I'm bringing this back up. A gentleman here asked me why his YF2100S was leaking fuel down when the engine was turned off (making it hard to start). Please see 4 images below for suggestions. In the first image, you see the recessed area inside the float bowl where gasoline must not be present. This bears a bit more explanation, I think. Your diaphragm (rubber) is sealed against the recessed area by the diaphragm bonnet. Now when your engine is running, manifold vacuum will be present in this recessed area. That vacuum is what raises and lowers your metering rod in your main jet...providing the "power circuit." Lower manifold vacuum (when the engine is under load) allows the spring to pull the diaphragm and also the metering rod up. This means the smaller portion of the rod is in the jet and that provides more gas into the engine for more power. Then when manifold vacuum increases back to normal, the diaphragm is sucked back down, the larger portion of the metering rod is forced into the jet and your engine gets less fuel and runs leaner. INCREDIBLY MORE CLEVER and predictable than the Rochester B's oddball power piston system. Earlier I explained that a leak in this area would make your carburetor run richer and that is a bona fide fact. Manifold vacuum will suck whatever fuel gets in here right into your intake manifold and if your diaphragm isn't sealed properly the vacuum will draw fuel into this area. And about the leak-down problem? Same thing. Please look at the second image. You can see a white twisty-tie sticking through the diaphragm vent (or vacuum if you please) hole through the bottom of the float bowl. And if you look at the third image, you see where the white twisty-tie goes back into the float bowl...and into the area where vacuum will be sucking against your diaphragm. So any fuel which runs between the bonnet, the diaphragm and the float bowl will pool in this recessed area but only long enough to make its way back into your manifold. The fourth image shows once again the areas which must be checked for level when your carburetor is rebuilt. Since I posted this, I've rebuilt a handful of YF carburetors...maybe 7. Over half of them have had warped bonnet pieces. All (in my opinion) from heavy-handed or ham-fisted screw tightening. And it must be said that the little gasket provided by the cheap rebuilding kits only makes matters worse. That gasket allows more room for the bonnet to be warped...as each screw is over-tightened, it pulls the corners of the bonnet down while allowing the center area to remain high. One bonnet was defective from the start. There was a casting flaw which had not been noticed when the carburetor was assembled and I think that's why the inside of the carburetor looked like it had never been used when I got it. Hope this is helpful. Please contact me with questions. Someday I hope we have a tech tip about this stuff or at least a sticky note to make it easier to find information...
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
