You could take your checking one step further with the filter off. Hold the choke open so you can see down the carb throat and operate the throttle linkage a couple of full strokes---you should see a good stream of gas from the accelerator pump discharge nozzle. You are giving it a few pumps before starting, aren't you? If there's no discharge that would be one more bit of evidence that fuel is not staying in the float chamber (without taking the carb top off). This isn't a fix or an elegant solution -- but if you kept an oil squirt can loaded with gas handy were the car is parked for days at a time you could give a squirt down the throat to get it fired up without having to do a lot of cranking. Maybe the float level is set too low, with just enough gas evaporating to not be able to feed the accelerator pump? -- Dave