Stihl MS-250 chainsaw engine bogging down

baymee(LehighValleyPA)April 14, 2008

My neighbor's Stihl chainsaw was bogging down so I said I'd take a look at it. It's not very old.

I took the carb off and dismantled it; blew out the jets, eyeballed everything. Checked the spark arrester, it's like new. Put in new gas mix. Chain spins freely. Adjusting screws at their proper position. Spark plug is new and proper type.

Went to start it up and it ran great. Revved it up and down and it had great high speed and smooth low to high speed acceleration. After about 30 seconds it was bogging down again. It has an external tank vent, and it is working well. Tried restarting and it starts easy enough, but just won't go from low to high speed. If I coax it, I may get it to high speed for one second and then it's bogging again. Tried adjusting the Low speed screw a little, but no good.

Any suggestions appreciated.

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ladylake(minnesota)

Try cleaning the carb again, could be a little crud got back in. Maybe put a new fuel line and filter in first. Steve

    Bookmark   April 14, 2008 at 7:46PM
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masiman(z7 VA)

Fuel and impulse lines can also be problems if they are old and or cracked. Fuel filter and fuel hose are other things to check too. Since you can get some throttling out of it I would not think it is the air filter.

Carb rebuild kits are cheap. The diaphragm could be stiff.

    Bookmark   April 14, 2008 at 8:42PM
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baymee(LehighValleyPA)

Will check again.

    Bookmark   April 15, 2008 at 5:53AM
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rcmoser

I would try to richen up the high mixture alittle see if that helped? If not My guess would be the diaphram also.

    Bookmark   April 15, 2008 at 10:26PM
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baymee(LehighValleyPA)

The diaphragm "appeared" to be good. The high speed jet has a limiter on it, but what is strange is that it ran perfectly for the first 30 seconds after I cleaned the carb. I tried it out again yesterday and it bogged down immediately.

    Bookmark   April 16, 2008 at 6:39AM
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masiman(z7 VA)

To make sure it is not the air filter, try running it briefly with the air filter off.

    Bookmark   April 16, 2008 at 9:09AM
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barbedwire

It seems that all the bases have been covered here. I`ll second the suggestion that you have a good look at the fuel line.

    Bookmark   April 16, 2008 at 9:26AM
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josephtm(CHN India)

This sounds more like a partially blocked pickup body(fuel filter)
Remove the pickup body and try the engine once more.If it runs OK, replace the pickup body.

    Bookmark   April 18, 2008 at 10:13AM
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baymee(LehighValleyPA)

I took the carb apart again, removed and blew out the fuel line and filter, all is clear. Removed the limiter on the high speed needle and tried that adjustment, nothing worked. The fuel line does have some cracks in it, but I applied a little pressure to the line and it doesn't leak through. I hate to tell the guy to get a new line, but that might be the next step.

    Bookmark   April 19, 2008 at 7:06AM
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josephtm(CHN India)

Check the four screws holding the engine pan and cylinder together.
They are located on the underside of the saw.
These screws becoming loose could cause a similar symptom.
External air gets in and causes the engine to lean out.

    Bookmark   April 19, 2008 at 9:46AM
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baymee(LehighValleyPA)

Good point, and will do. But why did it run perfectly for the first 30 seconds after I cleaned the carb internally and then start bogging? Subsequent restarts result in immediate bogging.

    Bookmark   April 20, 2008 at 11:21AM
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ladylake(minnesota)

That running good for 30 seconds is why I suggested to clean the carb again, could have been some dirt in the fuel line or somewhere that clogged it up again. Haveing cleaned the carb again it might be time to try a new fuel line, it could be cracked inside the tank. I suppose you could try holding it upside down, then if the line had a crack in the tank it would suck fuel instead of air. Steve

    Bookmark   April 20, 2008 at 8:56PM
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den69rs96(z6CT)

my older mcCollough would bog down and idle fine. I cleaned the carb. Worked ok for a while, tried adjusting the fuel mixtures and it still bogged. Found cracked fuel lines were the problem.

    Bookmark   April 24, 2008 at 10:51AM
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lawjort_msn_com

I had exactly same problems. Blew out the carb, cleaned air cleaner, new fuel pickup, changed spark plug. Same thing, not working. Changed the fuel pickup line to carb, works like new!

    Bookmark   June 6, 2011 at 8:54AM
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