Stihl MS-250 carburator adjustment for idle speed?

[cwatkin]

I have a year or so old MS-250 saw and it works well except for one issue. The idle speed seems to be just a tad too low. It idles fine most of the time but if I cut through something and let completely off, the saw will just die. It restarts fine and you sometimes have to give it some throttle to get it going again but that is it. I adjusted the low mix screw out some and that helped as well as the high speed screw (all the way out). The high speed jet seems limited, likely due to EPA crap, but the low isn't limited.

I do not see an adle speed adjustment. Does this saw even have one? There is no reference to it in the manual and I hate to tear a relatively new saw apart to find one.

I guess I could put a shim/blocker of some type behind the throttle linkage such as a folded over piece of paper to keep it from completely returning to home when I let off. I did something similar to the throttle return spring on my truck when we were using the alternator to power a large inverter off the battery/alternator. It didn't put out enough juice at idle but I folded over a piece of cardboard and stuck it in the throttle mechanism to rev it up slightly.

Conor

 

[asdf4240]

I'm at work and don't have the manuals in front of so bear with me I'm going off memory. The L (Low) and H (high) screws are both 1 full turn out. However, I believe the newer Stihl saws have limited caps on them so they will have to be removed to accurately set the L and H screw. The LA valve should be adjusted (screwed in) until the butterfly valve just starts to open. It should run at this point, so start it up and allow everything to warm up. Next I use a Tach to set the low rpm to 2800 rpm (I think this is the number) using the L screw. Any adjustments here should be less that 1/8 of a turn either way. And for high rpm I set it to 13.8K using the H screw, again should be less than an 1/8 of a turn.

My personal opinion is if you are unable to set the saws idle doing this there is another problem, air leak, clogged filter, spark plug, etc....

Good luck.

 

[cwatkin]

Thanks. Unless I am missing something, I don't see an LA adjustment on this saw like most others. I know that some carbs are non-adjustable for the most part and this seems to be one. The low (L) jet seems pretty open to adjustment but the high (H) adjustment is pretty fixed. I enrichened it just a tad and it hit its limit. The LA adjustment is usually just a screw that goes in or out and pushes against a plate attached to the pivot the actuates the throttle plate. If you screw it in, it holds this in a position where it can't close all the way.

As for the limit caps, this one appears to have the internal limiters, which are worse than the limit caps. I pulled the limit caps after installing a muffler mod on another saw of mine using a finely threaded screw. I just screwed this into the caps and gave it a pull. They were easy to remove. I understand there are ways around the internal limiters but they involve a pretty complete disassembly of the carb. I also have a Stihl BG-86 blower with the internal limiters. These are set very tight and I am sure this is partly due to the fact that this blower has a catalytic converter and those don't take kindly to rich mixtures. I opened them as far as they could go and that wasn't much. I feel that you get a little more takeoff when it is richer, plus you are running more lubrication through the engine, so it isn't a bad thing. This running everything so lean is to keep the EPA happy. I will say that this BG-86 blower runs almost as clean as electric model. Once you get it warmed up, there is no smoke or fumes.

Conor

I'm at work and don't have the manuals in front of so bear with me I'm going off memory. The L (Low) and H (high) screws are both 1 full turn out. However, I believe the newer Stihl saws have limited caps on them so they will have to be removed to accurately set the L and H screw. The LA valve should be adjusted (screwed in) until the butterfly valve just starts to open. It should run at this point, so start it up and allow everything to warm up. Next I use a Tach to set the low rpm to 2800 rpm (I think this is the number) using the L screw. Any adjustments here should be less that 1/8 of a turn either way. And for high rpm I set it to 13.8K using the H screw, again should be less than an 1/8 of a turn.

My personal opinion is if you are unable to set the saws idle doing this there is another problem, air leak, clogged filter, spark plug, etc....

Good luck.

 

[w8ye]

Here's the page from the owner's manual on needle adjustment

I set the low speed just low enough that the chain doesn't spin.

 

[cwatkin]

Ok, I found it! This was in plain sight once I realized it was being covered by some safety warning sticker that the factory or dealer had stuck on the air cleaner cover. After looking at this diagram, I could see that there was something under the sticker. I removed the cover and could see the adjustment so I peeled the sticker off since it isn't like I don't already know that a chainsaw can be dangerous.

I feel kinda stupid for overlooking this. I guess I assumed it was some type of EPA model where it wasn't adjustable as that is becoming the norm on lots of equipment now. My dad has a couple of blowers that had to go into the shop for some type of adjustment. The dealer essentially told him that they are making all sorts of stuff non-adjustable now. I almost wonder if they just don't want anything user serviceable now so that you must take it to the dealer for repair or just buy a new one.

Anyway, I turned this in just a tad and the world is good. I only cut a few logs after making the adjustment but it didn't die after I let off like it was doing before before.


Thanks again,

Conor

Here's the page from the owner's manual on needle adjustment

I set the low speed just low enough that the chain doesn't spin.