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Lawn Mower Compression Test

Lawn mower compression: this spark plug is a little bit blackened, a little bit fouled, you would say. Now the most likely cause of this is over-fuelling or a rich fuel condition.  Let me compare it here to a spark plug out of my car… Which is a very clean burning machine, especially alongside of my lawn mower. However a machine like a lawn mower sees such dirty service and has kind of a crude air filtration system. So you’re going to have problems like that.


Reasons for a blackened or carbon deposited spark plug would include things like too rich a fuel mixture. Or a sticking choke or a clogged air cleaner, you know. Anything that’s going to increase the fuel in the fuel : air ratio.

Also a good possibility is oil seeping past the rings on the compression piston. And getting up into the combustion chamber and fouling it. So what we’ll do is a quick compression test. And have a look and see what kind of numbers we’re pulling.


OK so check it out. On the left we have my low-range compression tester, on the right my high-range compression tester. The high range of course is for high compression diesel engines. The low range one is for a wide range of vehicles and machines. Including this tiny little four stroke engine we’re going to give a test to. OK you just push on this spot right here and that releases the lid… just remove these two screws. There’s another one in behind here. That’s why you need the long extension on your socket wrench. And as that one comes out it just falls away like that.

OK so we’ll have a look in here. There’s your money shot right there showing the throttle plate. So the procedure here is to put a screwdriver in this hole to block that open. Whatever you use make sure that it can’t fall in. And it won’t break apart and pieces of it fall in. Put my screwdriver on in there past the throttle plate. Make sure it’s blocked open before I pull on the cord. Thread it into the spark-plug hole, just do it about like that: hand tight. It’s a good idea to just put it down somewhere safe out of the way.

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OK so I looked around online to see if I could find some technical specifications for this engine. A new machine like this will make about eighty psi in pressure. Sixty psi on the other hand is generally agreed to be acceptable. Twenty psi is only a quarter of eighty psi so that’s twenty five percent. Most mechanics I’ve worked with agree on a range of acceptability of around fifteen to twenty percent. So twenty five percent is quite generous, now keep in mind that it would depend on the type of machine. Consensus seems to be that it’ll still cut grass acceptably under normal conditions. Even with a twenty-five percent loss in compression.

[Engine turns over]

Sorry about the bumpy ride.

OK I’ve done a dry compression test and the results are in. We’ve got eighty psi and its been holding eighty psi for five minutes now. 

OK we’re just going to put a drop of thirty weight oil in there. Maybe two shots. Put the compression tester back in. 

OK so now with a drop of oil in the combustion chamber we’ll do a wet compression test.

OK now the wet compression test is eighty psi. This ten year old lawn mower of mine is making the same compression it did the day it came out of the factory.

[Mower starts]

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