Expert replies to common questions by Brian Winch

Just Engines Online | Ignition Systems |  Expert replies to common questions by Brian Winch

Expert replies to common questions by Brian Winch

Can I convert a Saito 180 to petrol and how thick should the shim be and/or it has
been suggested that I could retard the ignition slightly to compensate for no shim??

Yes, you can convert the Saito 180 and, yes, you really need a shim as these engine have quite high compression and cylinder pressure. A metal cylinder base shim between 0.6 mm and 0.9mm would be the range of consideration, leaning towards the thicker would be my opinion. You need only remove the cylinder - simple job to locate the shim or make the shim in two halves with slotted holes and just lift the cylinder - complicated job. As to the retarding yes, I would experiment with this and the way to do it is to have the Hall sensor clamped on with a metal strap (hose clamp style or similar) in order to be able to move it around the front housing to select the maximum timing position.

Should I use an alternative carburetor when I change to petrol and what oil mixture should I use??

The carburetor on the engine will do the job quite well and you do not need a pump if you did not need one for glow operation. You will use a smaller fuel tank and you must use petrol resistant fuel tubing throughout plus a petrol suitable tank bung if you use that type of tank. Minimum 20:1 oil ratio and use a synthetic oil (such as is used for model fuels) or an oil designed for air cooled, high performance two stroke engines (motorcycle type typical). I believe you have an oil sold as Silkolene (or similar name) in the UK which might be worth considering.

Can I still use exhaust pressure to pressurise the fuel tank or will the heat cause the petrol to ignite?

Plumbing the exhaust into the tank has been a question for many years from way back in the early 70's but I have never heard of an incident wherein a fuel tank exploded. Webra sell a 150 spark engine and it is supplied with muffler pressure so it is not a concern to them. The hot gas would not be a problem but carbon might be however, I would consider the chances in the
million to one odds maybe even better. Besides, methanol is not too slow igniting with a hot glow look at glow plugs and we have been pumping hot gas into methanol for a long time. I would be prepared to say not a problem.

My glow engines is designed to run on 20% oil, how can I run it on as little as 5% oil when I convert it to petrol?

Petrol engines run at a higher temperature than a glow engine so you can expect and slightly greater expansion of mating bearing surfaces, conrods for example, and this reduces the friction coefficient to a small extent. Timing is another factor and this plays a very big part in the heat of the engine and the load on bearing surfaces. A modern petrol engine (CDI ignition) is timed automatically according to the load and speed. The sensor "reads" the RPM of the magnetic exciter and the information gained is used by the CDI program to vary the timing. A free running engine can run on the maximum advance but a loaded engine requires a retarded ignition. A glow engine (methanol fuel) is often (most times) running very well loaded(and over advanced) and there is no provision for timing compensation. As such, the extra oil is a big factor in carrying away excess heat. The final factor is the fuel. Methanol is a very dry fuel no lubrication properties at all so the engine depends entirely on the oil content for lubrication. Petrol is an oily fuel and provides considerable lubrication on its own. Combined with a small
amount of oil (of the correct ratio as specified) the ?petroil? mix is an excellent fuel and lubrication in the one package. Look at your motor car (if it is in reasonable condition and not an oil burner). The oil is splashed onto the cylinder walls and scraped off with the oil scraper ring. The amount left is almost zero but the petrol is then adequate to carry out the rest of
the lubrication job. As a simple test pour a few drops of methanol into a cup of water it disappears. Do the same with petrol and it sits on top of the water as an oily film. In engine technology regarding fuels, petrol has a positive lubrication factor methanol is almost negative.

How can I reduce the compression on my Saito engine given that they do not use head shims and the cylinder is all in one piece?

Make a metal shim to go under the base of the cylinder where it bolts onto the crankcase and this does the job very nicely. I am not too keen on card or gasket material for the job as I feel that the stresses on the cylinder might compress the material resulting in a loose cylinder. Tinplate is ideal as is brass.

Just Engines Online | Ignition Systems |  Expert replies to common questions by Brian Winch