Looking for experienced thoughts. This engine is in an excavator. Machine sat for about 2 weeks, temp was 90F. Fired machine and it lit as quickly as normal but immediately was stumbling. I let it idle like that for about 15sec, then turned up the throttle a touch and it cleaned up and not one issue after that. It's currently on a job and cold starts are fine. However, this concerns me because something was certainly up and I need that machine reliable. It never starts up like that! There is a lift pump/filter housing about 5ft from the tank. Fuel is pulled from tank, then pressurized to go through the filters and pushed to the injection pump. There is a small pressure hose on the filter housing that is wet with fuel, but not really active. I first thought it was on the vacuum side, thinking that is likely pulling a little air, but believe it is pressurized. However, I guess possible it allowed a little air in? Not entirely sure how, but.... I guess I am trying to pin this little issue down because stuff like this will bug me and probably pop back up at the worst possible time.
From your description, the problem does seem fuel-related. Fix the known issues (fuel leak) first.
Your 3126b doesn't actually have an injection pump, just a governor. If you want to go after it, change out your fuel filters and without pre filling them, see if you can get it to prime and start. Usually if theres a problem with the fuel supply, either restriction or sucking air, thats when it will be most pronounced. Be prepared though, those engines will run fine forever with a fuel problem if left alone, and will never start again until its addressed once you replace the filters. There's also 3(i think, it's been a while) plastic check valves in the fuel system around the mechanical transfer pump on the governor, that break up over time. Or you could just leave it alone until the job is done, and deal with it when you have time. If it happens once its an anomaly, twice and you have a problem
Only thing I recall on that poor starting event is I had recently drained 5gal of fuel from the tank for another project. I fail to see how that could relate other than possibly I didn't take the tank cap off in time and it pulled vacuum on the filter unit. This machine has an electric lift pump and I put a new fuel filter and separator on it about 60hrs ago. I looked and looked for a priming procedure but eventually just flipped the key (turn on pump) and cracked a couple lines to purge. Since that poor starting event, it hasn't missed a beat!
Double check the obvious easy to get at. Fuel filters tight. Fix that minor leak. Check fittings that you cracked for tightness fuel breather clean and functional. Had a truck die on the road once because fuel filters were too loose. Then theres always the two leavers rule. In life you have two leavers, Leaver A. Or leaver B. If its working fine, Leaver B.
The other thing that would cause the symptoms you have is the injector seats in the head starting to leak. This will allow some air into the head fuel passages around the injector and usually show up as either hard starting or rough running after startup. We would need a serial number to see which style seat and sleeves you have.
Yup, this was something also on my mind but I cringe to think of having to deal with an injector issue on that engine. Machine has sat for nearly a week now so I will fire it today and grab numbers.
Most 3126B engines have a checkvalve/fuel pressure regulator on the return port on the back of the head, very common for the oring seal on the plunger in that to go bad and that will manifest itself as extended cranking or stumbling for the first moments of running so check that first.