Keihin VD52 Carburetor Fuel and Air Circuit Identification and Porn

Here are some pictures of a stripped-naked carburetor body from various angles showing the plumbing of all the carburetor circuits.

The reason for this mess was due to a search for the secret hidden main air jet, never before seen by human eyes...And to a careful observer it would appear that these carbs do not have a main air jet, unless you care to cut one open.

The carburetor circuits come into play at different positions of the butterfly throttle valve and the functions are covered in detail in numerous manuals so i'm just trying to identify the location of the circuits within the carb body.

The carb bodies are cast aluminum with numerous internal passages, ports and conduits that require some sort of drilling, threading or plugging to form the final product.

Basically each carb circuit functions to mix air (blue) and fuel (red) in the required ratio to form an emulsion, or mixture (purple). Some circuits only deal with low pressure or vacuum (black).

The basic component is a precision drilled hole or orifice, known as a jet, which is usually made of brass, restricts flow to a controlled rate, and is either threaded, pressed, or swaged into the end of it's respective air or fuel port.

So the choke (enrichment) circuit, pilot circuit, and main circuit all have both air and fuel jets. Other circuits include the transition or transfer port circuit and the deceleration enrichment circuit.

The fuel jets are submerged in the float bowl, and the air jets are located somewhere in the intake air path, usually at the very front edge of the carburetor before the funnel down to the venturi. The venturi is the precision machined bore thru the carburetor just upstream of the butterfly valve. It acts like a large diameter air orifice that determines the maximum airflow rate into the motor in addition to increasing the air velocity and reducing the static air pressure with respect to the atmospheric air pressure.

Air comes in at the front, there is a diaphragm-separated vacuum and pressure chamber up on the top side, the fuel float bowl is down on the bottom, the emulsified fuel mixture exits out the rear into the engine, and left and right are with respect to the airflow.

Most folks pull the bowls off to look around, so let's just do likewise and start there:



So now that you know that there is a main air jet, how would you go about cleaning it without drilling out the big 6mm brass plug?

Seven out of the last eight VF700 / VF750 carbs that i cleaned had a clogged main air jet.

 

Nice write up and pics. Will a ultrasonic cleaner do the job on badly plugged ones?

 

That's a great idea, and so i tried it on a set of VD7B off of a VT700C Shadow with plugged up pilot and choke air jets and it cleaned them right up. i still can't verify the full diameter of the main air jet but i can blow thru it to verify its open. i always like to verify jet diameters using a GO/NOGO gage to ensure they are fully open to the correct size and not partially blocked--it makes troubleshooting a lot easier and quicker...

 

Yes but with a caveat. You also need to use a cleaner, it also needs to be heated, and you need to be conscious of Timings..