I also have the 240 system on my 122 and have not had the problem you cite,
although I didn't hook up the diode system either. Can you give a little more
concise description of your symptoms? Does it crank appropriately fast?
If not your battery or charging system may be weak, or your starter may need
brushes, etc. How fast it cranks is a pretty good indicator of how strong
your battery is, and also the faster it cranks, the less current it draws
so the more is available for your ignition. If the starter is good it can
carry a LOT heavier a load than your B18 or B20, since the B30 also uses
basically the same starter.

If the start is jerky, with the engine kicking back against the starter,
it is a sign that your ignition is too far advanced, and can be easily cured
by a minor adjustment of ignition timing.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma