Once again the motorcycle would not start. The fact that the Ducati did not start, unlike the Sanglas, was a physical wear and tear that took a toll on my spirits, since while the attempts to start the Sanglas ended when the battery ran out, the absence of an electric starter in the Ducati, forced to gather strength and desire before facing the daunting task of starting it. We will take the opportunity to learn how a carburetor works.
I was afraid that having disassembled and stripped the engine had damaged something that prevented starting and that I could not detect with the naked eye.
I rechecked all the elements that influence ignition. The battery was correctly charged and produced a spark, however he had to remove the ignition (points) to strip the engine, so he could have assembled it incorrectly.
On the other hand, the carburettor had been separated from the engine for a long time, and could have collected dust, so cleaning it was recommended, as well as regulating it properly.
Back to the ignition, I removed the crankcase cover where it mounts and checked the condition of the gears. It was then that I noticed the small dots that mark the correct position of the sprockets, something that I had not detected before, and I found that they were obviously mounted incorrectly . The correct location of these points, together with the correct timing of the ignition causes the spark to occur at the right moment in the piston cycle. With the wheels as they were, it was normal for the bike not to start. No matter how much spark jumps from the spark plug, if it does not occur at the right time, it is useless.
Once this was solved and the joints reviewed, it was likely that the Ducati would work again, however, he had already decided to clean the carburettor, and checking that the jets on it and the rest of the components were clean and not clogged were not going to do him any harm. damage.
I equipped myself with Zotal and the compressor, and carefully disassembled the Dellorto PFH 30 B carburettor that the Ducati Forza mounted (they could also mount an Amal).
It had not started and already had a problem, the conduit through which the throttle cable entered the carburettor was broken. This could cause the undesirable entry of air into the mixture, altering the operation of the engine. I repaired it with metal cement, however eventually I would have to replace the carburettor cover.
How does a carburetor work?
I have spent several paragraphs giving you the plate with the carburetor and, although I am sure you already know it, I have not presented it to you or explained how a motorcycle carburetor works . Go for it.
By the way, if you are interested in mechanics, we have written a book on basic motorcycle mechanics that may come in handy for you.
The carburetor is one of the most important elements of a motorcycle. Its function is to supply the engine with the proper mixture of air and gasoline, so that it can work. It works by the Venturi effect, thanks to which the air that enters the carburettor on the way to the engine sucks in the existing gasoline, and the mixture produced explodes when it comes into contact with the spark from the spark plug.
Yes, you are already lost, I know, let’s see it with an example. Imagine that the engine is an aspirator: the lowering piston works as such, sucking air through the intake nozzle, which is nothing more than the tube that joins the engine with the carburetor. In front of the vacuum cleaner we put a glass of gasoline – the carburetor. When inhaling air in contact with a gasoline surface, said air will drag part of that gasoline, entering the vacuum cleaner. If we were to go crazy and light a match (spark plug) inside the vacuum cleaner (engine), the mixture of air and gasoline would explode.
This explained here, although it has a very simple foundation, it has to occur in completely defined proportions to achieve proper engine operation. The proportion of gasoline in the mixture with the air has to be adequate, and the carburettor takes care of it through small ducts called jets . These fine tubes have one end submerged in the carburettor’s fuel tank, known as the vat, and the fuel rises through them by capillarity. It is at the upper end of the jets where the exchange between air and gasoline occurs.
There are several jets because there are several internal circuits in the carburetor, through which the gasoline circulates before being “absorbed” by the air:
1) High circuit: it is the one that works at high engine revs. In this case it is the central and larger jet. It is known as the main supplier. There is a needle inside it, which we move when turning the throttle grip, and which regulates the greater or lesser entry of gasoline into the mixture.
2) Low circuit: it is the one that works at low revs, when the motorcycle is idling. At this moment the needle of the main jet is completely inserted in the high jet, so no gasoline enters through it (we are not accelerating), but to keep the engine running a small amount of gasoline is necessary, which is performed through this low jet.
3) A third jet is observed , which in this case corresponds to the air circuit. With low temperatures, the capacity of the air to carry gasoline decreases, which is why an extra supply of gasoline is necessary, which is done through this third circuit.
I disassembled all the carburettor parts and submerged them in Zotal (Note: this method has always worked for me so far, but some people prefer to clean everything with gasoline, or with a specific carburetor cleaning product) to clean it, then I checked that the Air passed adequately through all ducts. The Zotal eats the plastics, so I took great care to separate all the joints.
I reassembled it once the glued piece had dried, and I went on to regulate the external screws of the carburettor. This is definitively regulated with the motorcycle running, since without listening to its sound it is impossible to determine if the air-fuel mixture is correct, however, for the motorcycle to start it was essential to at least bring them closer to their real position.
On the side of the carburettor there are two screws and this is usually common on classic carburettors. One of them, the one located on its axis, regulates the position of the needle at rest, that is, without accelerating. It is a conical screw that in its tightest position pushes the needle up, so the more we tighten the screw, the less we will have to accelerate to get more revolutions. The other screw regulates the idling of the motorcycle, the cadence of the engine when we are not accelerating, which is nothing more than the richness of the mixture of the low circuit. The tighter it is, the less gasoline will go through the circuit, and the slower the engine will spin, to the point of stalling.
I left them in an intermediate position, pulling rich in gasoline to guarantee starting, even if I had to tighten them later.
With everything assembled again, the ignition again stalled with the Top Dead Center and the battery fully charged, I tried to start it again.
And this time it did start.
It started and, unlike the first time, it did so with relative ease and once the carburettor screws were adjusted again, it achieved a stable and regular cadence, which it had not achieved the first time, most certainly due to poor carburetion and unwanted air intakes. .
After a while it stopped and did not start again, and after trying several times and looking for the possible cause, I found the problem.
The problem was spark plug shaped.
