Where are the jets in these SU carburettas.

[Revvy Kevvy]

Iv worked on normal "ford falcon 351 holly carburettas " many times but this is my first time with this SU carbi idea..

Where are the jets to atomise the fuel? is it this long chimney thing that the needle pokes into? How does this atomise the fuel going into the engine? does it all kinda flow out the top of the chimney? Holes in the chimney to mix the air into the fuel?

Where are the accelerator pumps? on a normal carbi when you hit the accelerator it pumps fuel in, I dont see where this carbi could do it.

Whats this other jet for, the one that was under the black hard rubber seal, and where does its fuel supply come from if the black rubber seal is in the way?

I understand the needle needs to move upwards by use of vacuum, not too sure where this vacuum comes from but should I put lubrication on the piston to make sure it moves?

Lastly, a choke is usually a seperate valve that makes the mix rich, how does this choke make the mix rich?

Still not got this baby working, I think the timing is ok now with the initial spark a couple degrees before TDC on both sides. and I know she sucks but as far as I can tell no fuel is getting into the engine. So, rebuild the carbi time

 

[CaptChrome]

Lots of questions! Let's see if I can help without typing a book.

Where are the jets to atomise the fuel? is it this long chimney thing that the needle pokes into? How does this atomise the fuel going into the engine? does it all kinda flow out the top of the chimney? Holes in the chimney to mix the air into the fuel?

The long Chimney is the needle jet (sometimes called the emulsion tube by other carb manufacturers). There are a number of air and fuel jets in this carb. The ones of most concern are located in the float bowl area (starting jet, pilot jet, needle jet and main jet. The needle jet is held in the carb body by the main jet and it controls fuel to the engine in the midrange throttle positions (the pilot jet and the fuel mixture screw determine fuel at idle and the main jet becomes effective at controlling fuel at wide open throttle. Fuel is drawn into the needle jet through the main jet by vacuum in the carb venturi and air is drawn into the small holes along the length of the tube atomizing the fuel. The shape of the needle and it's position in the needle jet determine how much fuel and air are pulled through. Once the needle is at maximum lift, the main jet determines the fuel flow.

Where are the accelerator pumps?

There are no accelerator pumps in the Mikuni BS34 carb. The bike performs well without the pump and it simplifies things for us as we don't have to worry about extra linkages, diaphragms, check balls and the like.

Whats this other jet for, the one that was under the black hard rubber seal, and where does its fuel supply come from if the black rubber seal is in the way?

This is the pilot jet. Fuel is pulled into this jet from a drilled passage between the rubber plug and the top of the jet. The rubber plug is important and is needed to be installed for the carb to work properly.

I understand the needle needs to move upwards by use of vacuum, not too sure where this vacuum comes from but should I put lubrication on the piston to make sure it moves?

Vacuum comes from the venturi area of the carb throat that acts on the top of the diaphragm and atmospheric pressure acting on the bottom of the diaphragm. As long as the piston is not binding in the bore, there should be no need to lubricate it. If you do feel the need to lube it, use very light oil.

Lastly, a choke is usually a seperate valve that makes the mix rich, how does this choke make the mix rich?

The "choke" on the bike is really an enricher circuit that adds extra fuel instead of choking off air. When you pull the knob labeled "choke" a valve is opened in each carb which allows fuel to be pulled up from the bowl through the brass tube on the main body and sprayed directly into the carb throat outlet. The amount of fuel is regulated by a starting jet that is pressed into the bowl (not removable) and the position of the enricher valve (either half or fully pulled out).

 

[Revvy Kevvy]

But you indeed did write a book and a good book you wrote. I am used to either fuel injection or the other style of carburetta (Holly or ford 250) with a venturi and pumps in the centre. I remember my grandpa having a Morris Avenger with these SU carbs on it but I was not allowed to play with that car. It was gramps pride and joy.

I am not too sure I see that advantages of this type of carburetta yet, my kit to rebuild it came on Wednesday and I spent friday installing all the bits and trying to find where to put the floats as there seem to be many different places, you tube just made me more confused. In the end I followed the official Yamaha manual at put it at 27mm which is really at the end of travel for the floats.

As for the diaphram piston I cleaned it up using white scotch brite and used some WD40 to make sure its sliding properly. Cleaned up the emulsifier and reinstalled it with new jet screws on the bottom.

The choke I established changes the vacuum for the piston so I guess it changes the mixture in someway using this.

I will reinstall the carburettas tonight after doing the usual domestic stuff on the morning and since the ignition is all set, the valve tappets are set and the carburetta is all new I hope she should fire up. Have me jumping like the aviator in mad max after the truck started.

I have a tin of starter spray ready incase it dont

Thanks for the reply, it makes more sense now but I still dont see its advantages over a normal carb.

 

[CaptChrome]

Thanks for the reply, it makes more sense now but I still dont see its advantages over a normal carb.

You are welcome. I guess that the advantage is some simplicity without the extra pumps and power valves, etc, and it is contained in a small body that can fit into the limited space allowed. There are a surprising number of carb variants out there but they all basically use vacuum and jets/needles to meter and atomize fuel.