I was given a 1983 motorhome on an Econoline chassis last summer. 75k miles, 460 ci. Starts and runs great when recently shut down. However, after sitting for a few hours (or overnight) it almost acts like there is no gas in the carburetor bowl. Automatic choke will be closed - but engine won't start until it has turned over for 20 to30 seconds. With pumping the accelerator, it seems like the system is empty of gas. Is similar to situation when a care runs out of gas --- after gas is put in tank it takes a bit of fuel pump action before gas reaches carburetor. In reading the service records for the old girl, this prolonged startup has been a problem for years. I had a full tune up last summer (including a carburetor rebuild). Nothing seems to help. Is there a check valve somewhere in the fuel system that may be malfunctioning? That really doesn't really make sense either as I assume the entry gas line entry level in the carb is well above the bottom of the carburetor bowl. Suggestions? Thanks.