Although the minimum instrument requirement for microlights is relatively small (Altimeter, Airspeed indicator and compass) there are many other instruments which may be fitted. Engine management and monitoring systems are normally considered fairly useful and are covered under the engine section none the less it is worth checking to ensure they work properly and are correctly marked with maximum limits.
ASI and Altimeter.
The Altimeter and ASI are simple devises and normally don't give problems but they can.
The altimeter can give problems and read incorrectly if it does not have a good static vent. Venting into the cockpit of a fully enclosed aircraft is
going to give errors when the cockpit vents are opened or a door is loose, but more importantly the cockpit will not necessarily be at the static outside
pressure and so read incorrectly.
The ASI is dependent on a static and dynamic vent, the dynamic vent is just a tube facing into the direction of flight so is susceptible to rain, water and insects
lodging inside it. The static port should be close by and is a tube with holes in the side and is less likely to become blocked. Sometimes the static port will
be on the side of the fuselage and still needs to be checked.
If the ASI appears to be sluggish slow to respond or just plain stuck the first line of action should be to remove the dynamic line from the ASI and blow down it
to remove the possible blockage. Secondly check the static ports for blockages, a 747 crashed in the pacific due to tape being placed over the static port while cleaning
and not being removed afterwards, all pressure sensitive instruments including the Altimeter, ASI and Transponder were rendered useless. If you do block a port for cleaning or painting make sure the port is made flight ready afterwards.
If the Altimeter is playing up try T-ing it to the ASI static port, this should ensure it works correctly and accurately.
Compass swing and checking is covered in section three of the AC below. AC43-13b also includes more detail on the pitot static system and gyro systems.
FAA AC43-13b Chapter 12-13.pdf Chapter 12 Aircraft Avionics Systems.