Small Engines (Lawn Mowers, etc.): Fuel Problem, craftsman lawn tractor, crankcase pressure

Question
Hi Marc,
I have a 1976 Craftsman Lawn tractor with a 12hp Tecumseh 4 stoke engine. Im having a problem in getting fuel to the carb. When trying to start the tractor after it has sat unused (usually for 24hrs or more, sometimes less)it loses the ability to pump fuel.It will start but once it uses the fuel in the float bowl it will quit. If I preasurize the system it will start.(I preassurize the system by blowing into the gas tank by mouth)It sometimes will not have any problems after I do this but if it is left at idle it sometimes quits again. Once started if I go to a fast idle or full thottle it will stay running and I have no problem until I try to start again sometime later . It acts as though it loses it prime. I have replaced the fuel line,fuel filter(a number of times), rebuilt the pump,replaced the hose from the pump to the crank case and cleaned the carb. I have checked to assure the float isnt sticking and have no problem with fuel flow to the pump. This engine has 110lbs of compression,burns no oil and runs very well. The oil is changed regularly and is kept at the correct level. The gas cap is not plugged(I have had the same symptoms with it off). Could it be low crankcase vacuum that isnt allowing the pump to work correctly? What is normal vacuum? Im at a loss. I have asked local repair shops for thier help but most dont feel its worth working on because of its age. They are more interested in selling me a different tractor. Any ideas would greatly be appriciated as Im tired of blowing in the gas tank and I cant convince my wife to do it.(ha ha)Seriously it is a good tractor and in great shape other than this annoying problem.
Thank you, Dennis  

Answer
If the crankcase pressure was low enough to stop the fuel pump from working, the engine's compression would be too low to run.  I would recommend you re-check how you assembled the fuel pump.  It sounds as if the diaphram is either pinched or one of the springs is on the wrong side of the diaphram.  Let me know what you find and write back.  Hope this helps.