Castor Oil Fuel in Saito Engines.

Based on our experience obtained over many years, MacGregor Industries Ltd. strongly recommend that Saito four stroke engines are operated using a Castor based fuel with 20% oil content. This is because most users in the UK upgrade from two stroke engines or have only seen two stroke engines in operation and are not familiar with the different sounds and characteristics of four strokes. As a consequence, they run with a lean mixture. The lean mixture leads to detonation and can cause overheating, marginal lubrication and subsequent engine damage.

With a two stroke and a rich mixture, the engine "four strokes" - fires every other revolution. As the mixture is weakened, the engine revs increase slowly at first and when the mixture is almost right, the revs suddenly increase and the engine zings into the two stroke mode. Small movements of the main needle peak the revs which can be clearly heard to increase and decrease as the needle is moved. The correct final setting is slightly on the rich side of the peak.

With a four stroke engine, things are quite different. As the mixture is weakened, the revs increase - but only slightly. It is very difficult to hear the maximum and usually the engine will suddenly fade as the mixture becomes too lean to sustain combustion. On the rich side, the engine just stutters slightly without the revs changing much.

Without the sudden increase in revs, it is very difficult to judge the maximum and our experience is that most modellers, attempting to get the absolute maximum revs, err on the weak mixture side. Of course, the correct needle setting for a four stroke is to maximum and then rich until the engine just starts to stutter. For those with a tachometer, Saito recommend that the mixture is set rich to drop the revs by 300 rpm from maximum.

Due to the lubrication being provided by leakage of combustion gasses past the bottom of the piston, a lean mixture can cause a seized and ruined engine due to excessive temperatures and inadequate lubrication. Our experience is that under severe load, marginal lubrication and high temperature conditions, castor oil provides the best lubrication and so helps to prevent engine damage.

The downside is that the castor goo gets all over the inside of the engine and tends to produce varnishing on the piston. Having said that, varnishing has never been a problem with Saito engines used or serviced by MacGregor Industries Ltd.

Of course, with the correct mixture, a high quality synthetic oil is fine. It just has to be remembered that there is no margin for error. The recommendations made by Saito, in their leaflet, assume that the engine will only be operated with the correct mixture.