It was used in salt water for part of its life. It had fairly new risers on it but I guess the manifolds could still be leaking. The thing that throws me is why only when warmed up?

Update!!! After new plugs,wires,cap,rotor and new coil didnt make a difference I pulled the risers to find no problems there either. Yeah guess I should have rebuilt the carb first lol. Chem dipped the carb housing and other metal parts and got a master rebuild kit. Running tip top again. Oh well at least It should be good for awhile now and I know all the ignition is new.