if its running rich, I'd check the float level. It may be to high, allowing too much gas into the carb, causing the bike to run too rich. Also what mix ratio are you running. Too heavy will cause plugs to foul. I have a 74 CR250M, that I've owned it since it was new, and it rarely fouls plugs. As far as I know (which is very limited), the 73-74 CR250M was not known for fouling plugs.
To get the bike running, 1st put in a new plug. Since you stated the bike seems to be flooding, I'd work with the float height. I don't think the problem is the jet settings. Kick start the bike several times. Now check the plug to see if it is wet. If it is, I'd say the float height is too high, allowing too much gas into the carb. Adjust the float height slightly lowering the level, small adjustments are all that is needed. Keep working with the float height checking to see if the plug is wet with gas. This is a slow process.
Good Luck