If seen small cracks in fuel hose from tank supply line to fuel pump suck air during cranking (slow pump actuation), but runs fine when started, and never leak a drop because its not pressurized. The hose by the tank can do the same thing. Soooo, after sitting for several days, a small amount evaporates from float bowls, needs more fuel from tank to refill, but its real slow in getting it.
If this is a Thermoquad, I doubt its a internal issue, it has 2 float bowls, but that sintered bronze filter, (or maybe replaced with a paper filter one) Griff mentioned is a prime suspect. Otherwise i would suspect and check fuel delivery, (volume and pressure).
You need 3 things , well really 4, spark, fuel, compression, and get them all together at right time. We assume the timing and compression are O. K. because it runs fine once it starts so that leaves spark and fuel, If you have a nice spark at spark plug to wire, that eliminates that. Got a fuel issue.
