Like HarleyT says, put up a post to your model OTC.
My humble opinion: OTC tools are usually good and right on. (do not do anything to void the new tool warranty)
We need to know the model because
if you got the Automotive compression gauge I won't read correct for chainsaws and small CC engines.
If it's the model for chainsaws and small cc engines,
first check the gauge itself and compare it too a 100 psi air compressor.
If the gauge reads correctly replace the Schrader valve and try again. (assuming the valve is at the spark plug adapter)
and the spark plug adapter has to be snugged down good (not tight but snug) against the o ring or it will leak around the threads and cause a low reading.
General chainsaw compression gauge info about trying to get a correct read listed below:
Here is a link to some good info about trying to get compression gauges to read correctly on chainsaws and schrader valves to use in the tester. Read all of the pages at this link.
http://www.arboristsite.com/community/threads/compression-tester-issues.310133/
I got 3 of the Napa 700-9556 compression gauge Schrader valves today
after the local NAPA ordered from their warehouse. The come 5 in a box and you can buy them each @ approx. $2.20 each. They worked A-ok. One was defective out of it's sealed package, was not spring loaded on back check due to a broken spring, noticed immediately before attempting install for test. Most NAPA guys will think you are crazy for not just using a tire valve, not realizing they are special valves for a OLD Model NAPA NLA (no longer available) compression gauge
One reason the wimpy schrader valves go bad readily is the spring is so weak that just a small piece of carbon can get in the seal and they won't re-seat fast enough to hold the complete high compression pulse even though they will hold a reading.
Note where I modded some auto tires valves to read correctly by cutting and weaking the spring, but the long modded schrader valves will not fit into some compression gauges fittings. (I just happened to have a pile of the long auto schrader valves in my tire flat fixing stuff was why I tried the mod and was not aware of the NAPA schrader valves at a reasonable price at the time I was building a homemade compression gauge for saws and playing with schrader valve spring pressure so as to get a homemade tester to read the same as my store bought gauge. I now have two different compression gauges that read same and correct. (one tester is homemade using the tips in the above link)
Some of them little special schrader valves are priced around $15 apiece plus shipping.
I had a guy (auto mechanic) bring me is own Stihl chainsaw, MS290 sometime back that he had put a new piston and cylinder into because his Auto compression gauge was reading low comp and read even lower with the new jug and piston. Saw would not start good or stay running when started. He did not know his auto compression gauge was not being truthful to him.
The saw had a bad carb. He will remember that one for a long time.
Just checked my gauge again after seeing this whole debate. This is my go to US Gauge tapped into my shop compressor both show a tad under 100 lbs.
On gauge tapped into my compressor both show 100 lbs. I am confident my gauges are real damn close.
