753 Fuel Priming

[DaleM 753]

Hi All, I'm still getting bugs out of my "new" 753 (SN 508613765, 1991). After the machine sets a while (greater than couple hours), it will loose its fuel prime / pressure. This is true if engine is cold or if hot. When this occurs, it takes a long crank time to get it to start and sometimes it stalls just after starting. If I simply use the prime bulb to pressurize the system a bit first, it starts perfectly - weather hot or cold. Once started and running, no issues with fuel supply. Why am I losing my fuel prime? Looking at parts diagram, I don't see any check valves in the tank supply line.... Also, some searching saw a similar item that noted a lift pump as the culprit??? Does that make sense? - I'm not familiar with it (yet)..... Thanks, Dale

 

[foton]

there is a check valve in the hand primer bulb ,maybe it has gone bad and leaking back.

 

[reaperman]

Dont overlook the fuel lines. A machine of that era could easily have the original lines still installed. Small cracks can form in bends of the lines allowing air in, thus loosing the prime. New lines are cheap and not to hard to replace. We had an ongoing issue with an excavator at work until a factory mechanic came out and simply replaced the lines. He said they just get bad even tho they still look good. It's not money wasted even if it doesnt work.

 

[DaleM 753]

Good info guys, I'll start with a good inspection of the lines and such. I notice the primer bulb is fairly "stiff" and not soft like a new one would be. As such, the lines are likely old as well. Would the check valve associated with the primer bulb be the only thing preventing the fuel from flowing back the line into the tank when its shut off? Or is the fuel pump design such that it would also not allow for back-flow? Thx

 

[foton]

i suppose that it is possible that the fuel pump diaphram has a pinhole in it letting crankcase air in, but I would also think if that were to happen there would be diesel fuel in the crankcase oil. So as a quicky smell the engine oil to see if it smells like diesel also look at the level to make sure it is not overfull becase of it. People have trouble with the plastic fuel pick up tubes cracking so that is a possibility too.

 

[ward]

From what you describe I would be looking at the lift pump not the primer bulb, however if the primer bulb is stiff replacing it would be a good idea also.
If the line between the primer bulb and tank has a small crack in it, remember these machinse are exposed to a lot of vibration, gravity is always pulling the fuel back in to the tank so the line can remain dry yet leak in air allowing the fuel to drain below the bulb, however the check valve in the bulb would keep fuel above it, in this case I would think you would get a start then die then have to crank to restart or a start then run rough until the air purges situations.

 

[DaleM 753]

Ward, I think you are on to something with your latter comment. When I dont prime it, it will fire after a short cranking, then run for a couple seconds, then die. If I don't manually prime it, I'd have to crank for a while to get it to pick up fuel and fire again. Seems like there is a small amount of fuel available initially, then I get an air gap till next batch of fuels gets drawn in. Sounds like time for new fuel lines and primer. Probably do tank pick-up tube also since its likely old. Thanks All, Dale.

 

[ward]

also make sure the unit is not making oil also know as a bad fuel lift pump that leaks the diesel in to the engine, my self if it was me I would just stick on an electric fuel pump and eliminate the primer bulb and mechanical lift pump, bleeding out the system would be a breeze.

 

[DaleM 753]

Worth thinking about Ward. I did check my oil / level. No signs of fuel going into oil. Level slightly down and no diesel smell. I've not dug into my fuel system yet. Is the lift pump the first component downstream of the primer bulb (Ref # / part #) and prior to filter?

22

6655216

 

[DirtRodeo]

This is the pump that I use on my 37hp Yanmar (sailboat) and on my 743.

Facet FEP304SV Posi-Flo 12 volt Fuel Pump Kit, 1/8 NPT Ports, 1 to 2 psi Maximum Pressure

It works great. Make sure that your hose clamps are correctly sized. If they are too big, they won't seal properly. Oetecker crimp on work best. Good luck!

 

[ward]

The mechanical pump is bolted on the side of the engine and should be the next connection after the primer bulb when you use the primer bulb you push fuel through the mechanical fuel pump, what we always did was remove the bulb and bypass the mechanical pump and leave it in place so we did not have to make a cover plate for the hole in the block, if you have a hard time mounting the electric pump you could remove the mechanical pump and use the gasket to make a outline on a 1/8 to 3/16 thick plate of steel and use the mounting bolt to mount the electrical pump, some pices of equipment came with electric pump and a cover plate for the mechanical pump location, light towers had them right from the factory, some machines have a spare bolt hole tht you can mount a plate of 3/16 to 1/4 in plate to and mount the electrical pump to as you don't want it just hanging lose.

 

[lmcl123]

Possibly the fuel pickup tube in the fuel tank. I had this happen on both a 1997 753 and a 1995 763. They get old and brittle over time. The 753 pick up tube snapped completely off just inside the top of the tank, that was easy to figure out. The 763 cracked about halfway down the pickup tube. When I was running a full tank to half tank it worked fine. When I got the low half tank it started sucking air in the cracked pickup tube. It took me a little longer to figure that one out. The machines were about 20 to 25 years old when this happened to them. The pickup tube isn't terribly hard to replace, as long as you're a contortionist , there's also supposed to be a fuel filter on the bottom of the pickup tube, that can sometimes fall off over time. Also, the plug that the pickup tube runs through when it comes out of the tank also gets brittle and worn and should be replaced.
Good luck