To clean the venturi:

Use the correct wrench slot to loosen the venturi top. Carefully unscrew it and remove it. Note the groove where the needle assembly fits...there are three very small holes placed 120 degrees apart. Check that the holes are not clogged (I need a magnifying glass for this). If any are clogged, use a small needle VERY CAREFULLY to unplug it.

Next, look at the needle assembly and make sure the needle hole is not clogged with congealed castor (or dirt, etc.).

Reassemble, being careful to not cross thread nor over-tighten the venturi on the nylon seat.

For anyone not familiar with a "sprinkler" venturi, the needle adjusts the fuel flow into the groove and the sprinkler holes distribute the fuel around the perimeter of the venturi. There is no spray bar in the venturi.

Plan B...If you don't want to disassemble the venturi, squirt some fuel in the fuel inlet with engine upside down and crankshaft rotated to the closed position. This should open it up unless it is clogged with dirt.

Once it is running, if it suddenly goes richer...it may have unclogged another sprinkler hole. [8D]

George