Questions about Stihl MS 251

[tomdcoker]

I recently acquired a Stihl MS 251 that is scored. I dissembled it this afternoon with plans to rebuild it. The first thing I did in the process was a pressure and vacuum check and there was absolutely no leakage. So since this saw has seen very little usage from the looks of it my first inclination is lean factory settings. I will check the fuel line and filter before reassembly.
My question is how to set the carb.? Where can I get the tool required and I know that it is probably not available except from Stihl and that is not going to happen? The next question is how can I remove the covers without destroying the carb. and will I then be able to adjust the carb. Any help would be appreciated. Tom

 

[HarleyT]

Here...

 

[jackjcc]

I’d bet more it was straight gassed, not improperly tuned from factory.


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[Franny K]

https://www.ebay.com/itm/Stihl-OEM-...943587&hash=item4aea7594bc:g:tUsAAOSw3xJVXg7M

Around page 94 or 95 if the pdf file attached above. But the pdf file says to use a drift to put it back.

You might look on ebay and see what parts for that saw sell for. When I got mine they were pretty new and folks seemed to pay about $100 for the cases. Might be better off selling it as parts and buying an new one. I do not recall using the screw driver the saw comes with to adjust the screws. It seems to me the kind of saw that I would be hesitant to buy used as I think high rpm and no load is not good for a two cycle and this one seems easy to run that way.

 

[Conquistador3]

I know those saws very well.
Unless the carburetor has been dicked with there are exactly zero chances it seized because of too lean fueling. You can easily check if the saw has been fooled with: if the H limiter cap is missing, the carburetor was messed with.
If that is not the case, just use the manual provided above to check the factory settings.

I am ready to bet a shiny sixpence that saw was straight-gassed: they may be little loved plastic clamshells, but those saws are really well made and far tougher than most would expect.

 

[tomdcoker]

I’d bet more it was straight gassed, not improperly tuned from factory.


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That is a possibility. There was not any gas in the tank so I could not check that way, but looking at the piston I suspect that it was straight gassed. It was scored severely on the exhaust side and a small piece of the piston was gone on the top edge with no sign of damage to the cylinder other than scoring. So in my opinion it was hot enough that it come off in small pieces or crumbled when it came off and hit the top. Thanks for the help and information. Franny K Posted a link to a tool to remove the caps that I have. I never thought about using it. Tom

 

[sawfixer]

I rebuilt one of these a year ago which had a hole in the piston due to old stale gas (low octane). The factory installed Zama C1Q-S296 carburetor which comes on this saw is about the worst I've seen with respect to restricting adjustment of the low and high speed jets. They are both encased in steel. What's worse is there's no way to clean the carb completely when you cannot remove the jets. I got fed up with the quarter turn maximum adjustment and got my Dremil out with a cutting well and cut about 2/3 of the OD of the outer steel cover and then used a flat bladed screwdriver to break of the outer covers. Just be careful not to cut into the needles. The ability to adjust the carb helped when I opened the outlet of the exhaust to improve the breathing.

 

[tomdcoker]

I rebuilt one of these a year ago which had a hole in the piston due to old stale gas (low octane). The factory installed Zama C1Q-S296 carburetor which comes on this saw is about the worst I've seen with respect to restricting adjustment of the low and high speed jets. They are both encased in steel. What's worse is there's no way to clean the carb completely when you cannot remove the jets. I got fed up with the quarter turn maximum adjustment and got my Dremil out with a cutting well and cut about 2/3 of the OD of the outer steel cover and then used a flat bladed screwdriver to break of the outer covers. Just be careful not to cut into the needles. The ability to adjust the carb helped when I opened the outlet of the exhaust to improve the breathing.

That is the answer that I have been looking for. Thanks. Tom