The clutch on my starter motor is slipping when I snap it over center. I adjusted it once and worked for a short time and now is slipping again, unless I back it off from over center . Other times it will come out gear.. 
I have to use 3/4 choke on the starter motor. to keep it running. Being as I use the tractor very rarely just for dozing, I use 91 non ethanol or aviation fuel which doesn't degrade.  

all petrol degrades. its diesel fuel that does not.
might need new clutch frictions but try adjusting it first.
and having the carb sonic bathed and fitting a kit to it wouldnt go astray. it can also be wear around the throttle and choke butterfly shafts thus creating a vacuum leak and a lean condition. you might be able to cheat by sliding a oring over the ends to create a bit of a seal. the other way is have it re bored in a mill and new bushes fitted.

One of my carbs had a significant vacuum leak around the outside of the packing in the carb body (rather than between the shaft and the packer or seal). Not sure how common this is, but I was able to fix up with some retaining compound nd it no longer needs choke.

 

Paul.