Looking for input on compression testing. My understanding is that it is a good way to check the health of the motor, I guess specifically the cylinder/piston?
How often if at all are you checking compression?
What is the procedure to check compression?
Which gauge would be recommended?
TIA
The only time I even check anymore is if I’m trying to diagnose a problem and feel like time on the engine might be a factor in whatever I’m worried about. If you even remotely follow rebuild guidelines from any engine builder you’ll be sending them the motor far before the falloff in compression is really an “issue”
You can get gauges pretty cheap off of amazon, just make sure you know the spark plug diameter sizing for your particular application. I’d still recommend getting something more in the middle quality wise…spend $50 or so on a decent gauge and you’ll have some peace of mind you have a decent tool, spend $80-100 on a good one and know you’ll have a life long tool.
to test you simply screw the gauge into the spark plug hole, make sure the o-ring is seated and turn the engine over to see what it reads. From there you have to have an idea of what a fresh / good motor makes to see how close this example is. Also if it’s not an engine with an on-board starter or external start you can use the results could be inconsistent if you’re trying to use a pull start to determine compression.
In my experience 2-strokes won’t run below 100psi. We used to check our Yamaha’s all the time because they were temperamental and a good WKA spec Yamaha would make 160-175 psi. I think I checked a KA once and it was like 180-185…again I send the KA’s in long before they are unhealthy at this point so it’s not something I commonly use anymore.
What engine are you running currently?
Compression readings are somewhat like dyno numbers in that they can vary by the measuring device/vendor, so it’s important to keep that the same across the board and establish your own baseline for best results.
Procedure is full throttle (or carb removed) and spin the engine over. If it’s on OHV or other pull start engine, you’ll have to spin it with an external starter to pass the compression release. A consistent cranking speed is desireable, logging air density can be useful too. You’re reading will be higher in your garage in cold winter vs summer for example.
IME it’s worth spending more on the gauge as I’ve found the cheaper ones do not take kindly to the higher cranking RPM of kart engines. The check valve tends to fail.
Thanks for the input.
Would a cold vs hot motor make a difference? Would running a compression test diagnose more than just a worn cylinder or ring? Would a compression test give an idea if reeds are working correctly?
Also, when cranking the motor over will the pressure continue to rise as it is cranked or will it only go so high?
I run KA but this tool and procedure seem like it would have use beyond just karts.
It can and the degree to which it will can vary. So again having a good baseline is important here. It’s a little trickier with a two stroke due to changes in lubrication which impact ring seal.
Or could point to broken reeds or a leaking head.
It will reach a peak, the gauge will hold that value and and that is your compression number.
Not to throw more things in the mix, but a leak down test might be a better way to go. This involves applying compressed air to the cylinder via the plug hole at a known pressure and observing the pressure drop.
What I like about this test is that it takes oil sealing out of the equation. After a few seconds of pressure the oil will retreat down the cylinder.
Another (arguably essential) test for a two stroke is a crankcase vacuum test/pressure test. This ensures that the crankcase is sealing properly and therefore is pumping correctly. Rule of thumb is to put the piston at TDC (port closed) and apply up to 5psi of pressure to the crankcase.
