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I'm running a 2002 Xc 700 sp
it has 500 main jets my plugs are dark brown and a tad oily when I remove them
just bought 490 jets
by going down 1 size am I in danger of "holeing" my pistons?
I don't think so but any input would be helpful

 

[99Indy500]

I wouldnt think so

 

[xc007]

Man, I can't remember the specifics, but I know that in one of the last couple Snow Techs, they had an article about the Mikuni jet sizes right around the 500 mark being a little off.  Meaning you think that one size is too lean, so you go up one which is actually leaner (could be real trouble).  I think it might be the 490 to 500 jump meaning the 490 is actually a little richer than 500.  Anyone else remember the specifics?  I'll take a look to see if I can find that article again.

Either way, watch the plugs and ideally get bend-a-light and look for piston wash.

 

[svanwormer]

Actually I just read the article 2 days ago.  It was the 520 jet that was actually leaner than the 510.  It has to do with the numbering system used by Mikuni that the smaller size jets overlap the larger sizes (for some reason) in the flow rate.  The specs for a 700 XC VES went from a 510 to a 530 for colder temps for that reason.