Rotax two stroke operation #63

Rotax two stroke engines are operated/tuned in a different way to the average four stroke & not following the rules is far less forgiving. Unfortunately, the Rotax manual only tells you the max RPM, EGT, CHT range, there is nothing on selecting the correct prop for the speed range of the aircraft and how strongly the load on the engine affects its tuning (Rich/Lean). It is one of the two most common reasons for seizure.
i) High temperature difference between the cylinder and the piston giving insufficient clearance & high friction to melt the side of the piston, smearing Ali onto the cylinder wall. 

ii) Lack of lubrication - with the same result. What prevents the seizure under normal running condition:- a]- oil mixed with gasoline.  b]- maintain temperatures under control - especially the piston which is cooled by the fresh air/fuel mixture entering the crankcase, and by keeping the combustion (EGT) low enough from setting the mixture for richer burning.

Leaning raises combustion temperature (EGT) and reduces the amount of oil for lubrication. Seizure is usually a combination of these two factors. Two strokes control the inlet fuel/air charge & exhaust discharge via piston porting & a resonant exhaust system plus the carburettor setting. Equally important is engine load & all these factors must be in harmony. You can't for example compensate for too low engine load (which creates lean condition - high EGT) by increasing the jet sizes to make up for it and lower the EGT, because now your carb. is out of step. You may manage to keep the engine from seizing, but fuel consumption will be high, temperatures will read oddly, peak power will be less and carbon will build up faster. Seizures are often caused either by too low engine load, typically:- A shallow descent at a relatively high airspeed with about half throttle will lean out the mixture significantly but does not necessarily show up as too high EGT because though the engine is not really producing much power it has a high difference between the piston and cylinder temperatures together with a low amount of oil entering. Recommended descending is at only slightly above or at idle rpm at slow airspeed somewhere between best glide and best sink.  Or static rpm too high caused by fitting too fine a prop, in flight the prop unloads and the rpm will soar so the throttle is reduced to keep the rpm in check: lean running results. This should be indicated by high EGT but often people don't pay enough attention and/or don't realise how dangerous this is until too late. Static rpm should be well below the max hp rpm, the optimum depends largely on the speed range of the aircraft, the higher the cruise/top speed the lower the static rpm, even if that means lower than max hp rpm in climb mode. Only on high drag aircraft with narrow speed range can you accept higher static rpm & close to max hp in the climb mode as you will not unload the engine on when you level off to cruise. A particular case is too large (diameter/blade area) prop , which requires a low pitch to obtain acceptable static RPM.  However even if low, the engine load will start dropping as the aircraft gains speed. At say 6,000 he blades could be at approaching zero angle of attack and zero thrust as soon as the airplane reaches about 55 mph.. Only very slow aircraft can use such a combination successfully. e.g. a hang glider tow plane or a powered parachute.