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Old 30 Aug 2010
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KTM 640 Engine Cuts on Braking & Hills


My 2006 KTM 640 Adventure engine has a problem. It starts and runs ok, however on heavy braking with high RPM it backfires like a shotgun. When I brake hard with idle RPM the engine cuts. The engine will also cut on idle when going down hills. Rather annoying when riding hard in traffic.

I've tried troubleshooting various things - fuel pump, choke cable, float height. etc. But can't figure it out. One thing I did notice when playing with the needle height is that the membrane slide in the carb seems to have a bit of forwards and backwards play in it causing wear on the needle - It will need replacing when I can afford it.

Does anyone have any suggestions?
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Old 30 Aug 2010
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Maybe an electrical problem not a carb one? The weight shift braking & downhill might be nudging something? I'd check the wiring under the tank for any rubbing. Maybe try bypassing the stand switch too.

BST carbs are bad for wearing out, apparently moto-lab.com is the best place to get a rebuild kit. However when I rebuilt mine at about 80K km it was well worn & generally running a bit crappy, but no cutting out like you describe.

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Old 4 Sep 2010
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Thanks for the advice regarding rebuild - once I get some money together I will probably start a rebuild job beginning with the carb.

I've not had a detailed look at the electrical connections but will keep my eye open for anything next time I have the tank off. To me, something screams bottom end mixture problem. I have discovered that the mixture screw is completely jammed solid so have had to lean the needle valve all the way out. The needle also seems to wear pretty quickly too.

My bike doesn't have a sidestand switch so I can cross that one off my list of potential gremlins.


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Old 23 Aug 2011
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i had a lc4 400 that did the same thing, especially when doing stoppies. unfortunately i never found the fault. sorry.
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Old 23 Aug 2011
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1. What was reported in the original post, sounds as if there has been some adjustments from standard. Namely carb settings. I would put these back to standard (all standard carb setting in KTM manual, with specs). Standard setting before trying moving away from those and then only one setting made to try. You will always have a known situation to go back to.

2. Try raising the tickover speed, 1400 to 1500 rpm is about right.

3. Mine used to stall/cut out sometimes for no apparent reason if the air filter is dirty/blocked, clean out airbox properly with blue roll/tissue and maintence spray and fit a new/oiled filter.
4. If you think its electrical - The underseat fuse box towards the back of the bike above the battery. The fuses have a habit of loosening off and being slightly baggy in the holders. You can push up tighter the fuse holder grips for the blades with a very small screwdriver or scriber. This makes them grip the fuses tighter. Could be that cutting out the ignition circuit with vibration, but you did'nt mention electrics cutting out at all in the original post.
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KTM 640 Engine Cuts on Braking & Hills-640-carb-settings.jpg  

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Old 14 Sep 2011
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Sounds like you need to check the float level in the carb.
There is a similar issue thread on advrider about LC4 bogging after jumps.
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