Carter YS Carb Mixture Adjustment

I have not had a chance to tear into the YS on my Daughters A1 yet so I am not as familiar with it as I am the YF but it is my understanding that the YS has vacuum operated accelerator pumps somewhat similar to the YF.

I would suspect that one of these diaphragms is leaking enough fuel directly into the intake manifold for the engine to idle without the fuel normally provided by the idle circuit proper. The YF can also manifest this same symptom when its pump diaphragm begins to leak.

Depending on idle / low speed jet size ( based on YF experience ) backing the mixture screw nearly all the way out may or may not produce a noticeable rich stumble.

I would gamble one or both of the pump diaphragm(s) are beginning to get tired and have started leaking. Its the only way to my knowledge fuel can bypass the normal idle circuit and get into the intake manifold.

 

My experience with M38A1 carbs would confirm Brian's thoughts. There is indeed a vacuum activated pump diaphragm, and it can leak when it gets old.

 

I didn't have issues with the pump diaphragm on my YF, but after not having been driven for quite a while, the gas lacquered up enough in the carb to cause the same issues. Pulled it off, torn it down completely and removed any rubber bits and soaked it all in lacquer thinner, ran thin wire through all of the different orifices, put it back together and that solved it. I was able to find the wire at the local craft store in the bead/jewelry making section.

 

the idle adj screw and seat may also need to be replaced as well as the pump springs. they both wear out of spec, and if you figit with that idle screw like I do, it will wear to the point of false readings

 

I'm having the same issue with my 52 M38A1. I have a rebuild kit on order (the last rebuild was 10 years ago and it sat a lot while I found a new motor) so hopefully that will solve at least one of my jeep issues.