Gauging your Rotax ultralight aircraft engine!

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Gauging your Rotax ultralight aircraft engine!

In the early days of ultralight aviation we used whatever we could find (or afford) to get ourselves into the air. In many cases our primitive craft were powered by a Mac 101 go cart engine, the 100 cc Yamaha go cart engine, two Chrysler West Bend engines belted together, or modified Pioneer chainsaw engines. The monetary investment in these engines could range from nothing, to several hundred dollars, and we generally got what we paid for!

With little money tied up in the engines, it didn't make any sense to spend hundreds of dollars in engine instruments to monitor them. We new they were going to quit we just didn't know when!

Today even the cheapest engine starts at over $2,000 and can run upwards of $20,000. When you consider the craft they are installed in might cost another $20,000 plus the time and effort required to built it , it makes common sense to spend a little money to watch over our investment. ofse instruments are required to insure proper performance, others are used to tune the engine, while others are used watch for engine problems such as overheating. Lets look at of instruments that should be found on most two and four stroke Rotax engines. (Remember a two stroke is a two stroke, and a four stroke a four stroke, no matter who the manufacturer is, thus this basic information can be used for other engines.

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.
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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.

Rotax 912



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.

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