I just picked up an 03 Z400 and it has an FMF Factory 4 slip on muffler. The bike is still running the stock jet, and although I don’t see any signs of it running lean, I think I should re-jet it? Which brings me to my question, what size jet should I run?

Oh yeah, rest of the bike is stock except the slip on.

Thanks

 

Until you do other mods ie intake - you probably dont really need to
You could go up a couple I spose but do the airbox first and then see !

 

i can't remember the exact size but its in the 60's somewere if that hepls you, probly allready new that but o well good luck on jetting it ( pro cuircuit T-4 is better)