Cheap compression testers?

[longhike]

Has anyone had any luck with cheap-ish compression testers, brands? I don't have any specific issues at the moment my motivation to get one is more educational and curiosity of how my gear is doing. So it doesn't need to be really good, but I also don't want to make costly repair decisions about my OPE based on a bad gauge. If it's luck as to whether they are accurate, can I use an air compressor to test the gauge, assuming that the gauge on my air compressor is accurate?

thanks in advance

LH

 

[StarTech]

For small engines you would one design for them. Basically the Schrader valve must have a lighter spring than the automotive type.

 

[Hammermechanicman]

The automotive testers are not all that good for small engines. Some testers have the Schrader valve not in the end of the hose. These will give low readings because the hose up to the Schrader valve increases to effective volume of the combustion chamber lowering the measured pressure.
A compression tester like this made for small engines will give more accurate readings. Engines with a compression release complicates things. I have both this one and the automotive type. I usually this this one.

 

[slomo]

The automotive testers are not all that good for small engines. Some testers have the Schrader valve not in the end of the hose. These will give low readings because the hose up to the Schrader valve increases to effective volume of the combustion chamber lowering the measured pressure. View attachment 54508
A compression tester like this made for small engines will give more accurate readings. Engines with a compression release complicates things. I have both this one and the automotive type. I usually this this one.

So what is the measured difference between the auto and small engine style? Can you test this for us all?

This might explain why my Wisconsin Robin is only reading 50psi.

slomo

 

[longhike]

For small engines you would one design for them. Basically the Schrader valve must have a lighter spring than the automotive type.

Would I need a valve or could I just use a gauge from an air compressor and watch the needle bounce?

 

[longhike]

Hmmm... since it’s just for play. Use the threads of an old plug and epoxy a tire gauge to the hole in the center
What’s your vote?

 

[bertsmobile1]

The other problem with small volume gauges is the volume of the spark plug.
My tester came with 4 different plug lengths but unfortunately all 14 mm
However it all depends upon on just how accurate you need the results .
What you should be doing is taking compression readings at regular intervals like servicing or every season end etc
You then plot them ( a graph is best ) and watch as the compression goes down over time as the rings / cylinder wears
Some time before this number is 20% lower than the first it is time for new rings.

 

[longhike]

>What you should be doing is taking compression readings at regular intervals like servicing or every season end etc

That was my hope. And even if the gauge is off, but consistently off it should be OK for me since it would still show a relative decline.

As for my extreme budget tester.... I found a video that was helpful to strip out the ceramic insert in the plug. I didn’t have a holesaw that would fit, but I found that a hacksaw around nut did the trick!

 

[Hammermechanicman]

So what is the measured difference between the auto and small engine style? Can you test this for us all?

This might explain why my Wisconsin Robin is only reading 50psi.

slomo

Here are two different compresssion testers on a 90cc hopped up chainsaw.
The 195 is correct. The automotive tester has a 12" hose from guage to spark plug adapter. The hose effecively makes the combustion chamber size larger when compressing the air like a spring. The smaller the engine the more pronounced the difference is.

 

[bertsmobile1]

And better still are the ones with a short solid tube .
The auto ones can work, but you have to pull the rope around 300 times to get an accurate reading and then you have to take the expansion of the rubber into account as well.
But because the amount pumped each time is so small after the first 10 or so pulls you don't notice it going up .
If you crank the engine with a drill on the end of the crankshaft nut you will gat a lot higher reading, till you burn your drill our.
Then there is the problem of back pressure from the tester between the schreade valve & the plug opening pushing the air backwards into the crank case when the transfer ports open