MS260 Carb Problem

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saxman

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I am still having a problem with the carb on my MS260. I have had it checked by a stihl dealer and adjusted (it is a one screw carb by the way). It starts and runs fine, it never bogs down in a cut(WOT) and idles fine most of the time. The problem is intermittent, it happens when idling or when I try to accelerate to make a cut and it just sounds like a saw running out of gas and dies. It always restarts and will run fine, but then it will act up again at some point. I am thinking that the problem can be;1. idle circuit plug in carb, 2. air leak somewhere, 3. hole if fuel pickup tube. Feel free to give me some advice and and all will be appreciated.
 
It may not be the carb. Just to be sure, get the saw hot, then set the L screw as rich as you can before the saw falters, then turn CW (lean) a small amount (say 1/16 to 1/8 turn). In this position, the saw is about as rich as it can be at Idle and there is no chance of having the progression from Idle to High too lean. The single screw carbs are a bit of a pain to diagnose.

Your points:

1) Maybe... but in general, if the welch plug is loose, you get an over rich condition, not a "lean" condition. If the transition ports (3) are blocked or partially blocked, the "falter" condition will tend to be permanent, not intermittent.

2) yes... you could have a slightly bad impulse hose that only fails occasinally due to a small crack.

3) yes...

2 and 3 are easy to check. Just take them off, screw in a 5mm screw (just borrow one off the saw) into one end, and pump up the other end with a bicycle pump or whatever to 10psi. Spray soapy water on the hose and see if it leaks.

Unless you're good with carbs, the easiest way to eliminate carb "problems" is just drop on another. I have a few brand new 026 dual screw carbs I can sell for low $. let me know by PM if you need one.


There is another possible cause related to air leaks. The oil seal under the flywheel fails, and at first fails intermittently. This affects the idle stability more then the aceelaration, but I've been both. You'll need to vacumm test the saw to figure out if this is failing. As it's tiny and often set too deep, it's a b???h to replace without the Stihl oil seal puller.
 
Thanks for the response Lakeside. I will keep you in mind on the carb, I really don't like the idea of the one screw carbs, although this one runs fine, most of the time. Are the carbs you have for the Canadian MS260's or for an older 026?
 
As they are, they fit any of the "compensating" air filters types like the 026-PRO, or, if you change the metering cover (I have them) to the non-compensating type, work perfectly on all 026's. If you need the carb body number I can stumble down to the workshop and check for you.

As for Canadian, no.... these are USA - just before EPA changes which is why you have the 1 screw carb on the earlier MS260 (they now have two screws again).
 
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