Start Model Engineering

Wiring as seen from the underside of the engine baseplate

Electrickery

As a result of a flood of emails, well just one to be honest, I have put together a few notes on how I tackled setting up the electrics on my two stroke. Let me say from the start my knowledge of the workings of all things electrical is severely limited. So trained and experienced electricians should look away now.

Working from Jan’s schematic these are the bits that I gathered together.

A 12V coil. The small type commonly found on classic motorcyles is ideal. I chose a 12V version rather than 6V only because I already had a serviceable 12V 2.2amp battery that had been used in an alarm standby system which would fit conveniently into the base of my engine. BTW I noticed during a visit to Maplin that they stock this type of battery.

From the same supplier of the coil (try ebay) I also picked up a condenser an essential component that acts like a tiny storage battery that charges and discharges rapidly. This I mounted on the underside of the engine base plate to provide good earthing.

A contact breaker in the form of a micro switch (Maplin ref GW73Q) complete with roller which is actuated by the cam wheel. This was mounted in position to bring the roller in contact with the cam wheel. Provision for ride height adjustment was allowed for when making the simple L shaped bracket. Important, when connecting up choose the higher of the two blade terminals and the blade terminal under the unit. This will give you a ‘push to break’ arrangement which is the required setup.

Whilst Jan provides details on how to make your own spark plug I opted to buy one ready made (Ref CM-6, Google Apache Aviation). You will also need an M10 x 1 tap, it will cost you more than the spark plug ! – buy just one, a 2nd which as you probably know has some taper and full width thread so is adequate for cutting the spark plug hole in the aluminium cylinder head.

You can carry out a bench test by connecting up all the components, observing the polarity of coil and battery. The spark plug body should be earthed by laying it on the engine base plate. If you have wired everything up correctly you will be rewarded with a healthy blue spark at the plug when you depress the micro switch either by hand or by rotating the cam wheel.

Flushed with success  I decided that I wanted some means of switching the ignition circuit on and off plus the facility of plugging in a battery charger. The simple answer is to fit a double pole double throw switch giving ON-OFF-ON. I made life a little more complicated for myself by opting for a two pole 6 way rotary switch (Maplin ref FF74R) to give the same result plus the glitz of ‘circuit live’ 12V LEDs. Wiring was very much a case of trial and error using a basic multi meter. (I wonder if Maplin will send me a Turkey at Christmas ? – no maybe not !).

To sum up, wiring up the basic ignition circuit is not difficult and providing you follow Jan’s schematic should present no problems. The switching, battery charging arrangement is very much optional. If anyone would like clarification on any of the above send me an email to john(spam)@start-model-engineering.co.uk.

Logic and a labelling system does makes life easier, use a test bulb to check each connection

The central poles are for positive and negative feeds. Check polarity after all connections added – mine were the wrong way round !

spaghetti junction – sorting out the wiring for my rotary switch was a case of trial and lots of error.

A double pole, double throw switch the easier option to switch battery between ‘ignition’, ‘off’ and ‘charge’

Wiring completed prior to securely mounting battery and coil in engine base

A chip breaker finning tool – a gift from the gods, boy did it do the job ???

It is vital to set up the ‘finning’ tool dead square, particularly so if you are ‘going in’ 6mm.

Careful and rigid set up is the order of the day when machining fins – maybe a trial run on some scrap bar might pay dividends.

Fins completed and resting on part finished pedestal cylinder mount – time for a well earned beer methinks.

Cooling fins

Now back to the engine proper. One job that I was viewing with some trepidation was the machining of cooling fins on the cast iron cylinder. Fortunately Bogs saw the potential problems and came to my rescue even before I asked for it ! I will attempt to pass on to you the key points in tackling this job. It is worth the effort in getting it right as, without wishing to fill you with alarm, a cock up at this stage will live to haunt you.

Whilst a standard 2mm wide parting tool may be OK I had the benefit of a ‘chip breaker’ cutting tool presented to me. The photograph shows the profile of the cutter which may be of sufficient guidance to enable you to grind up your own tool. Whatever cutter you opt for I would suggest you carry out one or two trial runs on a piece of scrap before starting on your cylinder.

The first thing is to ensure that your cutting tool is mounted with the cutting tip dead on the centre line, not a nadge above or below but spot on !

Next use a clock to make sure that the blade is square on to the centreline of the lathe. As the cutter does not have any side relief failure to do this will almost certainly cause jamming as you cut progresses deeper into the cylinder.

For maximum rigidity I would advise using your independant 4 jaw chuck. I have used mine quite a lot recently and setting up to run true does become much easier with experience. Make sure the jaws are tightened up really well. Tighten them progressively in opposites and to preserve your centering.

When the saddle is in the required position lock it down and make sure all gibs are tweaked up – there should be no slop anywhere in the setup.

Wind back the top slide away from the chuck (you don’t want to be running out of reach after you have cut your first groove).

Check how far your top slide travels for each revolution of the handwheel – mine gives 1mm of travel for each revolution which makes calculations nice and simple. After cutting my first groove 4 turns of the handwheel moves the cutter into position for the next one. Carry out a similar exercise for your cross slide – make a note of where the position of the handwheel should be when you have reached your 6mm depth of cut.

Lathe speed for your cast iron cylinder should be somewhere between 100 and 150 rpm. As Bogs says, don’t play with it, just give it a nice steady cut to keep it cutting. As soon as you stop that is when materials rub and form a hard skin and can make it difficult to get cutting again.

So near and yet so far. This is the stage I am at with ‘Debbie’, Jan Ridders Simple Two Stroke. Despite many days experimenting with fuel and ignition adjustments, remaking of components and every effort to eliminate any possible air leaks ‘Debbie’ steadfastly refuses to pick up her skirt and run. By the way, the tall pipe protruding from the fuel tank is a temporary affair and was used in experimentation of the air inlet.

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.

Despite all the problems I think this is a superb engine to add to my collection and this makes me all the more determined to overcome my present difficulties. I am most impressed with Jan’s take on two stroke engine design and I feel my difficulties are due more to my construction than to Jan’s design.