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http://www.ultralightnews.ca/hirth-engines.htm Page 14 of 31
The Ultralight News
The EGT gauges require probes which come with 100mm of lead. To install the probe, drill a 4.763mm hole
in the centre of the exhaust pipe at the recommended distance above. Install the probe into the hole, and
tighten up the clamp. As an option, manifolds are provided to suit the "bayonet probe" type. The terminal
cable can be lengthened to a maximum of 4.5 metres without affecting the accuracy of the instrument.
Route all wires away from the manifold and other heat sources. Support the wires every 300mm intervals,
using tie wire or similar. When going through firewalls etc, use rubber grommets to prevent damaging the
wiring. Do not route these wires with any wires from an AC source, such as a lighting coil, spark wires, tech
wires, etc. The EGT operates on .004 millivolts. Running near an AC source will result in incorrect
readings.
It is important that you verify the EGT readings by careful examination of the spark plugs. A reading on an
EGT of 1100 to 1200 degrees F should give a nice tan coloured spark plug. Lower readings should give
darker colours while higher readings grey/white colours.
Once you have verified a spark plug colour and seen where your gauge reads with the proper colour then it is
now more important to watch for a CHANGE in the gauge reading from this "normal reading."
It is important to set your engine up for proper fuel to air mixture, your EGT gauge and probes can aid in
doing this. A lean mixture will run hotter on the EGT. A rich mixture will run cooler on the EGT but will
make increasingly less power. Ideally the highest EGT temps should come in at settings usually 5200 to 5800
rpm.
Reason for EGT temperature changes include:
Air intake leak.
Air fuel mixture too lean
Air fuel mixture too rich
Changes in density altitude
Changes in relative humidity
Changes in air temperature
Pre-ignition or detonation
Fault spark plug or ignition system
Improper prop load
Poor quality or grade of fuel
Improper fuel air ratio, plugged main jet, failed needle jet, or clip
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