Engine Test Run, Unloaded

Posted August 23, 2003       Subj: Two stroke engine class

Hi all,

Since my knowledge of engines is nonexistent, I felt I would benefit from a 9 week class at the local college MATC, (Madison Area Technical College) on the fundamentals of the two stroke engines.

My question is: Do I need the re-drive and prop to run the engine? Is there another setup that would allow me to run the engine in the class? I'm sure he will not allow me to swing a 60" prop.

Thanks in advance.
Bill Nicholson


REPLY: Bob Robertson:

Hi Bill,

Take it from someone who works on these engines daily and has seen the result of running these engines without a load of some type (either a flywheel or reduction drive w/ prop etc.).

The engine can easily rev well beyond its design limits with just a touch of the throttle. Even a touch of the throttle can destroy the engine. As a matter of fact . . . I have one in a box under my workbench that is a result of exactly that.

The following story is true:

The customer placed his engine (a 503) on his work bench and set the points. He then decided that he would test run the engine (it had a type A gear box and an older model Air-Drive electric starter).

He clamped the motor mount to the bench, located a battery and a temporary fuel tank. He did not have the impulse fuel pump handy so he hooked up an electric fuel pump to supply the carburetor with gasoline.

Now, all this is happening inside his garage/shop . . . as we continue . . .

He connected the electric fuel pump to the battery and filled the carburetor bowl. He had jury- rigged the starter directly to the battery, so all he had to do was touch the engine ground to the negative post on the battery and the engine would turn over. He had planned to just short out the grounding cables to shut off the engine.

He hooked a small cable into the top of the carburetor to act as a throttle. He touched the starter ground to the battery and the engine came to life...... Here is where it all fell apart (literally) ...

He gently pulled up on the cable holding the carb slide. The engine instantly over revved.. The torque caused the clamp holding the engine to the bench to dislodge and the engine started to vibrate towards the edge of the bench. The owner then released the cable holding the carb slide open and grabbed the engine. Unfortunately the cable did not release fully and the engine continued the severe over rev.

The fellow could not release his grip on the engine to grab the shorting wires as he feared the engine would vibrate off the bench onto the floor. So he held the engine with one hand and garbbed a large hammer with his other and proceeded to dislodge the carburetor (a sure way to stop any engine).

Well.....he forgot the fact he had rigged an electric fuel pump to a battery to get this whole scenario started in the first place. When the carburetor and part of the intake manifold were dislodged from the engine, they fell onto the bench right next to . . . you guessed it . . . the battery.

Which, of course, was still connected to the electric fuel pump which was, of course, supplying fuel to the dislodged carburetor. Before our intrepid experimenter was able to release his now decelerating engine, the carburetor spewed copious quantities of gasoline over the work bench, the engine, and, of course, the battery.

He is not sure where the spark emanated from, but in short order he was faced with a fire that was being fueled by a battery-powered fuel pump, which was itself engulfed in flames.

Thanks to a quick-thinking neighbor with a couple of fire extinguishers, they got the fire extinguished. The engine burned really well; the work bench burned only so-so. The engine was destroyed and the shop needed a paint job. His ego was in about as good a shape as his engine . . .

When I asked him what he was thinking, he replied that he "didn't think it was safe to run the engine with the propeller indoors" . . .

Bill, trust me and others that tell you not to run one of these engines without some type of load.

Regards and happy landings,

Bob Robertson
Light Engine Services Ltd.
Rotax Service Center
St. Albert, Alberta, Canada