Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
The Carter YF was running rich, and there was a pretty bad stumble when opening the throttle. Also, after sitting a few days, it would take a long time cranking to get the engine to start. When I rebuilt the carb I noticed a tiny tear in the accelerator pump diaphragm. I figured it was at least part of the cause of the stumble, and thought no more about it. Then I did some research on the YF and how it works.
A couple of days ago I was thinking about all that, and suddenly it became clear; The pump doesn't squirt when it's pushed down, it squirts on the upstroke. There's a little hole at the side of the diaphragm that lets gas flow from between the diaphragm and its cover, down into the base of the carb and from there over and up to the discharge nozzle.
The metering rod is attached at the top of the pump stem, so that when the pump comes up, as the throttle is opened, the metering rod is lifted out of the jet causing a richer mixture.
Something I didn't know is that there's a passage from below the throttle plate into the area under the pump diaphragm. During light cruise manifold vacuum pulls the diaphragm down, which lowers the metering rod and also increases the capacity of the pump when the throttle is opened.
So that little hole in the diaphragm was causing problems in at least five ways:
It was decreasing the pump volume by not having it pulled down as far as it should.
It was decreasing the pump volume by letting gas squirt through it when the pump operated.
It caused a rich condition by not pulling the diaphragm down as far as it should, lowering the metering rod.
It added further to the rich condition by letting gas pass straight through it into the intake manifold.
It was letting gas seep out when the engine wasn't running.
The engine is running MUCH better now. It's very responsive to throttle changes, and I expect that the mileage will increase, too. So far I've only run about a quarter tank through it. I'll know more soon. According to the air/fuel meter, the mixture is about right for all steady state conditions.
But it's very plain to see the basic problem with carburetors; any significant change in throttle position causes wild swings in the ratio before it settles down in the new position. Functionally it's not too bad, but the advantage of fuel injection for economy and emissions is now quite obvious.