I'll swag a couple things together are causing your problems.
1) blown head gasket. A compression and/or leak down test will verify the innards.
2) Carb float not adjusted right. I know you said factory rebuilt carb but that doesn't always translate into a carb thats 100% ready to go. What carb do you have?
Sometimes it's all too easy to misadjust the valves after a Head R&R. Triple check your adjustments to make sure they are fully closing and not being held open. Also possible that whomever rebuilt the head didn't put those little seals on the valve stems. Without them you could get alittle too much oil into the cyl when running.
Good luck
