I have a small tiller I bought second had - 4 stroke. It ran well the
first year and then it developed a small problem. I can start it (fresh
gas, clean oil), but as soon as I pull on the throttle it stalls. when
it stalls I get a little blow back out of the throat of the carburetor.
Also if I take the oil cap off while it is running there is blow back
coming out of there as well.

I am not afraid to rip it down, as I can do minor engine repairs. I was
just hoping someone might be able to point me in the right direction.

I am thinking a gasket...

It's not worth taking in for repair as the labour would cost more then
the thing is worth.