weird carb problem

[nrp0450]

I have a Stihl MS250. It's around 10 years old. Over the years if it sat for a month without running it sometimes wouldn't run well (start but not idle or be able to rev up). If I cleaned the carb it would be happy again. After a few times I learned that all it usually needed was to take out the H needle, spray some WD40 in the jet hole, and set the needle back where it was. I could do it without even taking the carb off the saw. It would then run fine for several months or a year. In the last few months it has gotten to where I need to do that if it just sits overnight. It will run fine all morning. But the next morning I'll have to remove the H needle again, squirt WD40, and put it back to get it to run.

What would cause this? I don't think it's fuel because it's happened with two batches of mix made with different oil brands. I only use non-Ethanol fuel. And it runs fine on the fuel all morning if I clean the jet.

 

[ballisticdoughnut]

When you say clean the carb, did you replace the gaskets and diaphragms as well? You need to be careful spraying anything into the carburetor because you can blow the check valve out of the high speed nozzle.

 

[singinwoodwackr]

When you say clean the carb, did you replace the gaskets and diaphragms as well? You need to be careful spraying anything into the carburetor because you can blow the check valve out of the high speed nozzle.

Yea, ask me about blowing check valves...

 

[ballisticdoughnut]

Yea, ask me about blowing check valves...

Lol. I remember cleaning the carb on my CS600. Blowing it off with compressed air and I hear this little pop sound. Thought it was odd but didn’t think much of it at the time. Long story short, it was the sound of the check valve being launched into low earth orbit.

 

[singinwoodwackr]

Lol. I remember cleaning the carb on my CS600. Blowing it off with compressed air and I hear this little pop sound. Thought it was odd but didn’t think much of it at the time. Long story short, it was the sound of the check valve being launched into low earth orbit.

I have 4 261 carbs for cheap

 

[nrp0450]

When you say clean the carb, did you replace the gaskets and diaphragms as well? You need to be careful spraying anything into the carburetor because you can blow the check valve out of the high speed nozzle.

No, I just took it apart, checked that the gaskets and everything looked normal and cleaned any gunk that I saw (which wasn't much) and put it back together. I didn't know that spraying into the carb was bad. That makes sense though. What does the check valve do?

 

[ballisticdoughnut]

No, I just took it apart, checked that the gaskets and everything looked normal and cleaned any gunk that I saw (which wasn't much) and put it back together. I didn't know that spraying into the carb was bad. That makes sense though. What does the check valve do?

Check valve keeps air from being drawn back through the high speed circuit at idle. If it’s missing it will cause tuning problems, lean idle, hard starting, etc.

 

[ZeroJunk]

You can chase problems with those carbs until you are blue in the face. Get the number off the side of the carb, take it to the Stihl dealer, get a new one and be done with it for a while.
Run it every three months or so long enough to get the old fuel out of the carb if you are not using it.

If you let it sit I will almost guarantee you that it will puke on you.

 

[SteveSr]

No, I just took it apart, checked that the gaskets and everything looked normal and cleaned any gunk that I saw (which wasn't much) and put it back together. I didn't know that spraying into the carb was bad. That makes sense though. What does the check valve do?

Sounds like this could very well be yet another Zama with a sticking main nozzle check valve. This is a non-repairable fault.

A spray can of carb cleaner or WD-40 won't likely hurt the check valve but could temporarily un-stick it.

If the valve sticks closed the saw will idle all day but won't rev up. If it sticks open it won't idle due to the created air leak but should run fine at WOT.

 

[nrp0450]

Sounds like this could very well be yet another Zama with a sticking main nozzle check valve. This is a non-repairable fault.

A spray can of carb cleaner or WD-40 won't likely hurt the check valve but could temporarily un-stick it.

If the valve sticks closed the saw will idle all day but won't rev up. If it sticks open it won't idle due to the created air leak but should run fine at WOT.

This sounds like the culprit. I think it sticks open. I guess I need a new carb. Thanks guys.