Gas tester (part no. 343265)
11. Make a note of the reading and observe the display for a further one or two minutes to ensure that
the reading is indeed steady and within tolerance for the vehicle concerned. If the reading is not be-
tween the manufacturer’s recommended maximum and minimum, or is not below that specified as a
legal requirement, then adjustment of the carburettor or fuel injection system will be required.
12. If adjustment is required, make a small alteration to the mixture screw, and correct the idle speed by
adjusting the idle speed screw. Do this repeatedly in small increments, under these conditions the read-
ing should stabilize in less than one minute at each mixture setting.
13.When the test is complete REMOVE THE EXHAUST PROBE PIPE and switch off the engine. Allow a
period of at least 5 minutes or at least 10 minutes in still air conditions with little breeze, for air to
purge the exhaust from the instrument. This period in air with power on cleans the sensor before
storage and also allows a check to ensure the display returns close to the 2% setting indicating that
there has been little drift in calibration.
NB: A final reading of 1.8% in air for example would suggest that the last exhaust measurement
was approx 0.2% lower than the displayed level. While this is perfectly acceptable when setting to
approx 3% to ensure that a vehicle passes a 4.5% maximum legal requirement, it does represent a
more significant error if setting to a manufacturer’s recommended 0.5% CO. On occasions it may be
necessary to repeat a test if calibration drift is excessive (drift is reduced by keeping test duration
short and allowing a long warm up).
3. Further Information
It should be noted that an engine, even in good overall condition, will show a fluctuation in idle CO
over a period of time, of typically 0.5%. Bearing in mind this fluctuation, and also errors and drift in
the instrument, the user should aim to set the average CO reading to be midway between the limits
set by the manufacturer, or at a reasonable margin below the prescribed legal limit. If the CO level is
correct HC levels should also be low if general engine condition/adjustment is reasonable. Periodi-
cally, during the tests, examine the lowest point of the transparent plastic pipe to see if it contains
water to a degree that might impede the flow of gas, and if it does, remove the pipe at the instru-
ment gas inlet end and clear the pipe by allowing the water to drain out, then reconnect the pipe
and carry on with the tests.
If the transparent pipe falls continuously from the exhaust to the pump the automatic drain should
operate and keep the pipe clear of water. NB: Operation of the pulse pump will usually be audible
as the pulsations in the exhaust cause the internal diaphragm to vibrate. If the instrument ceases to
respond to changes in mixture setting or the sound from the pulse pump becomes irregular, check
the sample pipe for collected water.The calibration of the instrument may be checked at any time.
Simply remove the exhaust probe and wait at least five minutes or at least ten minutes in still air
conditions with little breeze, for the exhaust gas to disperse from the collector box. If necessary, the
calibration may then be adjusted using the calibration control knob. Periodically check the calibra-
tion of the instrument during particularly extended tests.
Some motor car engines will not readily “tick over” at idle speeds for long periods. The speed may
become erratic, and engine misfiring may occur. With the prolonged testing of such engines, it may
be necessary to occasionally “purge” the engine by, for instance, increasing the speed to 2000
rpm - 3000 rpm for 15 seconds. This may be done at any time during the tests but the exhaust probe
should be removed beforehand. VEHICLE REPAIR IS ADVISED.
