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jetting suggestions xr280

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Recently completed a 280 kit. Seems good! Damn thing can idle up my driveway in 2nd gear slightly up hill. More torque for sure . HP I cannot check yet until break in.

Only year slight popping on deceleration. Otherwise I am close with whatever is stock. Also a slight hesitation when whacking open the throttle.. probably pilot jet..

Thanks,

Cb

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Yeah, i think I'm running a 135 main jet with Gordon mods. So you should try a 140, 142, and 145 to see what that gets you. I'm still using the stock pilot jet, with 2-1/2 turns out on the mixture screw. And i lowered the needle clip one position, effectively raising it for more fuel. So i think lowering your clip on the needle jet 2 positions from stock would be a good bet.

And probably go up 2 sizes on the pilot jet as well.

My 250 absolutely rips now. But there's always that bog when you snap the throttle too fast. Since its not a pumper carb. Its better to jet it rich and work your way down so you don't run hot or melt a piston.

Pilot jet controls idle to off idle, needle jet controls off idle to midrange, and main jet controls top end full throttle.

Are you running any other mods, exhaust, intake?

Edited by I6CJ7

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Thanks,

No, the air box is opened up but stock muffler. Top end is nice to have but I want to make sure I get at the torque the kit has available... Of course getting 65-70 mpg would be nice as well.

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A stock late-model XR250 uncorked runs great with a 135 main.  A 280 will require a bigger main but not as much as you might think.  When Al Bakers XRs Only built my XR265 the main jets were nowhere near as big as I thought they would be.  I agree with I6CJ7 above on his suggestion and I'd say a 142 would be a very good starting place.  The increased displacement increases signal across all carb circuits automatically - The same effect as putting on a smaller carb.  You should call XR's Only to see if they can give you some guidance.

Edited by VortecCPI

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cany you help me out?  i am thinking of rebuilding my running XR 250 and want to add some power.  what is entailed in a 280 kit?  new crank (stroker)?  connecting rod (long rod)?  both?  or an overbore?

 

the overbore doesn't seem real my stock 95 bore is 73 mm  stroke is 59.5  giving 249 cc

 

increasing the bore 2 mm makes it a 263 cc

 

increasing the bore 4 mm makes it a 277 cc

 

increasing the bore 5 mm makes it a 284 cc.  

 

I found a 284cc piston but i need to know if that requires re sleaving?

 

also is a new cam needed?  i ride at 5000 - 10000 feet.

Edited by spomey

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  A 280 will require a bigger main but not as much as you might think.    The increased displacement increases signal across all carb circuits automatically.

 

I agree. This has been my experience as well. Chuckster says he has only very slight popping on decel, so going up 2 sizes on the pilot from what's currently in it would be too big a jump. Try to clean it up with the fuel screw first. If it takes 3 turns out or more to do it, "then" go up 1 size. Don't go throwing increased jetting at it til you see how its going to run with what's in it, especially if it was jetted well for the previous bore.

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The reason the main is not larger by percentage is because the venturi is the same size and this increases vacuum.  So with the increased vacuum in the venturi, more fuel is being drawn into the mixture.

 

If you increase the size of the venturi, then you would need a larger main because of decreased vacuum.

 

My advice is to focus on the pilot and needle setting to get it to start on one kick and to minimize the hesitation.  Then use Mikuni's roll-off method to dial in the main a bit on the rich side to aid cooling at full throttle. 

 

If you're not willing to remove the carb to change jetting four or five times,  you will not be successful.

 

Am I right or Amarilla?

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