Ensure the carb is well maintained and set-up. Get the mags serviced. Check the plugs, leads and compressions. Rectify each and every problem.
The C-65 is a very simple tolerant engine that should start first blade through in any weather if everything working right and can be a pig to start if not well looked after. If it needs 'a bit more throttle' to start or the primer leaving out, the engine is probably trying to tell you something.... Ask the previous owners of BTCH