Sorry, there are quite a few schematics floating around for the two stroke. I assume in yours they did not have any screen stopper resistor. This would normally connect between B+ and the screen on pin 4.
The grid leak would be the one on yours connecting pin terminal 6 to ground. The one from pin 5 to 6 is your grid stopper.
So, what the grid leak resistor does is reference the control grid to ground. If it is too high of a resistance, or if it comes loose from ground, then you start to build up a negative voltage potential between your coupling cap and the grid. This will cause your tube to cut off. And if there is a loose connection, then it can introduce noise. As for the screen stopper, this is mainly to protect the screen from drawing too much current, but it can also introduce noise. And if it disconnects from B+ then it can also drag the tube into cutoff.