I've been a Stihl dealer since the mid 1980's selling and repairing thousands of chainsaws, blowers, hedge trimmers, lawnmowers etc. I get pulled off the counter often to render a second opinion when my technicians are questioning and confused by strange compression readings. Two things I see wrong as I watch your video. First, the compression gauge you're using is better suited for use with large displacement four stoke engines (automotive, motorcycle etc.) but probably still ok to use with a 160cc lawnmower engine. The internal volume of the 10mm adapter coupled with two to three more feet of unnecessary hose probably exceeds the the 22.7cc displacement of the Stihl HS81 engine you're trying to test. For small displacement two stroke engines I prefer Echo's compression gauge because it is designed with a short hose having a small internal diameter. Also the valve stem is located as near as possible to the combustion chamber even when used with their very short 10mm adapter. I've found that using this style of tester will not only produce accurate results but also allows me to see how many pulls of the starter rope it takes to reach a maximum reading. A heathy engine will be at 90 percent or even higher on the first pull of the starter and peak out at a maximum reading after the four or fifth pull. A suspect two stroke engine will have a low reading on the first pull and require ten or more pulls slowly climbing to maximum which is usually less than 115 PSI. Engines with these test results will be hard to start, low on power, die when they warm up or won't start and run at all and should be inspected much closer for damage or wear to the piston and cylinder. The second thing I see wrong is your gauge only recorded a reading of 10 to 20 PSI maximum. It is IMPOSSIBLE!!!! for an engine to run with a reading that low meaning that your tester or testing procedure or both are faulty. I suspect that if you had been able to connect the tester to the 50 PSI stored in your air compressor the gauge would have read 50 PSI. My guess is that the valve stem in your 10mm adapter is loose and should be tightened or replaced. There could also be a piece of carbon or dirt holding the valve open allowing the compression pressure to escape back into the engine rather than be captured and building to a maximum reading of 130 to 140 PSI even in your overly long hose tester. Remember the compression in this engine is momentary not constant like the 50 PSI in the air compressor tank. After hearing the engine start and run it ran perfectly with a stable idle, good throttle response and a slight "blubber" at no load high speed. That is exactly what a premium piece of Stihl equipment should run like. I hope this has been helpful and insightful for you and everyone out there.
I'll add that compression guages are junk at Harbor Freight. They read way low even on car engines. I've had mine since the 90's, don't know what make it is, but it has a schrader valve right at the spark plug. I had to get a new valve recently, they're special low psi ones just for this purpose. Brand new Stihl BG50 blower has 150 psi. I never got more than 120 out of my Husky blowers, at less than 100 they don't start. Not much room for wear.
I've been a Stihl dealer since the mid 1980's selling and repairing thousands of chainsaws, blowers, hedge trimmers, lawnmowers etc. I get pulled off the counter often to render a second opinion when my technicians are questioning and confused by strange compression readings. Two things I see wrong as I watch your video. First, the compression gauge you're using is better suited for use with large displacement four stoke engines (automotive, motorcycle etc.) but probably still ok to use with a 160cc lawnmower engine. The internal volume of the 10mm adapter coupled with two to three more feet of unnecessary hose probably exceeds the the 22.7cc displacement of the Stihl HS81 engine you're trying to test. For small displacement two stroke engines I prefer Echo's compression gauge because it is designed with a short hose having a small internal diameter. Also the valve stem is located as near as possible to the combustion chamber even when used with their very short 10mm adapter. I've found that using this style of tester will not only produce accurate results but also allows me to see how many pulls of the starter rope it takes to reach a maximum reading. A heathy engine will be at 90 percent or even higher on the first pull of the starter and peak out at a maximum reading after the four or fifth pull. A suspect two stroke engine will have a low reading on the first pull and require ten or more pulls slowly climbing to maximum which is usually less than 115 PSI. Engines with these test results will be hard to start, low on power, die when they warm up or won't start and run at all and should be inspected much closer for damage or wear to the piston and cylinder. The second thing I see wrong is your gauge only recorded a reading of 10 to 20 PSI maximum. It is IMPOSSIBLE!!!! for an engine to run with a reading that low meaning that your tester or testing procedure or both are faulty. I suspect that if you had been able to connect the tester to the 50 PSI stored in your air compressor the gauge would have read 50 PSI. My guess is that the valve stem in your 10mm adapter is loose and should be tightened or replaced. There could also be a piece of carbon or dirt holding the valve open allowing the compression pressure to escape back into the engine rather than be captured and building to a maximum reading of 130 to 140 PSI even in your overly long hose tester. Remember the compression in this engine is momentary not constant like the 50 PSI in the air compressor tank.
After hearing the engine start and run it ran perfectly with a stable idle, good throttle response and a slight "blubber" at no load high speed. That is exactly what a premium piece of Stihl equipment should run like. I hope this has been helpful and insightful for you and everyone out there.
Real good info thx
Your compression tester needs a schrader valve located in the spark plug adapter.
I'll add that compression guages are junk at Harbor Freight. They read way low even on car engines.
I've had mine since the 90's, don't know what make it is, but it has a schrader valve right at the spark plug. I had to get a new valve recently, they're special low psi ones just for this purpose. Brand new Stihl BG50 blower has 150 psi. I never got more than 120 out of my Husky blowers, at less than 100 they don't start. Not much room for wear.