I have a CH and a Vanguard that don't like to fire off after siting for a while. I use an empty gear oil bottle to give them a shot of gas down the throttle bore and they seem to fire right off. That said once running I have no more issues. You said you are having problems when under load so that is why I asked about what you did to the fuel system. Just shooting in the dark, did you ever mess with the governor? Is it properly adjusted? Even if you have not touched it you may want to check it.
Edit: Do you have the correct type of fuel filter on it?