Yz250 losing power on low revs

Flit250
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Flit250
Edited Date/Time 12/21/2013 11:59am
Hi all I have just bought some Boycen pro series reeds for my yz250 2008 model. I. Took it for a blast yesterday and on the first session the bike was absolutely fine with tones of power however on the second ride as soon as the bike got warmed up it lost all power but only on low revs. If I get the revs up on it it performs great again, any ideas on what it could be...? I'm running it on a fmf gold series and shorty power core 2 if this helps. Only thing I could think of is maybe the power valve is sticking open or maybe the carb wants adjusting a touch. Any idea what it could be I would love your feedback cheers Ashley.
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FGR01
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FGR01 12/16/2013 7:39am
12/16/2013 7:39am
Since it developed the problem after you were in working on the reeds my initial suspicion is you have developed an air leak somewhere around the reed block and now have a lean bog. Many times air leaks around the reed block worsen as the bike warms up and things expand.
xnationalwrench 12/16/2013 8:14am
12/16/2013 8:14am
FGR01 wrote:
Since it developed the problem after you were in working on the reeds my initial suspicion is you have developed an air leak somewhere around the...
Since it developed the problem after you were in working on the reeds my initial suspicion is you have developed an air leak somewhere around the reed block and now have a lean bog. Many times air leaks around the reed block worsen as the bike warms up and things expand.
The other thing to check is the power valve if it hasn't been cleaned it could be gunked up and not allowing it to close properly
Flit250
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Flit250 12/18/2013 1:43am
12/18/2013 1:43am
Thanks for the advice guys I will check both. Many thanks.
lumpy790
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lumpy790 12/18/2013 7:25am
12/18/2013 7:25am
did you change the jetting? As I recall you need to after installing Boyesen reeds.

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Flit250
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Flit250 12/18/2013 8:25am
12/18/2013 8:25am
Yes I changed the jetting to what was recommended by Boycen but all that did was make it smoke like hell on tickover so I changed it back to standard. The bike had no problems first session out then second time out it happened when the bike got warmed up and same again on every other ride.
lumpy790
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lumpy790 12/18/2013 11:19am
12/18/2013 11:19am
As I remember the pilot jet needed to go leaner. Same with the needle.Increased / faster flow = more vacuum needing to go leaner

have you put in a new spark plug?
Flit250
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Flit250 12/18/2013 11:34am
12/18/2013 11:34am
Yes I have tried that, exactly what you said Boycen recommended to do but the bike smoked a hell of a lot, it ran as smooth as ever when rode on the track but once you pull up and no revs are on it just choaked up and bellowed smoke out. As soon as you open the throttle it clears after a few seconds so I went back to factory settings and was fine fore 30 mins. I'm thinking with the amount of smoke that was coming out maybe the power valves and ports just need a good clean as they may be sticking with all that oil it was burning...?
Flit250
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Flit250 12/18/2013 12:06pm
12/18/2013 12:06pm
I have also just done a full engine rebuild with brand new barrel crank piston ect but it ran fine, then after I new it was working great I decided to put pro series Boycen reeds in and now it's not. The problem only happens when the bike is hot so I'm going to clean the power value and take the reed cage out and reseal it and see if that makes a difference.
lumpy790
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lumpy790 12/19/2013 9:40pm
12/19/2013 9:40pm
leaning it out and smoking more does not sense..... unless you have not gone lean enough yet.
Aron213
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Aron213 12/21/2013 11:59am
12/21/2013 11:59am
I had a piece of epoxy get stuck in my reed cage once, it held my reeds open just a hair, so at low RPM the thing would want to die or fall on its face, but wide open it still ripped.

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