Gauges for your engine!

Most of the info below curtesy of Dave Loveman


The tachometer plays a very important role in engine ensuring proper engine cooling and performance. An engine allowed to over rev will run a lean fuel and can result in engine overheating and possibly engine failure due to seizure. If the engine is not allowed to rev up to its proper RPM this can cause engine overheating, or engine failure due to the engine not being able to burn the fuel they way it was designed to. A pilot taking off on an engine that can only develop 5900 RPM can attest to the fact that his engine will loose power and eventually quit. So what role does the tachometer play in all this? The tachometer is used to properly pitch the propeller, and a properly pitched prop means the engine will, if everything else is correct, provide the best climb, cruise and engine performance available.

To achieve this you most know what engine RPM your engine was designed to do in straight and level flight under full power application. Since you can't do this safely in the air, you must do it on the ground, with the craft tied down. There is one other problem since the craft moving through the air is provide clean air entering the prop at whatever speed you are doing, and a plane tied down only has the advantage of clean air into it, you must allow for this when setting the propeller.

A 503 and or 582 Rotax twin carbed two stroke engine should rev up to 6500 RPM in straight and level flight under full power. Experience has shown that this can be achieved if the craft is tied to the ground and the prop is set so that under full power the engine can only pull about 6200 to 6300 RPM. This should give you 6500 in the air. Since all Rotax engines are supplied with a break in procedure and it is required that it be done before the engine is put into service, the first time you will use your tachometer is to set the pitch on your prop for the break in. An engine that is allowed to pull 7200 RPM tied down on the ground is not likely to make it through the whole break in procedure, without seizing up!
Just as an engine that is set for 5500 RPM will start to loose power and RPM after about 10 minutes.

Exhaust Gas Temperature Gauge (EGT)
The next gauge that should be on all Rotax engines is an EGT, and you should have a separate gauge and lead for each cylinder! You can't just stick a probe in the center of the "Y" pipe and expect to get a proper reading. New Rotax exhaust manifolds provide a spot to install the probes. On older systems it is necessary to measure out from the PISTON, 100 mm and then drill a hole in center of the "Y" pipe on each side.
According to the factory exhaust gasses exiting the engine are the hottest on Rotax engines at this distance. (Picture a flame from a propane torch, it is hottest at the end of dark blue flame.) When routing the probes do not "COIL" them up try to run them as straight as possible without overlapping them.
An exhaust gas temperature gauge can show you whether your engine combustion chamber is running to hot or cold, lean or rich. It can also indicate problems with the engine.

For example:
If your intake manifold were to develop a crack this could cause a lean fuel mixture, which would result in a higher EGT on that side.
if the needle in the throat of the carb were to wear and drop down a notch this would show up on your EGT.
If your gas to oil mixture was wrong it would show up quickly on the EGT.
if something was wrong with your ignition system, a bad spark plug or faulty ignition cap/coil/wire this would show up as a colder EGT reading, since the fuel isn't being burned properly.
If your prop is not set correctly it will show up in your EGT temps
If a problem develops in one carb due too much or too little fuel this will show up in the EGT prior to take off it is very important that the engine be brought up to operating temperature, 1000 to 1200 degrees F you use EGT gauges to do this. The proper EGT reading for all two stroke Rotax engines is 1,000 to 1,200 degrees F.

Cylinder Head Temperature Gauges
Experience has shown that CHT's on two stroke engines are a waste of money. By the time a gauge reacts through the head to tell you, you have a problem your prop has stopped and you already know it. I would not fit them to "free air Cooled engines" at all.

In some cases, especially when using "hand held" radio's, the CHT sender wires, have been noted to causing interference in the reception. The sender wires act like an aerial, picking up the spark plugs, a sort of buzzing can be heard when recieving.

On a liquid cooled engine the reading is effected by the water circulating through the cylinder head, lowering the readings. They will however tell you when you are loosing coolant, or are low on coolant since the CHT temps are higher when the coolant is low. In most cases pilots will find that they will smell the coolant before they notice it on the gauge.

If you are going to install a CHT you still require one for each cylinder, don't "COIL" the leads, and it is necessary to remove the spark plug ring when installing the CHT sending unit. Failure to remove the spark plug ring may result in leakage due to improper sealing. It can also result in damage to the threads of the spark plug hole. With the spark plug sealing ring in place and the addition of the CHT coupler threads in lower part of the cylinder head are exposed and will carbon up. This will result in damage to the threads if a new plug is put in place without a CHT coupler, since the spark plug can now go all the way down into these carboned threads, usually breaking them off and depositing them into the cylinder.

Water Temperature Gauges
On liquid cooled engines these are mandatory. Rotax liquid cooled two stoke engines are designed to run at between 140 and 180 degrees F. With the optimum temperature coming in at around 160 degrees F. It is also necessary to bring the engine up to operating temperature prior to full power application. Failure to do some can result in "COLD" seizure. This can happen when an engine has been left at an idle for long periods of time just prior to take off or during a long slow approach from altitude, with full power applied suddenly for a go around.
What happens is the engine coolant has being cooled in the radiator, (super cooled in winter) full power is applied forcing this cold mixture up into the water jackets around the cylinders. This coolant shocks the cylinders shrinking them at a time when the piston is under load and expanding rapidly! The result the cylinder and piston react together to cause a seizure.
To prevent this warm you engine up prior to take-off, a thermostat aids in a faster warm up, and during long slow approaches, when the engine is left idling for long periods occasionally rev the engine to move fluid through the engine. AND if power is needed for a go around apply power SLOWLY to allow temperatures to come up gradually.

Hour meter
While some may think that this is not a necessary gauge I believe it is. When you sit around and chat with pilots you will find that they seem to "talk flying" more than they fly. Thus someone who has flown for years and has hundreds of hours under his belt will usually find that those hundreds are reduced considerably when checked against an hour meter.

All of the new dual CDI two stroke Rotax engines require rebuilding, which includes CRANKSHAFT REPLACEMENT not crankshaft rebuilding at 300 hours or 5 years. While many have been know to go on longer those that haven't usually destroy the engine when the connecting rod comes through the side of the block. An hour meter ensures that you have an accurate engine hour log.

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