Let me back up a bit. My Exmark is a year 2000 machine that I got really cheap with the knowledge that it may need an engine. It only had about 200 hours on it, which I believe based on the condition of the machine. When I got into the engine, it was pumping oil out the top bearing seal. Checking with a dial indicator showed that the crank had about .060" of play (it should be about .004"). I think what happened is that at some point the engine was run low on oil (which makes sense since it was missing the "O" ring under the oil dip stick and two of the plugs on the bottom of the block had cuts in the "O" rings). The upper bearing was the first to go dry, got hot and wore open, and ruined the top oil seal, bearing and crank journal. So I completely disassembled the engine. The bores and pistons were good, as was the block. The crank was worn down about .015" on the upper journal and the upper bearing was worn open about .050". I checked every other part and all measured within specs. The engine was good with the exception of the upper crank bearing and worn crank journal. I have a machining background, so I made a new bearing about .025 undersize on the I.D., pressed it in and then turned the upper journal diameter of the crank to get .003" clearance to the bearing. This repair has worked out good for over 200 hours. I have removed the shroud and checked the end play of the crank with a dial indicator several times, and it has not changed since the rebuild.
Since this machine was so old, bit by bit, I have been replacing anything that might fail due to age. I replaced the plugs at the time of the rebuild (several years ago). I have since replaced all of the fuel lines, fuel filter and pump, and all vacuum lines on the engine. I do not think that it has overheated at any point since I have regularly removed the shroud to check the end play of the crank and would clean the fins etc. at the same time. It really runs great, once it is running.
If I run one of the fuel tanks empty, and switch over to the other, it cranks about 15 seconds or so to get fuel into the engine. Does this seem normal? It is about the same time it takes to get it going on a cold start if it has not run for a day or so. It makes Berts idea about the fuel lines draining back into the tank sound possible. When it is time to start this machine up for the season, I will have to disconnect the fuel lines going back from the engine to see if they have air in them. Thanks again to Bert and Darryl for your input. Don