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johnmonkey

Cylinder Pressure PSI

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johnmonkey
I was looking over my junk 3410 and wanted to see if the motor (B&S) would turn over (it has been in a junk yard for a few years & is missing a carb, steering wheel, drive shaft etc.) I tumed it and it turned over fine. I checked the cylinder pressure and it read ~ 110-115psi. I was curious to see what the psi was for my AC 710 (Kohler) and it read ~ 90-95psi. Is the low psi a potential problem? Thanks John H

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Guest
It's probably not a problem. I don't know much about Kohler engines, but Briggs and STratton service manual doesn't even list compression pressure for their engines. Their reason is that they say it's difficult to turn them over fast enough (especially when there's no starter) to get an accurate reading. They also gave some other reasons that I don't recall at the moment. What they say to do is just give the flywheel or pulley a kick backward. If it bounces back forward when it reaches compression, then there's enough compression for it to run fine. You have to turn it backward because on those B&S engines with the easy spin starting, the intake valve is held open slightly at the beginning of the compression stroke which reduces compression. It's not held open if you're turning it backward. This isn't to say that the engine won't burn any oil from worn rings if it passes this test, but just that it should run with the amount of compression it has. It may need a little work.

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roma3112
i agree with dan, i have attended many briggs seminars/schools and it always struck me as odd that they never pushed going out and buying some $300.00 snap-on compression gauge. Just the opposite the good old finger ofer the plug hole was taught as a basic diagnosis procedure.

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Spyder
Standard Air Pressure at Sea Level is assumed to be 14.7 psi. This may change due to weather, but let's assume it to be a "standard". Then you multiply by the compression ratio for the engine, I think I read somewhere B&S is 7:1. 7 x 14.7 = 102.9 psi. Sounds like the Briggs is really doing well. Not good compression is a sign of 3 things 1. leaking head gasket 2. valves sticking or not seating 3. piston rings not sealing, broken, or worn (if it also burns oil then this is a common symptom.) Sometimes excessive carbon deposits in the cylinder will also reduce compression since the deposits reduce engine displacement by a small amount. Actually, 90 psi is probably alright for the Briggs, but not sure of other engines, depends on compression ratio for the engine. A 10% degradation in compression is not a concern, 20% would be. 102.9 x 0.9 = 92.61 psi. Your Kohler is right in the range if its compression ratio is similar to Briggs. hope this gets your engine running!! DaveG

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HubbardRA
John, I would recommend that you put in new rings (cost about $35 last summer for my 10 Hp B/S). This way you will know what you have. Smoke is the most common problem, they will run with stuck or worn rings since most only have about 6:1 compression anyway. Rod H.

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