December 9, 2023

Engines part one: two strokes

I’ve gotten to a point where I feel comfortable giving general advice on engines so I’ll write a series of posts describing my experiences. This series is intended for those who still think about engines as a complete mystery. These are my experiences and they are all things I wish someone had told me before I had to spend the time to find out for myself. (YS mechanics and engine doctors already know this.)

Part one is about bog-standard two strokes: un-pumped and uncomplicated.

There are two types of these engines, the plain bearing type (typically OS LA series) and the ball-bearing kind (typically OS FX series) The difference is obvious, the ball bearing engines have two ball bearings supporting the crankshaft. They are bigger, heavier and more expensive but provide more power and run better. The plain bearing engines like the OS LA series are extremely simple, light and inexpensive and good beginner engines. A .46 LA typically weighs no more than a .25 ball bearing type so you get almost the same power for less weight and less money.

O.S .55 AX ball bearing ABC engine
O.S .55 AX ball bearing ABC engine

Here’s some important advice on plain bearing types of engines. The crankshaft is able to move forward so you can pull on the prop driver and pull the crankshaft out a few mm. That means that when you attach a prop and finger it, it rattles back and forth slightly. Sorta like it’s broken, except it’s not. When running, the prop pulls outwards so there’s no rattling. What you have to consider, is that you must never use a starter with these engines. You will force the crankshaft back against the bearing, effectively grinding it, reducing engine life dramatically.

O.S. .46 LA engine - a plain bearing engine
O.S. .46 LA engine - a plain bearing engine

On both types of engines, tuning the engine to run properly is quite simple. When breaking in, use wide-open throttle. Turn the needle valve in so that the engine runs at full power for a few seconds, then turn it out until the engine runs slobbering rich, what is known as four-cycling. Turn the needle back in until it is lean again and repeat this for a few tanks of fuel. You’ll notice that after a while you have to turn the needle further in each time to make it run lean. When the lean setting is in the same place every time, it’s broken in and you can go flying.

After break in, it’s quite common to have engines run a bit too rich in the idle setting. You’ll know this if the engine stutters a bit when coming up to full throttle after extended periods at idle. To fix it, simply turn the idle adjustment needle in about a quarter or half a turn (clockwise)

A nice needle adjustment for flying is to tune the engines high-speed needle so that you have a steady lean run. Start rich, about 2 and 1/2 turns out then tune it very slowly about a notch or two at a time, leaving time for the engine to adjust to the new fuel flow until you have achieved a steady lean run, then go no further or you will run it too lean.

Coming up next: What difference does a pump make? Lots, as I discovered. Stay tuned for pumped-engine tuning…

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