Tecumseh 40 petrol rotary mower?

I had the same problem some years ago with a Tecumseh motor, and found that it was a manufacturing problem. The model was notoriously hard to start.
What I found was that the breather hole for the carburettor bowl was under the spindle of the choke butterfly. The spindle had a flat machined on it, to allow air to reach the breather, but when the lever was turned to apply the choke, the airway was blocked and no petrol was sucked up through the jet.
I drilled a small hole in the top of the bowl, and it fixed the problem (I wasn't using the motor in a dusty environment).
If the source of your problem is the same, you may be able to extend the flat around a bit further and still retain enough strength in the spindle to operate the choke properly. Hope it helps.....

Munro
May 2011