1. CO detector with digital display
2. one gallon jar or goldfish tank with opening large enough to admit CO detector.
3. large 60cc syringe from a farm supply store
4. short piece of 2mm vinyl tubing
5. 1 or 2 foot length of 2mm od copper tubing
6. 2 plastic sheet large enough to wrap over opening of jar or goldfish tank
7. 2 rubber bands to retain plastic sheets
Measure the volume of the jar/tank, by filling it with water and recording how much it took to fill to the top. Empty the jar, and dry it out.
Place the carbon monoxide detector inside the jar, with the cord extending outside. Cover the jar with one plastic sheet. Poke a small hole in the plastic sheet large enough to allow the copper pipe to fit. Cut a 3 inch length of vinyl tubing, and use it to connect the end of the syringe to the copper pipe.
Stick the copper pipe inside the exhaust pipe or muffler of the engine, and, using the syringe, suck in 60cc of gas. Remove the pipe from the exhaust, and allow it to cool for several seconds. Plug in the CO detector. Squirt the gas from the syringe into the jar, through the hole in the plastic. Cover the top of the jar again with the second sheet of plastic which has no hole, and retain it with the second rubber band. Wait 10 minutes for the detector to establish a reading. Plug your ears if the CO detector sounds its alarm.
Calculate the dilution factor by dividing the volume of the jar by the volume of sample gas, both using the same units, eg. cc's.
When the CO reading settles, multiply the number on the display by the dilution factor, and divide by 10,000. The result is the CO concentration in the original sample, expressed in per cent.
Try this on your lawnmower. You will be surprised by how bad these things pollute. I discovered that the types with the diaphragm carburetor make twice as much monoxide when the fuel tank is full compared to half full. A full tank gives about 6% CO in the exhaust, even with a new air filter and oxygenated gas.
Make sure to ventilate the jar for a few minutes between samples.
You can also use this setup to check for head gasket leaks, by sampling the air above the coolant in the recovery tank, but you should put the CO detector in a small plastig bag to reduce the dilution factor and make it as sensitive as possible.