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1996 Force 120, Fuel pump diaphragm

M

Midnight Oil Workshop

New member
Hi, My official factory service manual (and every other parts reference) shows that the fuel pump in my 1996 Force 120 uses 2 round diaphragms that consist of 2 pieces each - one clear plastic backing disc - and one black rubber valve disk - that are sandwiched together and attached to the pump with a clip in the center of each diaphragm. When I disassembled my pump to rebuild it, I found that it only has the clear plastic backing disc - but is missing the black rubber diaphragm itself. (it's been like that for years and the engine ran). My rebuild kit came with both pieces, so I installed the diaphragms as indicated in the manual - with the black rubber diaphragm aganst the pump housing and the clear piece on top of it to hold it down. When I went to start the engine I got no fuel and the engine didn't start. So, I disassembled the pump again and removed the black rubber diaphragm to put it back the way it was and the fuel flowed normally again.

The engine runs okay with only the plastic backer for the diaphragm, but it does stall after idling for a minute and I have to pump the fuel ball to get it going again. No noticable probelms at higher speeds. I have seen youtube videos of people rebuilding this same pump and they also only use the plastic backer for the diaphragm - omitting the black rubber piece. Can anyone shed light on this issue? This pump is powered by engine compression - which is on the low side, but runs just fine otherwise.

Could the slightly low (about 105psi on all cylinders) compression cause my fuel pump to not function with both pieces installed for the diaphragms?
 
Wrong assumption.----The fuel pump runs with the compression in the CRANKCASE .----The 105 PSI compression in the cylinder has nothing to do with the fuel pump !!
 
Did you find something wrong in the fuel pump when you took it apart ?-----Have you test run with another tank and hose ?---Have you inspected the reed valves ?
 
The old rubber gaskets in the fuel pump were pretty stretched out, but otherwise seemed okay - nothing blown out.

At this time, I am running it off of a new 6g portable tank - with a new fuel line and bulb (generic from west marine). Even though the fuel line came with a "mercury" style fuel connector, it does seem like I have to cram it on there to get it to stay on. All connectors are new.

Reed valves were recently replaced with Boyesen # 244 fiberglass reeds. Also just rebuilt (throughly cleaned) the carburators. In general, the engine runs way better than before. Smoother and quieter with no more immediate stalling when returning to idle like before. Starts right up when primed with no need to give it throttle. Only thing is that it stalls after idling for a little while. Squeeze the ball and it starts right up again. I have the timing and carbs fairly well tuned, but I still need to refine it under load.

Big mystery is why I can't install both layers of the fuel pump diphragms. Using only the plastic cover disc seems like it wouldn't make as good a seal - but as soon as I add the black rubber diaphragm part, I get no fuel at all. I've seen other people do the same thing with only the plastic discs - so it appearsto be a 'thing' - but seems like maybe a bandaid fix that isn't addressing the root cause.
 
Pump kits come with multiple parts to fit other engines as well. Cheaper to put all pieces in to one package rather than sort. So black rubber is for different model. So when you install both, you are blocking/preventing pump from lifting fuel.
 
haventaclue said:
Pump kits come with multiple parts to fit other engines as well. Cheaper to put all pieces in to one package rather than sort. So black rubber is for different model. So when you install both, you are blocking/preventing pump from lifting fuel.
Click to expand...
Yes, That was my first thought too. But it doesn't jibe with my official factory service manual which clearly shows both pieces used on the pump for my specific model. It's not like a Seloc manual which shows a bunch of different engine models in the same manual. This is an official factory service manual for this specific model, year and serial number range. It's a really fantastic manual - clear and accurate with no ambiguity. Something still seems wrong. I'll keep poking a stick at it until I figure it out. Maybe start by confirming that the fuel pressure is in the correct range. I already put in new fuel lines. Next I'll bypass the new fuel connector clip by connecting the fuel line diretly to the fuel pump to eliminate a possible constriction. I'll report back with my findings. Thanks!
 
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