Lawn Mower Compression Test

How to do-it-yourself instructional discussing troubleshooting an ignition with a fouled spark plug and demonstrating both dry and wet compression tests on a four stroke (four cycle) Tecumseh lawn mower engine.

Transcript provided for the hearing impaired:
[No dialogue] [Music]
This sparkplug is a little bit blackened, a little bit fouled, you would say. Now the most likely cause of this is overfuelling or a rich fuel condition. Let me compare it here to a sparkplug 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 and 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 and 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.
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]



32 thoughts on “Lawn Mower Compression Test

  1. Thanks Dave. All my videos are closed captioned for the hearing impaired as well as for translation in 58 different languages using the YouTube translate captions feature.

  2. If it had 80 PSI when dry why would you do a wet test with the oil? I'm not trying to be a smartass. I've just started working on small engines and I'm trying to learn as much as I can. Great vid by the way. I got to get me a compression tester.

  3. Ok I couldn't start my 22040 2 cycle Toro Commercial mower, took it to the shop and they said that it wouldn't start do to low compression. Now I will be pulling it a part this weekend to check the rings but is there anything else to look for as I do this?

  4. hey i was wondering if i do not have that washer on the spark plug will i have low or no compression. becuase i have an honda gx200 and it is incredibly hard to start (have to pull it like 50 times) and i think that might be the problem becuase there is carbon residue around the outside of the spark plug and when i do finaly get it started i can hear sort of pshs pshs pshs noise on each stroke, is this normal?

  5. Got a 4 cycle Toro 20332 that won't start. Compression is only 30psi but I did not open the choke. Will that make a difference in the compression reading? I have another mower that starts on the first pull and it's compression is 45psi also did not open choke. 

  6. The fact that the compression reading stays for five minutes has nothing to do with the condition of the engine. It means that your compression tester valve is holding the pressure.

  7. Good video, I have a very old Briggs and Stratton engine that makes 75PSI on compression and is almost like new inside. The spark is bright blue also but the thing just won't run. I am running it on propane and it was working brilliantly but I think some curse has been set on it as all it does is kicks occasionally or just backfires. Will not even run for a second.

  8. One correction: You noted that your compression tester "held" the pressure for some time. They make them like this so the highest reading stays long enough to read it. This is due to the valve in the tester – it has nothing to do with the engine being tested. You mentioned the engine held this pressure – that's not true. Any engine would leak down over seconds and would never "hold" pressure after stopping

  9. my riding mower back fires.hard to start.i adjusted the valves.clean the gas lines and tank.didva crab rebuild.and still no luck.any ideas?

  10. "It's been holding 80 PSI for 5 minutes." An engine won't "hold compression". It might for a brief moment if it stopped in the right place, but certainly not for 5 minutes. Compression gauges have a one way check valve at the end that screws into the spark plug hole. It allows pressure to enter the gauge but not exit. Without it you couldn't even do a static compression test. You could screw the gauge out of the engine and it would still read 80 PSI. That's the reason for the little schrader valve next to the gauge, to vent the pressure after you've finished the test (or between cylinders if you aren't checking lawnmowers).

  11. I was givin a craftsman riding lawn mower. The previous owner siad he had it running and it just died! He never had it running since. I first checked for spark from the plug – nothing. I got a new plug and then a new ignition module. Still I didn't get spark. I then tested it for spark with a simple screwdriver up in the end of the plug wire while being grounded on the engine. I only get a FAINT spark. I no doubt had cleaned the monting bracket for the module and the flywheel. I do not know what to be looking towards? I even took off the flywheel and cleaned it under that. I saw it only had 4 magnets and then seen other ones, the same model number on ebay with only 4 magnets. Another told me the the manets under the flywheel are for charging the battery and nothing to do with the spark for the plug. Any ideas??? Thanks

  12. Just wanted to clarify a comment you made that may mislead some. You said that the compression "held at 80 psi for a few minutes" This is not really true. What held pressure was your tester and hose assembly. No engine "holds" compression for any length of time after it stops turning and it doesn't have to. Whats important is the peak pressure during the test. Testers have 1-way valves so the peak reading is held. In fact you could unscrew it after test and it would still show the peak reading, as long as you didn't press the relief valve.

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