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Hello, my son’s KTM 50sx has been running great up until the last couple rides. Bike runs fine except when the powerband is about to cut in, the bike sputters and never allows it to cut in. Any advice? Thanks in advance.
 

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Welcome to the forum.

Here some things to have a look at:
- Make sure aircleaner is clean.
- Take carbi off and give a good clean, remove jets and check that they are clean.
- Replace spark plug.

If no help with those come back and we dig deeper,
 

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Discussion Starter #3
Hello.

Thanks for the information. I did all 3 and having the same issue. I even drained the tank, checked the fuel filter and added new fuel. The bike runs strong up until the point of when the powerband is about to kick in. Sputters like it’s starving for fuel. Bike doesn’t die but keeps sputtering at that point.
 

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The powerband starts at approx 8,500 - 9,000 revs and goes to 10,5 - 11,000 revs. Sounds like all is well untill 8,500 revs. At that stage the main jet takes over and the needle will be well on it's way out of the main jet.

As the bike has been running well untill now I suspect this is not an adjustemnt issue. So here a few suggestions:
  • You have cleaned carbi, but did you remove the jets and made sure they are fully clean?
  • The fuel level in the carbi is also critical. Too much fuel in the bowl and the main jet will suck too much fuel making the motor splutter.
  • Too low fuel level in carbi will make the bike run lean and loose power.
Here a good simple video re how to set fuel level:


Also have a good look at the tang on the float (the one that pushes the fuel closing valve). We had one that was badly worn and as result the needle got stuck and did not close properly making the fuel level in the carbi too high and affecting the running of the bike. If this is the case you can 1. use some 800 wet and dry to polish away the ridges, or 2. buy a new float.

Here some suggestions:
  • Move the circlip on the needle one step down so the needle sits higher up. This will provide fuel earlier. If no help, try the other way.
  • Buy a few main jets bigger and smaller than what is in the bike and test them
  • Is it perhaps time to fit a new piston and rings??
  • Is the choke working well or is it a fraction on?
Have you (or someone else) played with the air screw on the carbi? Normally the airscrew should be 1.5 - 1.75 turns open from fully closed.

Some general questions to consider:
  • Have you changed oil/fuel ratio? We have always run our bikes at 40:1. But I know others who use less oil type 50:1 or even 60:1. This will affect how the motor runs. KTM has a recommendation for your bike. Stay with that!
  • Have you changed fuel or moved to a different oil?
  • Has the weather changed? Got hotter or colder?
  • Have you moved and now race closer to the sea level or higher up in the mountains?
  • While you have the carbi off also take off the reed valves and check that they are in good condition and close properly. Any damage/wear on them will affect how your bike runs. If there is wear/damage you will need new reed petals!
If still no help come back and we dig even deeper!
 
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