The Stovebolt Site Forums The Shop Area Engine Shop Carter stumbles on acceleration

I agree with Jon/CarbKing. It sounds like it is running rich. Over the past 5 or 6 years I've posted some reasons why a Carter YF might be doing this. Some are rather obvious (improper float setting, cracked or split diaphragm, orifice of the fuel inlet seat too large, improper fuel pump pressure, and others). I mentioned one reason that people tend not to see nor think about...even though it is just as important as the rest of them and in some cases maybe more so. The bonnet that holds the diaphragm to the float bowl can be warped. It suffers the same problem as the Rochester B...4 screws hold it in place (at the corners) and it can be warped by improper tightening. Basic message is you don't have to torque these screws down. Some cheap rebuild kits include a gasket for correcting this...Jon knows about it and who created it. This was designed to solve the warping problem, but I think it makes it worse and for that reason I'd never use that gasket. Essentially the important thing to understand is there is always vacuum present below the diaphragm (the square-ish rubber piece that also operates as your accelerator pump). The design of the YF is that this vacuum will help regulate the position of the metering rod in the main jet...and when things are right it does that very well. Throttle position will also help regulate the position of the metering rod. Here's how the vacuum portion works: manifold vacuum is present under the diaphragm and that vacuum changes depending on the needs of the engine. If the engine is laboring and needs an extra bit of fuel, manifold vacuum will drop. The large spring on the diaphragm rod is then free to lift the metering rod up a bit. The linkage is designed to let this happen. This means the thinner part of the rod will then be in the jet and that means more fuel will be sucked into the manifold. If the engine isn't laboring but is just running along normally, vacuum is higher and that means the thicker part of the metering rod will be pulled into the jet and that means less fuel will be fed to the engine. In this way, manifold vacuum and the pressure of the spring will work together to regulate the air/fuel ratio of your engine. Marvelous design, but if your diaphragm does not seal correctly manifold vacuum will suck fuel right past your diaphragm and into your manifold all the time. How to fix this? I got a piece of flat wood...think it may have been a small piece of poplar and drilled a 1.25" hole in it about 1/2" deep using a forstner bit. Then I cut circles into a couple of pieces of sandpaper...one was 180 grit and one was 320 grit. Then I placed those on the piece of wood, put the bonnet on top and rotated until it looked like it had been machined smooth. In one there was quite a bit of warpage. I've found this problem to be present in about 6 or 7 YF models I've bought. Please look at the images below. The first shows the bonnet piece. The second and third show the vacuum passage leading to the recessed area in your float bowl that should be sealed by your diaphragm. The fourth shows the bonnet with the diaphragm and spring mounted. The fifth shows the bottom side of the diaphragm. The sixth shows the now-smoothed bonnet surface. The seventh and eighth show the little tool I made. A couple of folks have asked "will this affect the a/f ratio at all speeds?" The answer is yes and especially at idle, when the engine should be running at its most efficient mode and on deceleration (when the vacuum is higher). At those times, more raw fuel will be sucked into the recessed area and passed straight into the intake manifold. Just a thought and a bit of explanation of how the power circuit works on the Carter YF. Good luck!