If the throttle positioner screw is turned in too far; the idle mixture screw will make very little change from all the way in to removed and in your pocket.
Two and one half turns is wide open. Anything past two and one half turns should effect no change.
Suggest:
Start engine and warm to normal temperature
Turn idle mixture screw in until thumb-tight
Back out 1 turn
Adjust idle speed with throttle positioner screw
There are other possibilities.
Jon.
Jon -
This seems to be my issue, the throttle position screw is all the way out and it won't fall lower than 500 RPM. I can press down on the throttle lever to get it to fall below 500 but it is taking a bit more force than I am comfortable with. So, I have a problem somewhere with the internals causing the throttle butterfly not to close fully. Probably some adjustment I either overlooked or performed incorrectly. I'm going to take it apart again and try to see what the problem is.
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.
Jon G.-
Since I'll have it apart again, I'm going to check the diaphragm housing for flat like I should have done when I had it apart to put the part in that you sent me (which worked wonderfully by the way!)
