Carburetor

Physical phenomenon (weather):

You should know that the colder it is, the richer the air is in oxygen (the motorcycle is therefore automatically poorer).

Conversely, the hotter it is, the poorer the air is in oxygen, so the motorcycle is richer.

Atmospheric pressure also affects the setting, the higher the pressure, the better the bike will run

Air intake:

An air intake is a problem often present on Mécaboîtes, it is the number 1 enemy of carburetor adjustments for good reason, an engine that is not or not very airtight will not run "smooth" and adjustment of the carburetor is impossible.

Too poor:

(White candle / very light brown)
This means that there is a lack of fuel compared to the air intake, on a 2t the oil arrives with the fuel, if there is less fuel (too lean) there will therefore be less mixture (oil/fuel) therefore a risk of seizure or overheating and engine failure often at the level of the connecting rod head.

A lean engine at high revs has symptoms of leaking air, running better, then gradually running worse, revving too high, flaming, overheating/heating up faster, and lacking torque.
An engine that is too lean at low revs will idle higher than normal.

Too rich:

(Black/very dark brown candle)
A motorcycle that is too rich smokes, revs poorly, tends to stall, grazes a lot, can also make holes in acceleration like on a lean motorcycle, but it is important in a setting for an endurance/circuit style engine to always be slightly rich on all the carburetor circuits (low / medium / high revs) to avoid engine seizures and also improve the stability of the engine, because a lean motorcycle runs better but for less time or a rich motorcycle runs less but is more stable.

The 2 types of PWK carburetor:

On a PWK carburetor, you should know that there are 2 sizes. The “ small body ” is 19 to 28 or even 30 mm for some brands. They are recognized by a 50 mm inlet on the air filter side and 35 mm on the intake side.

For the "large bodies" these are carburetors from 30 to 34 mm, they are recognized by a 55 mm inlet on the air filter side and 38 mm on the intake side.

The influence of different carburetor components and how to test them:

Generally, the air screw and the idle jet operate at low revs, the needle at mid-revs and the main jet at high revs.

- To test the different elements, start by looking at the color of your candle; it should be light brown or dark. White is too poor, black is too rich.
- Then at idle, with the engine hot, apply some gas. If your idle speed remains a little high for a while before going back down, it is poor.
- For the needle, with the engine hot, put it in low gear, 4th, and accelerate gradually, check that there are no holes or that it does not chatter too much.
- For the main jet we often refer to the spark plug or whether it takes more or less turns than normal.

The powerjet:

On some carburetors there is a system called “ powerjet ” this system works via the suction principle of the Venturi effect of the carburetor (Venturi effect phenomenon of acceleration of the flows via a narrowing of a conduit) this system is used to inject gasoline at high speed to give thrust, in reality everyone removes it because it is not efficient and hinders the adjustments.

The air screw:

The screw located at the rear to the right of the idle screw is used to adjust the air supply to the idle jet (air richness screw) , the more you screw this screw in, the less air there is, therefore the low revs are richer.

If you have to turn this screw all the way in or more than 1.5 turns it indicates that your idle jet is too small (too lean at the bottom).

On the contrary, if you have to unscrew it more than 2.5 turns and at low revs the motorcycle “floods” and idles poorly except when cold, this indicates that your idle jet is too big.

Additional information:

There are one or two outlets that you should NOT CLOG because they are vents that allow the tank to remain at atmospheric pressure and therefore allow the fuel to be filled. If you block these locations, the carburetor will not be able to fill.

For Derbi, mainly due to vibrations, there is emulation in the carburetor, so it is strongly recommended to choose a flexible pipe, one of the references: Pipe mvt round outlet. But be careful, being more flexible they are less resistant, so will tend to break faster. Also check that your carburetor does not touch your engine casings. If this is the case you can either glue a foam underneath or prepare your casings by filing the part that touches.

(credit: ljt_hugo)