We use a couple of 4 stroke 40's at work. One of them has become hard to crank after it warms up. It will start just fine cold, idles and runs great at speed, and will run all day long as long as you don't shut it down. After it is run a while and warms up it seems to develop vapor lock or something when we shut it down. We can remove the fuel filter on the engine and drain the fuel out, then reinstall and it will usually fire right up and run just fine...until we shut it down again. We have been leaving it running all day long to avoid problems with cranking again. This is a very busy time for us and we can't spare it to go to the shop right now, if possible to avoid it. There is also a water separator filter inline, which is changed regularly. Any ideas?