I take it you haven't touched the carb settings at all?

The needles are there to be adjusted on, there are so many things that affect the fuel delivery that you of course have to optimise the carburetor settings for every installation, prop choice and fuel etc.

By using an extra clunk you reduce the pressure at the fuel pickup when the tank is full (compared to a regular set-up). But the advantage is that the pressure is now independent on the fuel level, as opposed to the regular installation where the pressure drops as the level drops in the tank.
So put in a new glow plug and tune the carb with the second clunk installed, it is as easy as that. For a description of the carburator you can have a look here; http://manuals.hobbico.com/osm/27481000-manual.pdf