2004 KTM 250 exc with 350 kit. Bike ran flawlessly all day Saturday after a fresh thorough cleaning (carb, air filter, oil, ect). Sunday it wouldn't run right. Played with the mixture screw but it either had no throttle response and bogged or had no power in the top end. Drained the fuel, replaced fuel filter and petcock, re-cleaned the air filter, replaced timing chain tensioner, adjusted the valves, still runs the same. I am out of ideas. What could possibly happen overnight that I can't figure out?
SOURCE: 2005 ktm 520 won't idle, cleaned carb new diaphram
did you try adjusting the air fuel mix screw?
SOURCE: 1995 KTM 250-EXC Buring Oil
you possibly have a bad seal on your crank. Its called a labyrinth seal and it will be on your clutch side because it is drawing in crankcase oil. the other side would make it run lean by drawing in air by the stator. if it is eating crankcase oil and not leaking it, it is most likely your problem. you can do a leakdown test on it to confirm the problem. put the piston at bottom dead center and compress the cylinder to about 10 pounds of air using a leakdown tester or a compression tester hose that can be used to hook up to an air compressor somehow and hold the pressure, but do not go above that because you could damage those seals with too much pressure. Check to see if the cylinder will hold the pressure for a good amount of time on its own. Check for air coming out of the crank vent hose or the oil fill hole. Also dont overlook the simple things like your premix ratio. try 42:1 or 50:1 ratios of gas to premix oil.
SOURCE: bike only starts with choke on and wont idle
If your bike ran good previously - and nothing has been changed that would require rejetting or a needle adjustment, I would look for problems elsewhere. Such as:
Has the bike been stored for more than a few months at a time? If so, the carburetor jets could be plugged with fuel varnish. Only being able to start with the choke applied - and an inability to idle are both classic symptoms of this. Remove and disassemble the carburetor. Clean it and the jets out with carb cleaner and compressed air, then reassemble. Also make sure the fuel in the tank is fresh and that the fuel strainer (usually with the fuel petcock) is clean.
There are three ingredients required for combustion: fuel, compression, and a spark (at the right time). So long as you've got each of these - it has got to run.
SOURCE: I bought this bike this winter, it had sat for 3
Your lower rpms are controled by the mixture screw, pilot jet, accelerator pump and the slide needle. So your problem is probably in one of these-though racing 4 strokes are known to have a little stumble at the bottom, but not what you're describing. I'll have to assume the shop has the pilot jet clean and the mixture screw adjusted properly.(roughly 1.5 turns out)
First, check your accelerator pump. Remove the carb enough to be able to look inside the throat, but leave the cables hooked up. Work the throttle a few times and make sure that there is a small squirt of fuel in the throat of the carb each time you twist the throttle. If not, your accelerator pump is clogged or damaged.
If everything is fine there, the next likely problem is the needle adjustment. It's mounted in the slide. Once you remove the needle, you'll see a small clip around it. Try moving this clip up one notch. This will lower the needle and slightly delay the fuel feed from the main. If this fuel comes in too soon, it will cause a way rich condition and cause a bog off idle.
Outside of carb issues, valve adjustment(too tight) can make for a rough idle and hard starting.
SOURCE: Ktm 250 excf 09 bogging
Check the fuel filter in the tank, (above and connected to the fuel tap). It may well be blocked.
Hope that helps.
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