My Stihl ms250(9 yrs old) has gotten difficult to start and won't idle. But once started it runs strongly unless I let the speed go to idle. It then dies and won't restart. I tried new gas, new spark plug, carb kit to no avail. The fuel lines look OK... not stiff , no apparent cracks. Adjusting the L screw on the carb has absolutely no effect. Is it likely that installing a new carburetor would fix my problems? Thanks, PRS
When you let off the throttle, does the saw race before it dies? At 9yo an new fuel line is in order, and I hope you already replaced the fuel filter. How stout is the compression? We also have the possibility the carb kit wasn't installed properly.
Really it's hard to diagnose the saw without being there.
Thanks for the reply.
When I let off the throttle, it just dies... no racing.
I replaced the gaskets and diaphragms and sprayed out with carb cleaner. The diaphragms weren't brittle, but I replaced anyway. also changed the needle valve and screen filter. After this the saw runs OK at fast idle(1/2 choke position). Previously it died after about 15 seconds at fast idle.
Compression is good. I haven't changed the fuel fileter, but wouldn't the fact that it runs well at high speed indicate that the fuel filter is OK?
What about the issue of the L carburetor screw adjustment having no effect?
By the way, to cold start, I have to crank at least 10 times at full choke, then switch to 1/2 choke. I get the idea that I have a low speed fuel starvation problem
Thanks, Preston
Try turning the "L" screw counterclockwise about 1/8-1/4 turn......and then adjust your idle accordingly
Andy is right. Time for fuel filter, fuel line and impulse hose.
At 9yrs that stuff is 4-5yrs overdue. Especially with the amount of ethanol in fuel nowadays.
Thanks for the input guys. At your recommendation, I will replace the fuel lines, filter, impulse line & see what happens. However I still thinking that if there is enough fuel supply for high speed, then there should be enough for idle. Is that not necessarily true?
Thanks, Preston
Just because the saw is running at full throttle, doesn't necessarily mean it's getting enough fuel. Does the saw 4 stroke a little at full throttle? A small air leak won't necessarily effect how the saw runs at full throttle, but can it idle.
A new carburetor is not expensive for that saw. Your thinking about the fuel is somewhat correct. You must remember that the fuel pump is driven by the pulses of crankcase pressure. At idle, those pulses are minimal and therefore the fuel flow into the metering chamber of the carb is low. At full throttle, the fuel pump is working with more authority and it can deliver much more fuel. It is not uncommon for the impulse line to come off on the back side of the airbox. I have also seen plenty of 1123 saws with crank seals getting hard. Any good dealer can press/vac test that saw in about 15 minutes, and save you the expense of replacing parts that are not faulty. Has your idle stop screw possibly fallen out?
Sometimes an idle problem can indicate compression loss and cylinder problems. Pop the muffler off and take a look at the piston. Vertical scoring means it needs attention.
Thanks to all for the good tips. In response to some questions: the engine doesn't 4 cycle at high speed, idle screw is in place. I will report back after changing the fuel line, fuel filter , impulse line, and if that doesn't work, the carburetor. Preston