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YZ250 with Big Bore Jetting Help Wanted

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I just installed the wisco 290 big bore kit on my yz250f. I have a power bomb pipe /power core 4 silencer. Will somebody help me out with my jetting I have stock on the bike's carb right now. I have a stage 1 dyno jet kit. I need some advise on what jet to run were. Thanks

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I just installed the wisco 290 big bore kit on my yz250f. I have a power bomb pipe /power core 4 silencer. Will somebody help me out with my jetting I have stock on the bike's carb right now. I have a stage 1 dyno jet kit. I need some advise on what jet to run were. Thanks

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I just installed the wisco 290 big bore kit on my yz250f. I have a power bomb pipe /power core 4 silencer. Will somebody help me out with my jetting I have stock on the bike's carb right now. I have a stage 1 dyno jet kit. I need some advise on what jet to run were. Thanks

Where are you located? I posted some jetting suggestions in my post above. That could help get you started. You won't need to be too far off from stock though.

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I just installed the wisco 290 big bore kit on my yz250f. I have a power bomb pipe /power core 4 silencer. Will somebody help me out with my jetting I have stock on the bike's carb right now. I have a stage 1 dyno jet kit. I need some advise on what jet to run were. Thanks

Please create your own thread in the future. I have moved your post to its' own thread. This time.

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What is the exact jetting settings you currently have in it?

What problems are you having?

Dyno Jet jets are not the same as OEM jets and any jets you need, you have to get from DynoJet. A dynoJet Main that says 175 is not the same as a Yamaha/Keihen 175 so you are going to be in for a tough time.

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I just installed the wisco 290 big bore kit on my yz250f. I have a power bomb pipe /power core 4 silencer. Will somebody help me out with my jetting I have stock on the bike's carb right now. I have a stage 1 dyno jet kit. I need some advise on what jet to run were. Thanks

You'll have to go leaner. I don't have a big bore but have been reading up. Try a smaller size pj and main and maybe a clip position up and go from there. What throttle position are you having problems?

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It all stock I'm having a lot of popping after I let off the throttle. Sorry I posted wrong I thought it was related to the thread. Thanks for setting me up my own thread.

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Thanks for the help. I'm having a lot of popping after I let of the gas.. That is my only issue. It did not do that until I put the bb kit on.

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Popping is normal on a race bike. But you can check for exhaust leaks as they will make things worse.

You have a race bike. Racers do not coast. It is WOT then 100% on the brakes.

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Pabricio30: Please do not ask the same question in multiple forums. I will merge the posts from here in to your thread in the jetting forum.

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I need some help guy's Will a 2005 yz250f cylinder head fit a 2004 yz250f Thanks.

Respectfully,

Randall

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I installed an Athena 290 kit on my buddys bike and had to go with a 50 pilot and needle clip in middle position. Very minimal popping on decel now. 3000 ft elev.

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I live at sea level Wilmington NC. I just ordered a stage 1 dyno jet kit. Also I ordered a power bomb header and have a power core four silencer on the bike along with the Wiseco 290 bb kit. Anyhow I got a really good deal on the header but it did not fit my manifold to big. So I put it to the grinding wheel and smoothed it out real good. The header fits real tight right now i think there could be a possible exhaust leak at the manifold could that cause the real bad popping when I let off the gas? Because my track is riding distance from the house so I coast to the track and when I let off the gas it's constant pop pop pop pop pop pop? I heard the dyno is very difficult to dial in with the stock carb on the Yz250f's. Any suggestions on this issue would be much appreciated thanks.

Respectfully, Randall

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Your pilot jet is too lean. Keep going up on the pilot until you have minimal popping and don't forget to redial your fuel screw each time; and then work on getting the needle clip position set. Finally after you've dialed those in you can change your main jet as needed.

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I really appreciate your time and knowledge. I took your advise and the popping has stopped. Thanks so much. I am a new member and can't figure out how to post my own thread. Anyhow On one of my yz250fs one of the intake valves spun around and broke off. It left a little ding right by the spark plug. Would this affect the performance of the head?

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