Walbro Carburetors and how they work

[JRgraham]

[JRgraham]

So lets take a look at our carburetors

Nothing really magic here, their design is virtually identical to the carb that you'll find on any fuel powered yard implement or gas powered RC airplane you may have

They serve the same purpose as the carb on a glow powered model, they mix the fuel with air in precise amounts so that it can be fed into the motor for combustion.

They have an internal fuel pump to get fuel to the carburetor, they have a throttle to control the engine speed (by controlling the amount of fuel being mixed) and some versions have a "choke" to make starting the motor easier. One thing that the versions for helicopter use have that the ones on the RC airplanes don't is a primer system to get the fuel into the carburetor faster for startup.

These are vacuum fed carburetors, the fuel is not pumped or forced into the motor as with a fuel injection system, the low pressure that's created in the intake side of the motor literally sucks the fuel out of the carburetor. Technically they are known as "diaphragm carburetors". It will become obvious why a little later.

Here are views of a typical carb, in this case a WT-644 which includes both a primer bulb and a "choke" system:

WT644 Intake Side - Choke Open

WT644 Engine Side - Throttle Closed

WT644 - Fuel Primer

WT644 - Fuel Pump Cover

WT644 - Exploded View



Pump system (fuel inlet)

[JRgraham]

[JRgraham]

[JRgraham]

[JRgraham]

The fuel needs to get from the main fuel reservoir into the carburetor throat so it can get into the motor. There are separate high speed and low speed control systems in the carb which will be covered later but there are also common parts for fuel delivery control.

those will be covered here

On the motor side of the carburetor is the throttle valve (butterfly). Here you can see it in the carb and in the closed position

Throttle Plate - Closed

And here are the components of that. Note that the throttle plate has a small hole in it. This ensures that even if the throttle is fully closed that a small amount of air can still pass through the carburetor. Because of this, depending on the mixture the motor can actually still run with the throttle fully closed

Throttle Plate Parts

And on this side of the carburetor is the "choke" or engine primer plate. You'll see in the low speed control discussion exactly how this works closing this causes much more fuel to be pulled into the motor

BTW - if you remove the choke shaft, there is a small spring and ball at the bottom of it that acts as a detent. When you remove the shaft those parts will fall out so be ready to capture them. Easy to reinstall.

Choke Plate - Open

And here are the choke parts. Again even fully closed there is a small hole to allow some air to move through. This is needed to ensure fuel flow through the fuel control systems.

NOTE: the two small indentations on the choke plate serve as alignment guides for attaching the plate to the shaft. Because of the shape of the venturi of the carburetor, this plate has to align precisely when installed. When the indentations align with the shaft, the plate will be aligned.

Choke Parts

So in the last section we talked about the main fuel reservoir being filled with fuel. That fuel has to get into the carburetor. Note that there is a low speed and high speed control system visible but they are capped off. If they weren't, the motor could pull unlimited fuel through the ports. In this case it would flood with fuel.

So a more precise way to meter the fuel is needed. That way is implemented in the form of needle valves which can be adjusted to allow a precise amount of fuel being allowed to flow

So the fuel has to get from the reservoir to the control needles. It does this by being pulled through the needle feed housing. This is pressed into the carb body and is not serviceable

Fuel Circuits

It does however contain the small composite plate. Basically it acts as a one way valve by only letting fuel into the needle feed cavity. If you look at the previous photo you can see this plate just inside the needle feed port

Whats inside the needle feed housing

If you remove this (and this was a destructive process) you can see that at the bottom is two ports, one directed to each of the needle valves

Needle Feed Ports

If you look into the carb body where the high and low speed needles install, you can see that deep inside there are seats machined into each. This is where the needle valves fit. Also the needle feed ports drop into these cavities on the needle side of the seats so fuel flow control into the rest of the system will be controlled by how open or closed the needles are.

High/Low speed needle seats

Finally, here are the actual metering needles. You can see that the tips fit into the seats inside the carburetor and that they have slightly different shapes which account for different fuel flows

High/Low speed needles



these carburetor components provide very fine control over the amount of fuel entering the fuel feed circuits.

The Low Speed Circuit

[JRgraham]

So here's where the fuel control parts covered in the last section all start to work together.

This carburetor has two basic fuel metering systems, this section will focus on the operation of the low speed fuel control system

As discussed earlier, this particular carburetor has a "choke" to help prime the motor for starting. In that config, the choke plate is closed as is the throttle (remember its never fully closed off)

Here's the fuel path. With the plates closed a large vacuum is created in the carb throat. This causes fuel to be pulled from all fuel control systems to prime the motor for starting

Fuel Flow Path - Choke Closed (engine prime) - Diagram from Walbro Service Manual

at idle, only one fuel port is "open" and its inside the throttle (see shortly). Here's the fuel flow for the idle configuration

Fuel Flow Path - Engine Idle - Diagram from Walbro Service Manual

Also note that at idle, the air moving throught the carburetor actually "backs" into the exposed high speed and low spped ports and acts to push fuel through the idle port.

Here's how the fuel gets there. For this photo I removed the cap over the low speed fuel cavity to expose the ports. BTW, this cap is known as a Welch plug (so named because of an invention by the Welch Car Company). They have to be pryed out and "deformed" back into position. More on this in the problem/maintenance segment.

you can see where fuel metered by the needle valve feeds into this cavity and will be waiting under each of these ports.

NOTE - there are no screens here

Low Speed Fuel Cavity

At idle, only one of them is exposed to engine vacuum, the idle port. Here you can see that when the throttle is closed, this port is still exposed

Idle Port in carburetor throat

As the throttle is opened more, the additional low speed ports are exposed as is the high speed port further back in the carb throat

Here you can see the fuel flow for partial throttle openings.

Fuel Flow Path - Partial Throttle - Diagram from Walbro Service Manual

And here are the additional ports inside the carburetor throat.

Low Speed ports in carburetor throat



The carburetor has a dedicated fuel control system for idle and low speed throttle settings. As the throttle is opened more and engine speed and carb vacuum increases fuel is pulled through the low speed control circuit.

The High Speed Control Circuit

[JRgraham]

Finally a control system is needed to deliver additional fuel when the engine is operating at high rpm's. The exact throttle opening at which this circuit will begin to deliver fuel will be based on a number of circumstances and needle settings but typically its providing some fuel at half throttle opening and more

Here's the fuel flow for the high speed fuel delivery system

Fuel flow at High speed - Diagram From Walbro Service Manual

The high speed control cavity is covered with a brass plug. This plug must be pryed out and would need to be replaced if removed.

Metered fuel is delivered into the high speed port cavity via this hole, which is at the end of the high speed needle valve

High speed fuel feed from needle valve

and here's where its going. This port is much larger than the low speed circuit ports.

High speed fuel port

Its also covered by a screen. This small screen is held into place with a circular clip. This part is serviceable but very difficult to remove. Its been pointed out to me that it also provides a capillary function to allow fuel to pass through it but not air.

High speed circuit screen and retainer

If you look in the carburetor throat, you can see the high speed fuel port. Its more on the intake side of carburetor in its narrowest part. If you're familiar with the concept of a venturi you'll know why

High speed fuel port in carburetor throat



The high speed fuel control system works with the low speed system to delivery fuel to the motor at high speed operation.

So there you have it, we've covered every component of the Walbro carburetor and how it operates. The theory is almost identical to that for a common glow motor carburetor, its just implemented in a more complicated way.

Problems and Maintenance

[JRgraham]

[JRgraham]

the chamber "caps" are typically called "welch" plugs. The rebuild kit I referenced has replacements.
walbro supplies two tools that you can use to remove/replace welch plugs and they look like this:

Available Walbro tools

you use the sharp edged tool to "dig" into the caps and pry them out and the flat tool is used to compress the new plugs.

I haven't found that the sharp edged tool works very well and not at all on the brass plug you see. I got them out by drilling a hole in the center and then using a screwdriver to pry them out. HOWEVER.............this puts drill chips in the chamber and maybe in the ports and more importantly if the drill goes THROUGH the cap it will have already damaged the carb body. There is practically no clearance between the two

so thats why I recommended that if you've cleaned the stuff you can get to easily and it still doesn't work correctly, you'll invest far less time to just replace it, they are not expensive.