.  In response to your questions: it idles and starts fine.  The chock does work and is required to start it when cold.  The plug is heavily coated with soot after running for a period of time.  When cleaned or changed, it runs like it is a new engine.  When operating, it starts smooth then it will start to miss occasionally then the miss becomes more consistent until it almost stops running.  If I pull the plug and change it, it runs well again before a repeat performance of missing.  Gas has been changed, fuel and air filters replaced, vacuum lines have been checked and the only thing left is to change out the carb.  Thanks again.

[Mech]

Ok. That's a much more informative explanation.. It does sound like carby, not the idle but possibly the needle or main jet, or float height, but before I went pulling it off again(since you sound like you know your stuff), I'd check the spark was a good one at the plug. If the plug cap comes off the lead, you could take that off and jury rig the wire on the plug for a while, a small split pin can be poked into the wire and fitted to the plug end perhaps.

The other thing is, are you sure you have the right heat range plug for your riding conditions ? Have you tried one one step hotter ? Are you using NGK plugs ?

 

Edited August 4, 2022 by Mech

[fyredude]

Thanks again for the input.  It really helps.  I am going to try a hotter plug.  I am running NGK - currently a 7 and will try a 6.  I have acquired a replacement carb also.  When I removed the original one, I spoke of the brass plug preventing access to the pilot adjustment.  As suggested, I removed the plug.  The screw was there but that sucker will not budge.  I tried everything.  I think MIKUNI glued that thing in.  Anyway, I appreciate the help and rest assured, I will have it running like new shortly.  One problem is consistent and that is some engineer designed these carbs so you need magical powers to adjust the pilot screw with the carb installed.  Most have worked for Ford at some point.

[fyredude]

Richer mixture - counter clockwise screw adjustment?  Obviously then, clockwise would lean it out.

[Mech]

Yes screwing it out makes the idle richer.

They are mongrols to adjust alright.  I made a tool to do it.. there's a picture in here somewhere. Probably a bit much trouble for occasional use, but the recommended settings is just a starting point really.they do run nicer with the mixture fine tuned.

https://www.quadcrazy.com/atvforum/topic/16053-carby-adjusting/

[Jd101506]

Had a similar problem with my 335 actually. The hotter plug did help but I don’t think my issues were as drastic as yours. Would foul and run a little rough between oil changes. Dropping down a plug helped.