Carb barrel spring

Why does my FA-125 have no barrel spring in the carb?

It's not a stupid question. But it is a testimony to the excellent machine work in the Saito carb.

When you work the throttle lever you'll notice the throttle barrel also moves in and out of the carb body. This is what makes the metering (low speed) needle vary the mixture as the throttle setting changes. By moving nearer and farther off the LS needle the fuel port gets larger and smaller, changing the amount of fuel delivered.

Now, since we are depending on the consistency of the variation we have to be sure it changes only with turning the barrel, and not by the barrel moving straight in or out. The spring prevents this, keeping the barrel pushed toward the lever end.

Now we get to the excellent machine work. There is a screw, often the idle speed stop, that rides in a spiral groove on the throttle barrel. As the barrel turns, this is what makes it go in and out. With the outside of the barrel and its bore in the carb body smoothly finished, there is not much friction preventing the barrel's turning. Add the well finished side of the groove for the idle stop screw, the spring is easily able to push the barrel out of the carb body, thereby seeming to be spring loaded to full throttle.

All twin needle carbs except the cheapest possible ones, have this spring. But most others are rough enough in their production that you don't see the barrels being pushed out by the anti-rattle spring.

Please note that an ordinary air bleed carb does not need this spring. Saito air bleed carbs do have one, but they aren't ordinary air bleed carbs either. They could be the subject of another discussion if anyone is interested.

Carb barrel spring revisited.

Since writing the last note I’ve given the spring and the excellent machine work more thought.

No matter how well machined the throttle stop screw and the groove in the barrel are, they are subject to wear. When the groove gets the first few thousandths free play, then there’s going to be the hammering from vibration causing the barrel to vibrate sideways, this will make the wear even faster.

When the wear is sufficient to allow movement in and out without turning the barrel, the mixture control will also be less precise.

The spring on the barrel not only holds it firmly to one side of the groove for precise mixture control, by its very side tension it keeps the barrel from hammering on the edge of the groove.

You say there’s a three year warranty on the engine? Yes, there is. Wear is not covered.

When I get one of these engines without a barrel spring I’ll have to get a spring to fit the throttle barrel.