Idle Circuit View

This is all Larry (LRCXed) but I figured it needed an official post here too.

OK everyone,

Here is what I have been working on with the help of Blindstitch. I think this is way overdue in the explanation on how and why the idle circuit works, as well as why they need to be cleaned out so well to work properly.

I have done a cutaway for you to see how the inside of the systems work. This is just a sample of the things I'm incorporating into the book.

I hope these pictures will help folks understand why it is so important to get the insides of a carburetor so clean, and why it does not run right when it's not. This should help also in the understanding of how it works.

First up is a view of the air bleed system and the fuel pickup paths. As well as how they mix with the air to emulsify fuel for proper delivery into the motor. Please note all of the slightly small areas around the emulsion tubes where the air is passed into the center to mix with the fuel. This is a key point in proper running on the system. It's how the air/fuel mixes to an atomized mixture.

In case your not sure, the right side is the main jet passage. And the left side is the secondary jet passage that is controlled by the needle in the slide.

This second picture shows a very intricate picture of the idle circuit and all the related circuits. Each of them must be clear and functioning properly for one another to work.

I want you to notice one thing here. Even though it may seem big in this picture, all these passages that are drilled into the body are very small, and are step drilled. This means that as old fuel and calcium deposits build up, the passages, jets and airways get restricted very quickly. ANY restriction in one or another will effect the function of the entire circuit.

As you can see, the .018" hole in the low jet would not take very much to plug it up. Also notice the area around the 3 idle holes at the throttle plate and for the idle mixture screw. Lots of places for junk to build up and plug the circuit in this isn't there. Imagine dried up fuel deposits finally getting loosened up and not dissolved or removed all the way. Those strings of gelled fuel and loosened calcium go straight to the only outlet they have. And it's the holes that you need clear for the engine to run below 5000 RPM or at idle. If that tint area at the needle gets plugged, it is why you can't get the idle needle to make a difference when you screw it all the way in. One of these 3 hole's slightly open, is enough to keep it at an idle. But beyond that, it will suffer due to fuel starvation.

One more thing. The air cutoff diaphragm controls the extra air bleed in the center hole. As you let off the throttle all the way, the added vacuum signal from the motor travels up to the top side of the diaphragm through the hole in the cover that holds it in, and pulls it up against the spring. And the brass plunger in the center shuts off that center air bleed. This in turn will cause more fuel to be pulled through the low jet (just like if the other air bleeds were plugged on the other side) and adds extra fuel to the motor at off idle. This is how it keeps the engine from having a lean popping in the exhaust when you let off the throttle after high RPM's. It pulls more gas in due to restricted air flow to the system making the off throttle run rich enough for the fuel in the exhaust pipe not to combustion and cause popping.

I'm sure I have missed a few points to which some will have questions to. But I hope this helps you understand the insides of your carbs a little better.

Thanks Blindstitch for the help in editing these photos.

Larry