Skeptical of compression tester

[ronaldt]

I started tinkering with a mid 70s XL-12 that I acquired about a year ago. It appears to be fairly low hours by the paint and overall condition. I decided to do a compression test and it wasn't very good (105-110psi). The piston and cylinder look very good. Hmmmm. I decided to compression check a few more saws for comparison. The last one I checked is a late SXLAO that runs and cuts great with plenty of power. It had 115 psi according to my tester. This can't be right can it? My tester comes from China. Maybe you get what you pay for?

[edju1958]

I think it's possible your compression tester wasn't calibrated properly?Or maybe just plain defective?I've got 2 testers,the later one I bought I like better than the 1st one.

[blythkd]

The readings could be right but I'd suspect it's probably reading a little low. I'm sure it didn't come with a calibration certificate!

[rowdy235]

China is not known for producing accurate gauges lol.

I’d be tempted to put some compressed air with a known pressure (eg by reading the gauge on the compressor) to it and see what it reads

[tangobravo]

If checking cold, run a little oil in the spark plug hole and cycle it a few times with the pull rope. I was surprised how big of a jump in pressure occured.

[ronaldt]

Nov 30, 2020 21:17:58 GMT -5 tangobravo said:

If checking cold, run a little oil in the spark plug hole and cycle it a few times with the pull rope. I was surprised how big of a jump in pressure occured.

I think I will start doing that on all old saws.

[sawnami]

Is the check valve in the end of the threaded adapter that screws into the cylinder or closer to the gauge?

[ronaldt]

Do you mean the little valve to release the pressure? It's on the gauge

[sawnami]

No, the check valve that looks like a tire valve core.
If it's closer to the gauge, the hose volume is taken into consideration as part of the cylinder volume effectively reducing indicated compression pressure.

[5terrysupersaws]

  The type of Schrader Valve used on the end of the Compression Tester can also influence the 
accuracy of the test, a typical tire valve used in that location will give an inaccurate result .

[blythkd]

I like to use a hand held gauge with a tapered rubber tip that you just hold in the plug hole. The valve is right on the end of it. That way you're not losing any compression in several inches of hose if the check/release valve is near the gauge.

[ronaldt]

So this is what I have. The Schrader valve is in the end of the hose adapter.

Attachments:

[ronaldt]

So I filled the cylinder with penetrating oil for 3 days. I just rechecked the compression and did pick up 5 pounds. That's still just 110 lbs. I really think my compression tester is off

[sawnami]

Dec 2, 2020 18:46:25 GMT -5 ronaldt said:

So this is what I have. The Schrader valve is in the end of the hose adapter.

So the adapter that screws into the cylinder is open with no check valve in the threaded end?
If so, your compression will consistently show low because the adapter acts as an extension of the combustion chamber and lowers the compression.
Its not an issue in automotive applications with larger displacement cylinders.
When the coupler is connected to the adapter, the valve inside of the coupler in your pic is unseated. It doesn't act as a check valve

[ronaldt]

Thanks for the input everyone! Is there a certain brand that is better for little 2 strokes?

[sawnami]

Brand is not a big deal. The main thing is to get one that has a check valve in the threaded end of the adapter. Lisle, Milton, OTC, Mityvac, have nice gauges that are accurate. There are many others that will do a good job.

[ronaldt]

Dec 3, 2020 8:44:00 GMT -5 sawnami said:

Brand is not a big deal. The main thing is to get one that has a check valve in the threaded end of the adapter. Lisle, Milton, OTC, Mityvac, have nice gauges that are accurate. There are many others that will do a good job.

Ok thanks! I like shopping (for tools).