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Ye Olde 92 WR 250 - Can't sort pilot circuit

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First I should note I've no two stroke tuning experience at all. I'm comfortable doing basic jetting on four strokes and have been trying to use the same technique on my two stroke but failing miserably.

The bike is a 92 Yamaha WR250. Stock Mikuni VM carb, engine has a fresh Eric Gorr 'mo-betta' ported 265cc top end with the head worked for pump gas. I'm running 93 pump gas mixed 40:1 with Belray H1-R.

Current jetting is a 35 pilot, 390 main, needle is unknown taper, middle position. I think it's the stock needle and slide? Float level is even with the bowl seam.

The plug reads black, part of that is because I wasn't getting it on the pipe that often at Southwick this weekend. The bike starts cold within 3 or 4 kicks with choke, then idles on it's own off choke. I suspect my idle is high however, I can't gauge the RPM by ear, and the bike has a rapid staccato at idle, brrring-brrring-brrring rapidly. It responds to throttle instantly loaded or unloaded. If I let the revs drop down as long under load, as long as I feed some maint throttle it'll lug right along. If I lower the idle on the stand it just peters out unless I keep it alive with throttle blips.

Playing with the pilot mix screw has no effect what so ever. All the way in, 4 turns out, I can't hear any change in the engine's behavior. That can't be right?

I suspect I have needle adjustments to make as well as at lower revs under load and light throttle (up a steep hill in 1st, trying to maintain walking speed rather than blasting up for example) it sounds like the bike might be firing every other rev. As you feed in more throttle it startes to catch some of the 'missed' revs and really want to take off when doing so. If I open it up, it comes alive that's usually when I panic. :smirk: I don't want to muck with the needle though till I get the pilot sorted.

Anyone got pointers to a good vid or audio clip of a 250 idling correctly so I can get a feel for the sound/revs I should be shooting for?

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Sounds like you actually have a good idea how it should sound/run. At least you know the rapid brring-brring-brring is not right. I agree, if the air screw adjustment makes no difference there's something amiss there.

You know it's an "air screw" right? Unlike the pilot screw on a 4 stroke, the pilot screw on a 2 stroke meters air, not fuel. So when you come out with it, you're dialing in more air, not more fuel.

Sounds like where you ride there would be plenty of other 2 stokes around. Listen to those bikes to get an idea of proper idle and hit up the owners for help in advice/tuning yours.

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