Lambretta starting and carb questions

[Piatti]

Hi,
Some general questions, please bear with me if they sound a bit basic

I have just restored a 150 Special, it’s a standard engine with a KBA 42mm Clubman exhaust running a NGK B8ES.
The carb is set up as below, any deviations from factory standard are expalined

Carb fitted: SH1 18
Choke size: 18mm
Slide: 5914-1
Atomizer: 5899-2
Main jet: 106 (factory setting was 101)
Pilot jet: 45
Starter Jet: 50
Float: 5gms

I was advised to up the main jet (106 from 101) because of the exhaust.

The scooter will start without the choke and I find that I have to take the scooter off the stand and stand over it leaning the body over to the carb side so I can get full travel on the kick start.

Does this indicate anything

I took it for a ride out yesterday and it cut out, (It was my fault, not used to the gears yet). I pulled
over and tried to kick start which it wouldn’t so I ended up bumping it and riding home.
When I got home it wouldn’t start again.
I took the air elbow off the carb and it started.

What does this mean

I read up that a worn slide can cause starting problems so I was in the garage at 8am this
morning (freezing) stripping the carb to inspect the slide, pictures are below.

Do I need to replace the slide





My basic understanding of Lambretta carburettors is that the fuel fills the float bowl which causes
the float to rise to the top of the bowl shutting off the fuel supply with the float needle (red tipped).
Upon starting the scooter the throttle has to be open to allow the slide to be raised allowing a mixture
of fuel/air to be sucked into the engine to be compressed by the moving piston and sent towards the
sparking spark plug.

How does the carburettor know which jet (pilot jet, Starter jet, Atomizer/main jet) to suck fuel up from

Any answers gratefully received....

Piatti

[coaster]

The engine 'knows' which jet because it relies on the speed of the air entering the carb passing over it. on starting with the choke on it will suck from the choke AND idle jets. It will also TRY to suck from the main but as the air isn't moving over it fast enough, no fuel will be sucked up. As engine revs rise so will the speed of air through the carb and so the main jet eventually starts to come into play (around 3/4 throttle onwards).

As for your starting problems, check that your choke is seating properly, there should be a little bit of play in the cable when it is off. Also, when starting, leave the throttle alone until it has fired, the cut out in the slide is there to supply air on tickover/starting, twisting the throttle as you kick is not recommended. Just a couple of thoughts