1st of all we should know what the check ball & check weight is for. When you press on the gas to accelerate, the engine needs an extra spurt of gas to get you accelerated until the cruise circuit can catch up. This is where the accelerator pump circuit comes in play. Pressing the accelerator causes the accelerator pump diaphragm to move up forcing a small amount of fuel out through the main discharge. The force of this fuel forces the check ball and check weight, or in some cases just the check weight up, allowing fuel to flow through the main discharge. When gas isn't being pushed out the discharge, the check ball closes off the fuel so that vacuum from the intake doesn't siphon fuel through the main discharge.Some Carter YF carburetors have a check ball with a check weight on top of it. See # 24 & 25 in the illustration below. Notice that the check weight is rectangular shaped. This check weight is nothing more than a weight. It probably weighs less than 1 gram, but that's OK. Too heavy and the check ball would not lift off the hole.
Other YF & YFA's might have just a check weight, but this check weight is tapered on the end and this taper acts the same as a check ball does. See #26 in the illustration.
Check weight w/o check ball. There are 3 different lengths depending on the carburetor. Most use this one captioned here. After placing in the main discharge you should have 1/16 - 1/8" free space for the check weight to lift off the seat. You can grind the check weight down to fit.
Check weight w/check ball. Make sure you have about 1/16" - 1/8" of free space for the check ball to lift up. If you need more, simply grind down the check weight to fit.
When rebuilding your carburetor, test the accelerator pump circuit before installing the carburetor on the engine.
Add liquid to the float bowl. Push the **accelerator pump** down, then pull up. Pulling the pump up will force fuel out of the main discharge. If you don't see fuel coming out then you have a plugged passage. Use thin wire to clean out the passage way.
Using a brass drift punch and holding the check ball, or weight down to plug the hole, pull up the diaphragm again. You should feel a little pressure. If you get very little pressure, or fuel is coming out around the check ball, then you need to seat the check ball. Do this by inserting a brass drift punch on top of the check ball and tapping it a few times. This will seat the check ball. Now I said tap, not bang on it. You don't want it to get stuck.
Check balls are often placed in the wrong hole, so don't assume you will find the correct size in the particular hole. To figure out which check ball goes in which hole, assuming you have two different sized check balls, look into the bottom of the holes and you should see which hole is bigger than the other. Simply place the bigger check ball in the bigger hole. Be sure each hole has a trough for the check ball to sit into. The check ball has to stop the fluid from going in one direction, so it will need to sit into a beveled edge hole. When there is no bevel, then it probably doesn't need the check ball. Once the check balls are in place you will need to test them to make sure they seal correctly. Use a test fluid, like carburetor cleaner (NOT WD-40), and a small punch to gently hold down the ball on it's seat and depress the pump in it's well by hand. If you get a hydraulic lock then you are good to go. On holes other than the accelerator pump simply watch that the fluid doesn't leak out. When the check ball isn't sealing, then you will need to re-size the hole by tapping on the check ball a couple of times. DO NOT use the new check ball for this. It could become mall formed. Use the old check ball if it isn't corroded or beat up.