Carb assumptions

After years of mechanical experience on all sorts of bikes strangely a couple of things I thought were gospel are not.
Thought the reason for the fuel being lifted into the tract was the presence of the Venturi shape but now realise that the drop in pressure is also a major contributor?

Again thought the fuel level was below the main Jet but again this can’t be correct. The needle doesn’t seal the jet? And it can be moved a notch proving this so the fuel level must be the same as the float level which is not far below the inlet tract and above the jet? Additionally on some T3 carbs there is a spring ball valve the spring in direct opposition to the slide spring, is this a form of acceleration pump injection but appears to work all the time not just under hard acceleration. Anyone know the ins n outs of this thing on my Ducati the pump operates with a lever but again every time the slide rises not just hard acceleration. Seems wasteful and not ideal or have I missed something as normal?
Regards Ratt

Hi Ratt,
I hope I have understood your post correctly.
You were right first time. The venturi shape causes a drop in pressure so that fuel is sucked from the float bowl through the jets. Also, the main jet sits at the bottom of the float bowl which is why any any water in the carb will be sucked up first.

Have a look here, all will become clear


Thanks for replies, first the Venturi effect I thought was the main cause for fuel lift but there is the suction as in a vacuum cleaner so assume a combination of both.
Petrol level while working on a Falcone I discovered the needle in the float was adjustable and the float jet had a nut inside the float chamber why? The conclusion I came to was it could be adjusted affecting float level. A leak occurred behind the main slide through the air entry hole due to poorly seating “cone” on float valve. It’s then I realised the float level is higher than the main Jet in the carb body so the level is over the main Jet not under it as I had assumed for years suspect others do as well

Regards Ratt

As far as I know, yes the main jet (at least) is submerged below fuel level in every case. Also starter jet. :smiley:

Fuel cannot be sucked up otherwise, only air.

Yes that’s my assumption but the Falcone level is higher than the main Jet so if the float needle is not closed I think it’s above the jet? Next time I work on the beast I’ll check this out.

Regards Ratt

Are you confusing the main jet with the needle jet?
The main jet is at the bottom of the float chamber and will always be submerged.
The needle jet protrudes slightly into the venturi but the fuel level should be below the top of this jet.
The venturi constricts the area of the inlet tract therefore increasing the the velocity of the airflow. Increased velocity lowers the pressure so atmospheric pressure acting on the fuel pushes it up from the needle jet into the airstream.
The needle jet meters fuel flow in the middle range of throttle openings whist the main jet limits the flow at full throttle and a bit below.