As the throttle is let off, the pump diaphragm moves to the down position. The check ball at ‘A’ drops down and fuel fills the pump diaphragm housing. It is recommended that you NOT try to remove the check ball at ‘A’. You probably won’t get it back in.
As the engine accelerates, the diaphragm moves to the up position, forcing fuel through hole ‘C’, which leads to the main discharge ‘D’.
As the diaphragm moves up, the check ball at ‘A’ is forced up blocking the fuel from re-entering the float bowl.
Fuel at ‘D’ enters the carburetor bore through a small hole at E.
Hesitation can be caused by electrical, but we aren’t addressing that here. Aside from electrical hesitation is usually caused by the accelerator pump circuit. 1st do a quick pump test.
This video illustrates the accelerator pump circuit:
If you aren’t getting a good squirt then you will have to look further. Remove the carburetor from the engine. Remove the carburetor top. Remove the check weight and/or check ball from the main discharge hole. Fill the flow bowl with mineral spirits for testing. Press the pump diaphragm all the way to the bottom of the bowl. This should fill the pump with fluid. Pull the pump diaphragm up. Fluid should come out of the main discharge hole. Put the check ball and check weight back in the main discharge hole. If the check weight has a pointed end, then not check ball is used. Hold the check ball down gently with a drift punch. Push the diaphragm down and up again. You should feel some resistance and no fluid coming out of the main discharge. If not, then tap the check ball down with a few gentle taps to seat it. You can do the same thing on models that only use the check weight.
Warped pump housing?
This will cause gas to leak out of the diaphragm back into the fuel bowl, causing a weak accelerator squirt.
Run a flat file across the bottom of the housing to insure it is flat. Filing at the corners will help.
Do you use any adhesive? NO, NO
Main Discharge Relief
Some YFA carburetors use a relief hole to relieve the pressure in the main discharge. You can plug the hole off only as a last resort to fix your hesitation problem. Test the pump circuit before you consider plugging this hole with a check ball. Epoxy will most likely fail due to the ethanol.
Which check ball and can I use the pointed weight?
Use the bigger of the 2 check balls we provided in the kit. The check weight with flat end goes over the check ball. See illustration above for note about clearance.
You can substitute the check ball with the pointed weight, but you will need to make sure it doesn’t leak. Tap is gently to form a good seal. See the illustration above about how there needs to be a space between the check weight and the top of the bowl. It needs to lift up.
Here is another type of pump intake.