After restoring the entire electrical system, and with a spark in the spark plugs, I had a lot of cattle for the start of the Benelli. If the operation of an engine depended on the intake of a mixture (air and gasoline) from the carburetor and the intake of sparks through the spark plug, it had already gone halfway. It played carburetion of a two-stroke engine.
Lebrijana posed two new challenges for me: it was the first time I had faced a twin-cylinder, and also the first two-stroke motorcycle to come through the workshop, so I was going to have to work hard.
It would have been quite useful for me to have had a book on motorcycle mechanics for beginners at that time .
After verifying that there was a spark at the plugs, it was the turn of the carburettors. Both were in a sorry state on the outside and I didn’t want to imagine how they would be on the inside. The cable that connected one of them to the throttle grip was broken and the same was the case with the starter of the other.
The starter , or simply “air”, is the mechanism that allows additional air to enter the carburettor when the motorcycle is cold, to facilitate starting. (More air that enters, more gasoline that will drag).
I disassembled the air filter, which was common to both carburettors, and immersed it in gasoline to clean it. The filter was located in a metal box that would have to be painted.
The function of the air filter is to prevent the entry of dust particles and other solid elements into the carburettors and engine, which could damage them.
With the carburettors accessible I set out to disassemble and open them. What I found inside it attested to how difficult the motorcycle would have if it wanted to start. Its inactivity for so many years had caused a greenish layer of dirt and decomposed gasoline to form, clogging all the jets and preventing gasoline from rising through them.
I dipped them in Zotal and waited for the cleaner to take effect.
Regarding the cable that transmitted the order to accelerate from the grip to the carburettor, since it was a two-cylinder motorcycle, it forked in two to reach both carburettors. This was achieved by means of a cylindrical piece that was in good condition. To get a wire of the exact size, I cut a standard wire and prepared a terminal with cast tin.
While I was doing the boot tests I would have to fend for myself and, at first glance, it seemed that it fulfilled its function.
How a two-stroke engine works
Unlike the Sanglas ( Primitiva and Raimunda ) and the Ducati ( Fortunate ), the Benelli was a two-stroke motorcycle .
The main difference between a 2-stroke engine and a 4-stroke engine is that, while a four-stroke engine (an engine that any conventional car has) has to make two turns of the crankshaft to complete one cycle (the piston goes up twice inside the cylinder per each explosion), in a two-stroke engine the cycle is completed in a single revolution of the crankshaft (the cylinder rises only once).
Some notable differences between the two engines are the absence of valves in the two-stroke , since it is the piston itself that allows the mixture to enter or not, or the way the engine is lubricated , which is done through the crankcase oil. in four-stroke engines, and by means of oil mixed with gasoline in the two-stroke.
For the same displacement, the two-stroke engine develops a higher power , as an explosion occurs at each turn of the crankshaft. So much so, that the Benelli, being 250cc, develops 32CV, compared to the 25CV of the Sanglas (400cc) and the 22CV of the Ducati (350cc).
On the other hand, they are much more polluting engines , as they burn oil together with gasoline, and with a higher consumption .
If you are interested in the subject, we explain the actual cycle of a four-stroke engine in this post.
After this theoretical explanation, the carburettors should already be clean, I took them out of the Zotal, rubbed them and cleaned the jets with compressed air.
Once the carburettors were mounted and the accelerator cable fixed, all that remained was to cross our fingers and activate the kickstarter.
I had no hope that it would start, since to get it done with the Sanglas or the Ducati I had had to spend many long days in the workshop.
I put my battered foot on the pedal, pushed it and the bike started first time. It scared me to death, because I didn’t expect the bike to react so quickly to the restoration, but without a doubt, the simplicity of the 2-stroke engine had helped to achieve such early results.
