Building an IC (internal combustion) engine, whilst calling for similar machining techniques to the construction of a model steam engine, has given me a sharp lesson in the need for greater machining accuracy. I would strongly urge beginners to serve their apprenticeship with the construction of a number of steam engines before moving on into the more exacting world of IC engines.

Having built a dozen or so steam engines I felt I might just have sufficient experience to tackle a simple two stroke – just how difficult can it be ? I opted for Jan Ridders ‘Debbie’, a simple two stroke to a design where Jan has removed as many of the difficult bits as possible. There is an easy to construct vapour carburettor and those tricky to make piston rings have been dispensed with altogether.

I guess it took me around four or five weeks to complete the build working on average three or four hours per day – not that I was in any hurry. It would be nice to say that after adding a drop of unleaded ‘Debbie’ sprang into life – no such luck !

It is now two months later, during which time when there were many days when it was just too cold to venture into the workshop, and still ‘Debbie’ refuses to run for longer than a few seconds. The following are steps taken in my quest to get ‘Debbie’ into the ranks of runners.

Piston/cylinder bore. This is the heart of the engine and calls for a sliding fit where the cylinder internal bore is no more than 0.05mm (that’s 500ths of a millimetre !) greater than the diameter of the piston. I had several attempts at achieving this level of accuracy before I was satisfied. It was during this process that I learnt the correct way to use an adjustable reamer. I felt the need to dismantle mine to remove sticky storage wax and the way it worked became apparent on disassembly.

Ignition. Another key aspect of the build. Initially I opted for a battery, coil and contact breaker setup. This worked fine but it soon became apparent that the micro switch contact breaker had a very short life expectancy and I got through half a dozen of these during my efforts to get ‘Debbie’ to run. Having lost faith in micro switches I wondered if a change to electronic ignition might not be the answer to my problems. £60 later I had a more reliable set up but regretfully this was not the answer to ‘Debbie’s’ problems. One bonus of the switch to CDI ignition is that with the coil and larger battery dispensed with, a shallower base is now practical and this, in my opinion, results in a more pleasing overall look to the engine.

Ball valve. This is the device fitted between the carb and engine to restrict fuel vapour to flow in one direction only. According to Jan the malfunction of this valve is frequently the cause of poor or non running of the engine. I have lost count of the days spent focusing on this component in my bid to get ‘Debbie’ underway. Following a suggestion from my mentor Bogs I modified the design to a square edge ball seat rather than a taper and this certainly improved the sealing properties. A light tap with a hammer on the ball helps to form a good seal. Discard the ball after tapping and replace with a new one.

Fuel. Jan suggests using either unleaded petrol or Colemans camping stove fuel. He recommends Colemans for improved running characteristics and less odorous exhaust fumes when run indoors. It was during my many hours of adjustments to the ball valve that I discovered that there was a definite relationship between the fuel level in the carb and the willingness of ‘Debbie’ to fire up. The other feature I noticed was that to get ‘Debbie’ to fire the air valve on the carb had to be almost fully closed giving a very rich mixture. Then the penny dropped ! Air was clearly being sucked into the inlet system somewhere after the carb, thereby weakening the mixture. Close examination revealed that the seal around the piston rod appeared to be the prime suspect. I also discovered that the level of fuel in the tank directly affected the engine’s willingness to fire. I know I am not alone in having problems getting this engine to run and I am beginning to think that the piston rod seal must be high on the list of potential problem areas.

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