Heat percolation does not seem to be the issue if the engine is having problesm when warming up....??? Another possibililty: It may be choking too rich in the warmer weather. Does it have the 'heatwell' type of choke control (not electric)? (This is where the small rod to the choke plate goes to a metal 'lump' bolted to the manifold. ) If so, remove the 2 bolts and pull the choke spring assy out of the heat well and see if it has a small nut on the side with a round plate with several small marks, with L (Lean) at one end and R (Rich) at the other end of the marks. Loosen the nut and move the pointer so it is closer to L to lean out your choke for summer operation, and tighten the nut and re-intstall. Reverse in late fall to enrich the choke.