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Off idle bog

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My bike, a 2007 300 XCW, has been acting very weird lately.  A few months ago, it was in great shape.  Perfectly jetted, nearly new top end, etc.  Perfect power.

But the last two rides were cut short.  On the first, the bike was running perfectly for about 25 miles.  Then, suddenly, it started to randomly bog down at up to 1/4 throttle.  If I could get it past the bog, it lit up and ran fine.  Only to bog again a few minutes later.  Made low speed twisty uphill turns nearly impossible.

Naturally, I thought "Damn pilot jet must be clogged from crud in the float bowl".  Unfortunately I didn't have a wire small enough to clean it (it was clogged badly).  When I got home, I replaced the pilot and cleaned the clogged one.  Rebuilt the carb and added a fuel filter.

Thinking I was now good to go, I went out this morning only to find it's behaving exactly the same way.  I checked the pilot and carb and everything was nice, open, and clean.

What could it be then?  Could a sticky power valve behave like a clogged pilot jet?  Something electrical perhaps?  Even now, every once in awhile, it runs perfectly.  Only to bog randomly and usually at the worst time.

Thanks!

Baffled as usual,0

XL600

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Check for water in the fuel. Next time that happens, immediately if you can, drain and catch the fuel in the bowl for inspection.

Make sure all carb vent lines are clear.

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Hadn't even thought about that possibility.  I did drain the fuel completely (my truck doesn't care...) so I'll have new gas next time.  I'll check the float though to see if anything is left.

 

Thanks!

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I was so looking forward to draining the bowl and the last little bit from the tank and seeing water.  But no, nothing.  Just perfect looking gas.   All hoses clear.  Are any hoses supposed to route upward or do they all point down?  Some carbs have an upward vent opening but over the years, I've lost track and just snake all 5 down through the swing arm.

 

Bummer, and still baffled...         

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What does your spark plug tell you?  Could be stuck open power valve.  Just open the side port behind the pipe to know.  Could also be reeds going bad, as well as the plug or even cap.  Coil and cdi likely aren't the cause with those symptoms. Might also make sure no water is stuck in your pipe. 

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Spark plugs looks good, but I replaced it anyway.  No excessive carbon buildup or rich indication.

Power valve seems to move freely with no sticking when manually moved (spring disconnected of course).

No water anywhere (tank, pipe, hoses).

I'll try reseating the plug cap and checking the reeds.  Those are all original so could be my issue, but it seems odd it runs fine sometimes then bogs at other times.  So very frustrating.

 

Thanks for the ideas!

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It's best to T at least one of the carb vent lines and run it up high for water crossings sake. Prevents the engine from cutting out/stalling when the lower ones go submerged.

I had an RM250 years ago that was doing this. Turned out it was a chunk of carbon from years of build-up that had come loose from inside the pipe and was intermittently blocking the small stinger end of the pipe, killing all power.

You might check the exhaust too to see if someone is messing with you and put something in it or to see if you have blown packing that might be causing an issue with blockage. Just a thought.

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Well, thanks for all the advice.  I finally did find the culprit, but it wasn't my first guess based on the symptoms.

The bottom reed was all but destroyed.  I suspect it was intact enough to allow the motor to feel 'well' sometimes.  Then, when bent sufficiently, the bog would occur (feeling exactly like a suddenly clogged jet).

As soon as I replaced the petal, the old 300 tractor was back to it's normal outstanding power.  Happy days!

 

IMG_20170911_062945~2.jpg

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