One thing I discovered earlier this summer was that some of these YF models will have a warped diaphragm housing (could be called a diaphragm bonnet, also). If this happens, it is a very good possibility your carburetor will run rich and this could partially or largely or possibly totally override the effect your mixture adjustment screw has. Please let me explain: There is manifold vacuum constantly under the diaphragm and there is a spring which regulates the movement of it on the diaphragm rod. The diaphragm should seal completely around the diaphragm housing. There is fuel constantly above the diaphragm (in the float bowl). There is fuel inside the bonnet (which provides fuel for your accelerator pump--when the diaphragm is moved upward against the spring pressure, it forces fuel up through the passage, past the check needle and into the throat through the little nozzle). Now, if the diaphragm housing has become warped (and out of 5 I have checked lately, 3 have been warped), then there is a high possibility the constant vacuum will suck gas out of the float bowl and straight into your manifold. It sucks gas right underneath the diaphragm where it fails to seal to the float bowl surface. Obviously this will make your engine run richer. It could obviously also play havoc on adjusting using the idle mixture screw depending on how messed up the surface of the housing is...and I hope this makes sense to you. I've brought back to the top the post where I explain one way to straighten the diaphragm housing. The title of the post was "Carter running rich? A possible surprise for you" or something like that.
In a for sale post of two Carter YF 4145 carburetors, I mentioned they had been rebuilt. I bought 3 of these last spring/early summer. The housing in all 3 was warped and the housing i another YF I purchased was also. This has now become standard procedure for me to check using sandpaper and the little wood piece I made. It only takes a minute to see low/high spots and correcting them is easy.
