Compression, I need a lesson

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Metallichg

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Could somebody please explain compression in terms of my situation:
I bought a Stihl 070 with a 090 cover, sold as an 090. I checked the compression of the 070 and got 50 PSI when pulled over many times without using compression release. it turned over easy. I changed out the PC and clutch on this saw using an expensive Mahle 090 PC. after putting it all back together, I checked compression again. I barely had enough muscle to turn the saw over. I pulled the starter 5 times and was only strong enough to get one compression stroke each pull. my compression Reading was 100PSI. Is this good or bad? I have read posts from people that are getting 185 PSI on their saw. How is that possible? I may be a 6'1" whimp, but I can't possibly be that weak. What am I not understanding about compression that would make this make sense? I fired up my saw with new clutch and PC. I thought it ran great. Definitely needed compression release to start, but started up from zero gas in about 3 pulls. Was kind of poppy when idling and had tons of muscle with a 36" bar, 404 chain, 8 pin sprocket and some big maple, ran about 3 tanks of gas (in about 30 min). Can somebody please give me some info to explain it what am describing regarding compression, and does this sound normal for a 090 with new PC? Since I don't know, I would rather ask the experts than make guesses on my own. thanks very much-Joe
 
Sounds like your guage is bad. Most says won't run with compression under 90 psi and are pretty stubborn to start at those numbers.
 
I could have a junk compression tester, just bought off Ebay for 50.00 it's got an orange flexible cover. I'll have to get another and repeat. I never thought the Guage could be junk, but that is a very good possibility. the saw ran good with 50psi, so I will replace it. thanks for the good ideas!
 
I suspect that you aren't pulling the saw over fast enough to get a good reading. You said you only got one compression stroke per pull. If that's true then you likely have a lot more than 100psi.
 
Hmmm. My brand new compression tester is showing me 60 psi all the way up to 135psi on my saws. I never considered that it may be bad because it's newish, but cheap.
 
I think too many people rely on compression gauges. If it starts it's got enough to run. If you try cutting with the saw and it's low on power it's low on compression. At this point the quality of his tester is pointless since he can't even pull the saw over,meaning too much compression.
 
I think too many people rely on compression gauges. If it starts it's got enough to run. If you try cutting with the saw and it's low on power it's low on compression. At this point the quality of his tester is pointless since he can't even pull the saw over,meaning too much compression.
Or a bind in the starter mechanism, or new seals that make it hard to turn over?
 
there is different shrader valve (think i spelled that right) for different app. a car compression tester will have stronger shrader valve as a diesel tester will have a way way way stronger shrader valve. also if i remember correctly you compression will go up after running 5-10 tanks of gas. send me you email address @Metallichg and i will send you the stihl engine fault manual will help with checking cylinder and ring gap when checking a used cylinder or a new one. but i do think 100 is kind of low for a new p&c
 
there is different shrader valve (think i spelled that right) for different app. a car compression tester will have stronger shrader valve as a diesel tester will have a way way way stronger shrader valve. also if i remember correctly you compression will go up after running 5-10 tanks of gas. send me you email address @Metallichg and i will send you the stihl engine fault manual will help with checking cylinder and ring gap when checking a used cylinder or a new one. but i do think 100 is kind of low for a new p&c
The valve needs to be in the tip that goes in the spark plug hole or you will have a false low reading.
 
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