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HELP! I have an 06 KTM 525 EXC with the stock Keihin FCR-MX 3900F carb. I have a problem where right at the most delicate moment, between idle and first application of power, I get WAY too much acceleration. We are talking an imperceptible movement on the throttle, (much less than 1mm) and a surge of at least 1000 rpm if not more in an instant. The result is nothing but a jerk on first acceleration and it makes it difficult and miserable to ride the bike with any kind of finesse. Let's be sure, I understand KTMs are abrupt, but we are talking a real technical problem here.

The carb has now been gone through over 10 times. Every jet size in the book has been tried, it has been JD Kitted, accelerator pump has been tested out, slider plate has been tested, idle jet has been tried in every position, and 3 different needles have been tried in every clip setting. As much as the bike now runs pretty dang good, NOTHING has affected this surge problem.

I have at this point fully eliminated carburetor problems. I have also now eliminated the throttle position sensor as I have tried it disconnected and tried a replacement TPS.

What other electrical component could cause this kind of problem?

 

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I have the same bike as you and had horrible running issues. My problem turned out to be the carb mid-body gasket. I am big on throttle control while riding and I was ready to get rid of the bike till I figured it out. Do a search on mid body gasket. It might answer your problem. Hope this helps.

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Thank you all for the responses. The throttle tamer has come up, and I will now get one, simply because I am out of things to try. I still insist that it is a technical problem vs the characteristics of the bike. Mostly because it is an instant "jerk" while all the rest of the acceleration is perfectly smooth. I should add that after the bike has warmed up, it seems to run erratically right at the idle/first slider open point.

I would like to hear more about this mid body gasket...exact symptoms, any way to visually inspect the gaskets?

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If you take the float bowl off you will see 4 torx screws in there holding the top half and bottom half of carb together. That is where the gasket is. There is a discussion here on TT about it with great pictures. Do a quick search on mid body gasket.

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Thanks for the info. I have a good understanding of what it takes to split the mid body gasket. I am much more interested in the EXACT symptoms you experienced. Hesitation? Warm/Cold problems? Starting problems? Cruising problems? Any information is helpful. 

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2 hours ago, Pete Janemark said:

Thanks for the info. I have a good understanding of what it takes to split the mid body gasket. I am much more interested in the EXACT symptoms you experienced. Hesitation? Warm/Cold problems? Starting problems? Cruising problems? Any information is helpful. 

If your motor idles fine and starts fine and runs fine any of the time, it's NOT the mid body seal giving  you problems, period.

You need to replace your worn out needle jet (emulsion tube), check the apump system for correct operation (fine 1 sec squirt that always misses the slide) and clean all your ground harness connections, from the stator out......and replace your plug and inspect the cap for cracks. 

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The symptoms I had were very inconsistent idle and air screw had no effect even with stock jetting. Some times when I twisted the throttle it would hesitate for a moment and hit all at once. Occasionally I would start the bike and it would idle perfect until I touched the gas and it was a coin toss as to if it would fall on its face or try to jump out from under me. I had tried everything and was ready to get rid of the bike. Not sure if this is your problem but it's only about $30 out of pocket.

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Before replacing the midgasket it would seem to run a little better after I rode it for maybe a half hour and as long as it was hot it wasn't awful but still way off. I cleaned and inspected the carb sooo many times. I'm not saying its for sure your problem but after what mine looked like after cooking in there for 11+ years it was way overdue. It took me more time to clean up the mess in there than to replace the gasket.

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Note to all. This turned out to be a game changer and a life changer. The solution (after 2 years of reading forums and trying everything) turned out to be the loose plate part of the throttle slider (the part that falls off) was installed upside down. PAY ATTENTION AND FOLLOW THE PICTURES IN THE BOOK when reinstalling it. Do not rely on witness marks or how it looks like it should fit.

The symptoms experienced were:
Horribly abrupt acceleration from idle"
Lack of smoothness when accelerating out of turns
Randomly difficult starting, sometimes not at all
Sometimes start on manual kick, but not by electric start
Slow return to idle
Difficulty adjusting idle
No feel or understanding if too much choke needed or not enough

All of these symptoms were random and occasional. Each symptom has plenty of forums and a rabbit hole of suggestions and solutions. None mentioned this plate.

It was VERY, VERY hard to solve this problem and in the process I spent large amounts on every suggestion, and really hated the bike. It literally changed my life once resolved. I now ride 10-20 hrs per week and it ALWAYS starts immediately and runs like a champ.

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