A little more info is needed.
Compression test: a simple way is to hold the pull cord like you are going to start the saw. A strong motor with good compression tends to either stay in place or slowly fall as the piston moves. If the saw just falls down the motor is worn.
But compression increases as a motor warms up so I don't think this is your problem. If it was the saw would be very difficult to start like my dad's old Stihl 041 Farm Boss from the early 1970s.
20 minutes is a long time to run without a problem.
Consider cleaning the carburetor (or a small can of goof off) with carb cleaner - but don't get it on plastic or rubber parts only metal.
Smoke: How much smoke is there? If there is no smoke the mixture might not have enough oil causing the motor to overheat. Also, many small engines recommend regular not premium gas.
could be a coil as weak coils tend to die off as they warm up try running it till it dies and check for spark right away it should create a bright blue spark not orange also if your running mixed gas that doesnt have enough oil run engine wide open for about a minute or until engine is hot then check spark plug if the tip is white your running to lean and must run a richer mixture of gas oil
Interesting possibility. If he doesn't see the spark he could touch the plug and give it a very gentle pull. If he doesn't jump that coil is tired. What about the spark plug? A weak coil would lead to more carbon build up.
could also be the pick up line in the gas tank sometimes the hose has a hole part way up and after the fuel gets lower than the hole the saw sucks air instead of fuel when the saw stalls out try filling the fuel tank and restarting the engine as if it were cold ie.choke
as far as the plug building carbon from a weak coil it is unlikely due the engine temp. at the time of failure,normally when a coil is beginning to fail it will work normally until it overheats causin the saw to stall out but by this time the engine is @ full operating temp. so the build up of carbon tends not to happen
curtis interesting suggestions. I didn't think about a hole in the fuel straw. I'm going to un plug from updates on this discussion as the original poster appears to have abandoned the quesition.
OKay, there are 2 obvious reasons for failure, and they are both in the same system, Ignition. The sparkplug must be matched for the specifications of the Unit that is why they install a certain plug when its manufactured. Some run cool and ignition is spotty. Others run too hot and foul out completely. The other is the ignition coil. Chainsaws generate a lot of debris and oily mess when operating. This mess tends to accumulate inside the case, around and near the vicinity of the coil magneto surfaces. Routine maintenance keeps this area clean. Wipe these surfaces with a rag dampned in brake cleaner or alcohol and let dry and then wipe again with a clean dry rag before reassembling the unit. 80% of the time this procedure beats the expensive outlay of replacing the ignition coil, when it is not needed.
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