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06' YZ250 detonation @ 1/4-1/3 throttle

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Bone stock except for a FMF turbine core II. Air screw is set to 1 1/8 turns, otherwise jetting is untouched from the factory. I ride primarily at 1000-1500ft, 70-80f, minor to no humidity and the bike flat rips. Throttle response is good, never fouled a plug and pulls like crazy on 91 octane and Amsoil Dominator @ 32:1.

I've noticed what I think is detonation just before the bike gets 'on the pipe' at 1/4 to 1/3 throttle. I've never had a 2-stroke that pings before so I'm not exactly positive thats what it is, but I'm failrly certain. I dont notice anything prior to 1/4 throttle or after 1/2 throttle so I'm unsure if its the pilot circuit, the main or the needle. I'd prefer to run pump gas, but not willing to burn a hole in my piston. Anyone have experience with this on the new YZ's? Any advice?

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Keihin provides tuning information that describes which carb circuits are effective at different throttle openings. Detonation usually indicates lean, so it sounds like you need to richen the 1/4 - 1/3 throttle range. Looking at the chart, you might try to raise the needle (lower the clip) one position and see if it helps. Alternatively, you can try higher octane fuel.

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Another alternative is the optional N3CJ needle (Yamaha PN 4SR-14916-CJ-00) which is 1/2 clip position leaner than your stock N3EJ needle. You can try it one clip position richer than the standard clip position on your N3EJ needle.

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Seems odd that it would deto with such small throttle openings. In my experience, deto generally happens only with the high cylinder pressures found with WOT. Just a though....

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The optional needle took care of the pinging on my 06 yz 250. I had the identical problem you did. Your bike should have came with the N3EW, I believe. This needle, in the second position, is slightly more rich than standard.

The pinging will also go away with race gas.

I had a jd jetting kit from my old yz 250. It worked very well in my new one and I am currently using one of the jd needles.

Just keep in mind the jetting is only a band-aid. The real problem is less than good fuel. Maybe we are not paying enough at the pump to get good stuff??? LOL

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