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I've just opened up the intake and drilled out the exhaust on my 05 crf250x. I got the JD kit. All runs just OK. Some issues at idle to 1/8 throttle. Stock was a 40 slow jet, JD kit had a 42 with 160 main, Bike ran hotter then stock(lean). I put in a 45, the fuel screw made no difference. also still hotter than stock. So I went to a 48 with a 165 main. Runs very good with 1/2 turn on the fuel screw. Soooo... While putting in the latest 48 slow jet I noticed that it screwed in very nice and easy for perhaps 4 turns, then it gets harder to turn, it doesn't seem like its seating but i don't want to "force" it. Oh yeah, I took out the jet to check. threads look perfect. I'm using a tiny screw driver so cant put too much torque, i think....I don't know what to make of it. Is the slow jet top out up against something spring loaded? The choke maybe? Is there a pressfit threaded sleeve that i have broken loose and now that is turning together with the jet? Any answers or ideas?
Thanks.

go with the 42 as recommended and also compare with other jet for thread length make sure your boot between air box and carb is sealed and look for any place air mite bypass the filter. the jet should screw in till it stops with out any force at about 0.3 N.m or 1.5lbf-ft

Your right, so was James Dean(JD jetting). My eyes not what they used to be. I meant to start with a 42 but installed a 45 by mistake. Finally got back to jetting my bike and... 42SJ with a 160MJ 1.75 turns out on the fuel screw. I've got a 250cc rocket.

no backfires

no bog at all.

Also the slow jet threads fine I just wasn't used to the feel. Does seem to snug up at the last 1.25 turn before it seats.

Thank You