I have a Cummins Onan generator with a Kubota D722 in it. It will start and run for a few minutes then stale. It will always restart but it's hard starting like it runs out of fuel. Has a code 36 which indicates fuel related problem. Fuel pressure was low so I replaced the filter and the 12 volt pump. Same thing is still happening. I checked for leaks in the fuel system and also pulled down the fuel tank to check the pickup. All good. One weird think I notice is if you crimp the fuel return line the engine will keep its fuel pressure and not stall. I had it running for half hour yesterday, but as soon as I take the vise grips offthe return line it stalls in less then a minute. Is there some type of check valve in the injection pump that is stuck open maybe and not letting the pump build up pressure???. Any help would be appreciated.