MS171 Compression

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Can you recommend an appropriate small engine compression tester?
Well if it is reading around 160 in good saws it is good.
I use a Snap on .
How about a picture of the piston through the exhaust port, you said it looked good, but can we see it. You want to call me, I will PM you? Something is being lost in translation here.
 
just opened it up to clean and reseal. I still think this thing isn't broken in yet. I will also test some line trimmers and blower.
Here are pics of the piston as I removed it. There is a little oil on it from my testing. Couldn't get a good focus on the cyl. but it looks just as good. Ring isn't cracked and looks new. I agree, I expected higher reading, especially with the Lyle tester as it is one single hose instead of a coupling like the Snap-on one. I'm going to re-assemble and see if I can start it. Have to get a spark plug - it's one with the small threaded end. One pain - the bolts that hold the engine to the housing also hold the bottom clamshell on - so you have to bolt it together to pressure test, then remove to re-assemble. I will wait until the sealant cures. Seems like the 170/180 was a better design.

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just opened it up to clean and reseal. I still think this thing isn't broken in yet. I will also test some line trimmers and blower.
Here are pics of the piston as I removed it. There is a little oil on it from my testing. Couldn't get a good focus on the cyl. but it looks just as good. Ring isn't cracked and looks new. I agree, I expected higher reading, especially with the Lyle tester as it is one single hose instead of a coupling like the Snap-on one. I'm going to re-assemble and see if I can start it. Have to get a spark plug - it's one with the small threaded end. One pain - the bolts that hold the engine to the housing also hold the bottom clamshell on - so you have to bolt it together to pressure test, then remove to re-assemble. I will wait until the sealant cures. Seems like the 170/180 was a better design.

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Looks like the machine marks are still on it, I don't think it's very worn at all upon first glance. So this one uses a small plug what plug size do the other saws with more compression readings have, did you need an adapter for the saw you are working on because that would explain it. If all things are equal, it doesn't make sense.
 
Looks like the machine marks are still on it, I don't think it's very worn at all upon first glance. So this one uses a small plug what plug size do the other saws with more compression readings have, did you need an adapter for the saw you are working on because that would explain it. If all things are equal, it doesn't make sense.
Yes, no wear, not even dirty. Yes, had to use a small adaptor on the 171, not on the 170/180, but they only read 125. The 171 read 125 with oil - I know it should be dry - the other two and the 361 were. I'll check the opening on the adaptor and compare to the regular fitting. If it's much smaller, then try to restrict the regular fitting to see if it makes a difference.
 
Yes, no wear, not even dirty. Yes, had to use a small adaptor on the 171, not on the 170/180, but they only read 125. The 171 read 125 with oil - I know it should be dry - the other two and the 361 were. I'll check the opening on the adaptor and compare to the regular fitting. If it's much smaller, then try to restrict the regular fitting to see if it makes a difference.
The adapter can give low readings. I wouldn't worry about it from the looks of that piston. I'm thinking once you properly fix the leak, it will run. What was the cause of the leak again?
 
The adapter can give low readings. I wouldn't worry about it from the looks of that piston. I'm thinking once you properly fix the leak, it will run. What was the cause of the leak again?
The leak was my fault - didn't seal the upper intake port well enough. That wouldn't affect compression as both ports are open during compression test. When I pulled the engine, I made a plate to cover all three openings on the block. With the mounting bolts installed in the bottom clamshell, no leaks. I don't see how a leak on the bottom end would affect compression. I've had saw with leaks still have good compression. I could see iIf it was a leak on the top end, but then it wouldn't hold pressure. I'm plugging off the exhaust and intake, including the impulse port and testing through the spark plug hole.
 
171 uses an adapter?
You mentioned that the shraeder valve is 1” or so away. That’s enough in a small saw to give a low reading. It needs to be “in” the end of the adapter that goes into the combustion chamber.

But if it’s oem, the cylinder isn’t scored up, and the ring end gap isn’t huge, it’ll make enough compression to run fine
 
I have no idea how many tanks this thing has had but I’d guess quite a few. I bought it 2 months ago and it looked like it lived a rough life. Original bar had most the paint smoked off it and had so much dirt and grit in the nose sprocket you’d have thought the original owner used it as a shovel. The fuel line had been replaced at some point with tygon and that had rotted out in the tank so now on its 3rd fuel line. Still runs like a raped ape. I didn’t count how many pulls but probably 6 - 8. The psi came up pretty quick. Mityvac MV 5530.
 
171 uses an adapter?
You mentioned that the shraeder valve is 1” or so away. That’s enough in a small saw to give a low reading. It needs to be “in” the end of the adapter that goes into the combustion chamber.

But if it’s oem, the cylinder isn’t scored up, and the ring end gap isn’t huge, it’ll make enough compression to run fine
That makes the most sense so far. The 171 has a smaller spark plug hole, see photo of adaptors I bought, it takes the smallest one in the set. So, the schrader valve is back a ways, effectively increasing the size of the combustion chamber. This would have a more effect on smaller cc engines and reduce compression readings. On the 170 and 180, no adaptor is used so the schrader valve is right in the chamber and reading are higher. Lone Wolf, this is probably what you were trying to tell me earlier - sorry it just didn't click at the time. Mea Maxima culpa. So, I will attempt to measure the amount of increase caused by the adaptor and compare it to the 170/180. I'll fill the space in the adaptor with water when installed on my tester and compare to the cc of the combustion chamber. The percentage of increase is probably equal to the drop in pressure between the 171 and 180. I may attempt to machine my own adaptor so the valve is farther into the spark plug hole. I will also let the company that makes the adaptors know to take the increase into consideration and advise customers.
Funny, I've been working on cars over 50 years, ASE Master Tech for 35, building saws for 27, and this never occurred to me. It's all Stihl's fault for going to that smaller sparkplug opening. :rolleyes:;)
 

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Here's my solution. First pic shows the relationship of my tester to the adapter :angry:. I took an old rubber tire valve and stripped off the rubber, measured the brass, and bored out the adapter, then machined the tire valve to a snug fit and assembled with red loctite. The shrader valve is now as close to the compression chamber as I can get. I did use the schrader valve from my tester, not the tire valve. I applied pressure to the other end of the tester hose and no leaks. I will assemble the saw with the broken parts and retest compression. Assuming this works, I will notify the adapter company.
 

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Compression is now 130. Not 185 but still a big difference and it matches the other two saws. I'm not going to try starting it, I know it will run. Thanks to all for your patience and for sticking with me on this. Now to find one with all the parts I need. Found one in the UK but he won't ship to the states.
 
Here's my solution. First pic shows the relationship of my tester to the adapter :angry:. I took an old rubber tire valve and stripped off the rubber, measured the brass, and bored out the adapter, then machined the tire valve to a snug fit and assembled with red loctite. The shrader valve is now as close to the compression chamber as I can get. I did use the schrader valve from my tester, not the tire valve. I applied pressure to the other end of the tester hose and no leaks. I will assemble the saw with the broken parts and retest compression. Assuming this works, I will notify the adapter company.
FWIW, DO NOT use regular tire schrader valves. You need a special one with low spring tension as the pressure to open the valve will effectively get SUBTRACTED from your reading on the gauge.
 
FWIW, DO NOT use regular tire schrader valves. You need a special one with low spring tension as the pressure to open the valve will effectively get SUBTRACTED from your reading on the gauge.
You are correct. As stated in my post, I didn't use the tire valve shrader - I used the one from the tester. I made it bold now so all can see.
 
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