Stihl chainsaw almost locked up-?

[Gator6x4]

Two cycle engines are different from four. They are some what simple. They have an intake port and a exhaust port. They also have a reed valve. The reed valve open to let gasoline into the combustion chamber on the upstroke or compression stroke, the gas vapors are compressed and ignited. The piston is forced downward and the hot gases are exhausted out the exhaust port. No intake or exhaust valve. If the exhaust port or muffler is clogged the air or gas has no where to go and the engine will "Lock" up or "Freeze". Also the majority of two strokes do not have a cylinder head like a four stroke. The cylinder is a one piece cast affair that the piston travels up and down in. For a good schematice of a two stroke motor go to the below web site. A two stroke will also lose power and not achieve maxium RPM's if the exhaust port or muffler is restricted.

HowStuffWorks "How Two-stroke Engines Work"

 

[MJPetersen]

A better "starting fluid" for 2 cycles is WD-40. It works great when you are trying to sort out a carb problem. It also cleans the varnish build up of dried oil/fuel.

The "flexible thing" behind the carburetor is the intake reed valve. As the piston goes up on compression a vacumn is created in the crank area and the fuel/oil/air mix is drawn in, when the piston goes down on the power stroke then the mixture is compressed in crankcase, exhaust ports are uncovered and the spent gases are exhausted, and intake ports are uncovered and the fresh fuel/air is brought into the area above the piston, which then travels up to compress the mixture. OK pictures are easier!

2 cycle engines are simpler in that they have fewer moving parts, and they can be temperamental, but with proper care and a bit of understanding they can be very reliable and tremendous powerhouses for their weight.

Mike

 

[jmc]

OP here. Thanks, everyone, for the help so far. Got a chance to play with it somemore last night and got the muffler off. There's no reed valve at the exhaust, no carbon buildup at the exhaust port, and I can blow through the muffler. (love that taste!)

Its very apparent this is a trapped fuel mixture problem and not a simple sluggish rope or mechanical block of the piston. To those of you who have been reading these posts closely, that flex I mentioned at the bottom of the carb during the compression stroke didn't look like a cantilevered leaf spring flexing. It looked more like a diaphram bulging or being sucked in- hard to say with limited visibility.

Another quirk that may be telling is this: when turning over slowly by hand, it gets difficult near top dead center on the compression stroke of course, then snaps easily over center for maybe 20 degrees, then starts to become difficult again (but not as bad) until about 30 or 40 degrees past TDC.

Anyway, is that intake reed valve made from a piece of sheet metal and sandwiched between the carb base and the carb gasket? Hope you all have some more thoughts.

John

 

[Gator6x4]

OP here. Thanks, everyone, for the help so far. Got a chance to play with it somemore last night and got the muffler off. There's no reed valve at the exhaust, no carbon buildup at the exhaust port, and I can blow through the muffler. (love that taste!)

Its very apparent this is a trapped fuel mixture problem and not a simple sluggish rope or mechanical block of the piston. To those of you who have been reading these posts closely, that flex I mentioned at the bottom of the carb during the compression stroke didn't look like a cantilevered leaf spring flexing. It looked more like a diaphram bulging or being sucked in- hard to say with limited visibility.

Another quirk that may be telling is this: when turning over slowly by hand, it gets difficult near top dead center on the compression stroke of course, then snaps easily over center for maybe 20 degrees, then starts to become difficult again (but not as bad) until about 30 or 40 degrees past TDC.

Anyway, is that intake reed valve made from a piece of sheet metal and sandwiched between the carb base and the carb gasket? Hope you all have some more thoughts.

John

What you are looking at through the bore of the carb is the reed valve. When the engine is rotated the reed valve flexs inwards to allow the gas to enter into the combustion chamber and then provides a restriction to prevent the hot exhaust gases from blowing back through the carb. Most reed valves are made out of stainless steel and are calibrated for the engine they are installed on. I do not know of anyone who has ever fabricated one out of sheet metal. Did you have the spark plug out when rotating the engine? If you did not have the spark plug out this sounds normal. If you did have the spark plug out, then that sounds abnormal. Look at the diagram I posted for your review in an earlier post. Watch the rotating engine and you will be able to observe how a two cycle engine works.

 

[MJPetersen]

Hard to pull problem (if there is no damage somewhere) is usually a flooded condition. What is the condition of your fuel? Old fuel does not start well--you might get away with it after it is running but it will not start well on old fuel. put in fresh fuel and use WD-40 to get it firing. Wide open throttle and no choke, spray in some WD-40 and see if you can get a pop. Once it fires it will clean itself out of the old excess fuel.

Mike

 

[jmc]

OP here. Mystery solved, for the most part. The last seemingly probable problem, the intake reed valve, was working freely.

Although the gas drained from the tank looked normal, the gas pumped out thru the plug hole was syrupy. After about 30 pulls, it was all pumped out. After putting the plug back in, the engine was acting normally again. I'm guessing that fuel prevented a necessary amount of blowby required to be loose enough to start, which is probably similiar to what MJPetersen was talking about with the flooding issue. Next step is finding some of Carykong's fuel additive.

Thanks, everyone, for your help and interest.

John

 

[westbrooklawn]

Pick up some "Sea Foam" and add to your gas. In addition to being a stabilizer, it works wonders at clearing up fuel delivery problems.

 

[WBWI]

My craftsman push mower was exhibiting the same problems. The last time I used it it smoked like crazy for a few minutes then ran normally. It sat for 2-3 weeks and when I tried to start it it felt like the piston was frozen. I pulled the plug and was able to cycle the engine but when I put the plug back in I could not pull the starter rope. It was also spitting out oily crud when I pulled the plug out. Do you think I have a chance a getting it running again? I did buy a replacement so it doesn't really matter.

 

[Treemonkey1000]

Sounds like you may have leaky valve seals or some blow by in the piston. With the other guys fix though you might just get by with pulling the plug and spray either starting fluid or carb and choke cleaner down the plug opening. Then cranking it over. Although be careful vaporizing that in a garage or something and having the spark plug connected. That might flash it off if you are a little close. I haven't had it happen to me but still...............

 

[MJPetersen]

WBWI, What engine does it have? Is it stored level or tipped? What is the oil level? Did your fuel level drop durring storage? You are getting excessive oil or fuel on the piston. It is either coming from below the piston up past the rings or in the intake and valve. It is probably as simple as turning off your fuel when you store it. (small carbs often have leaky needle valves.) Or draining the oil and filling to the correct level. (I would do that anyway if you had fuel leakage.) Nothing major is probably wrong with it.

Mike