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05 KX250 High RPM Flutter

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Hey Guys,

Been involved with TT for a while now, but mostly in the CRF450R forum. I recently picked up a lightly used 05 KX250 and I'm having a blast. My 2t knowledge is a bit rust having been on the thumpers for a while now, so I'm hoping some of you can help me out.

While riding moto, I'm hearing this thing break up/flutter at high rpm. It's in those odd spots on the track where you need to shift, but don't have time before the next jump/corner. I'm wondering if the high rev flutter is reed float or ring flutter? I don't think it's a jetting issue. I'm at stock jetting, and there is zero spooge coming out of the silencer.

The bike has a 51t rear sprocket. I just picked up a 49t to bridge the gap I described earlier about odd shift points.

Any thoughts?

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I always go up one on the main when it has a cackle or flutter as you're describing..it's only a couple bucks. With the cold weather coming, make sure it's not getting too lean.

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Are you at 100% throttle angle when it happens? I ask this for the purpose of jetting. What does the color of the plug look like?

It may be as simple as replacing the plug with a new one.

In my experience, when the jetting is close it will not cause the issue you describe so if the plug is close in color it is probably not a jetting issue.

Certainly a cracked or broken reed will cause what you are describing.

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It wouldn't hurt to check the reeds, but in my experience a chipped reed will give you a noticeable off idle bog.

A friend of mine recently picked up a 02 kx125 that started out with the same problem he is describing. After cleaning the carb, multiple different plugs, and checking compression the symptoms progressively got worse. (it never had any issues below about 5,000 rpm) I finally pulled the reed cage and sure enough, there was a piece missing off one of the corners.

Basically I'm saying I believe reeds can cause a variety of issues and many times lead you in the wrong direction, so just keep it in mind and it only takes a few minuets to check.

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detonation? My 05kx250 is sensitve to low octane

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detonation? My 05kx250 is sensitve to low octane
All two-strokes are. And the '05-'06 is known to be detonation-prone. The compression was lowered slightly in '07 by changing the shape of the combustion chamber.

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Here's some follow up - I pulled the plug and the carb to have a look into things.

The carb had been jetted up 1 from stock, with a 168 main and 48 pilot installed. The plug looked great terms of color, but I noticed that the ceramic insulator had what looked like a crack running around the circumference, about a millimeter below the electrode.

I ran a compression check also. 230psi...with 2 gauges. I didn't belive it at first. That's race gas territory and i'm surprised this thing hadn't come apart. I've always run 93 octane in it, but never really noticed any detonation. I can't believe this thing is pumping 230psi in stock form. Has anyone else had readings this high.

BTW - I should have yanked the reed cage to inspect. The more I think about it, the more it's likely to be the reeds. At idle, this thing sounds like it has an air leak, which has been the previous tell tale signs of bad reeds on all my previous two strokes.

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I think mine has read 210-220 psi, yes they have a lot of compression and like race gas. As said above, I'd do a top end and check the reeds. Mine loves the JD jetting red needle.

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I raced the 2 strokes back in the mid 90's and I remember mine blowing around 220, (that's after some prep work to raise comp.) so your good. My son's small 2 strokes are pushing 215. If there are no signs of detonation on the plug and top of piston I would keep doing what your doing.

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It's been a while and wanted to follow up - I checked compression (230psi) and checked my reeds (no cracks or fraying). I installed a smaller rear sprocket and hit the track again. The high RPM flutter wasn't as prevalent, but did occur in spots where I really revved the bike out. I'm led to believe that the spots on the track where I was having the flutter was due to the engine 'falling off the pipe."

All is good.

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It's been a while and wanted to follow up - I checked compression (230psi) and checked my reeds (no cracks or fraying). I installed a smaller rear sprocket and hit the track again. The high RPM flutter wasn't as prevalent, but did occur in spots where I really revved the bike out. I'm led to believe that the spots on the track where I was having the flutter was due to the engine 'falling off the pipe."

All is good.

FYI, my yz250 makes 250psi of compression with stock internals. I have around 100 hours on it and I'm on my 3rd piston, running 91 octane exclusively. I've had 0 detonation.

Cranking compression is different than compression ratio.

Edited by GMO397

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