Fuel pump pressure

[Darton]

Need, some help on an 80 h.p. mercury, I have lost fuel pump pressure, started out thinking it was carbs, cleaned them, checked fuel delivery from internal tank to fuel pump, all good, installed new fuel pump diaphragm and check valves, still won't pump. Pulled fuel pump from block and cranked engine over testing for plus-minus pulses from block, have some but doesn't feel like much, I can block it with slight finger pressure, thought it was enough to function pump but it won't, also blew air back into block port , seems clear, wonder if anyone would have some idea where to look next, I'm sure baffled on this, which ain't hard to do. Thanks and ideas are appreciated.
Darton

[vector mike]

Check the diaphrams to make sure there aren't any holes in them.

[Darton]

Vector, already checked diaphragm, matter of fact the old one was good, I thought maybe it was streched and weak but the new one isn't any better, thanks anyway. Darton.

[Raceman]

If the condition of the engine was bad enough that there wasn't sufficient pulse in the block port to operate the pump, I doubt that it would run. That pretty well leaves from the port outward in my opinion, although I've never seen the problem before. To say 80HP covers a lot of territory from the early 60's foward, but assuming it's fairly late model, the basic function should be similar. It'd be interesting to know how and when you determined you'd "lost fuel pump pressure". After a long period of storage? Suddenly, as ran fine one day, wouldn't the next? After some type of other mechanical work was done on the engine? If it was after some kind of service are you certain the valves were re installed properly? How did you determine that the pressure is inadequate? Are you absolutely sure there are no restrictions in the vent side of the fuel system? Depending on the configuration of the pump, are you sure there are no leaks on the pulse side?

Of course the easy fix would be to eliminate the mechanical pump and install an electric pump regulated to about 4psi, but the system is simple enough that it shouldn't be hard to diagnose.

[Darton]

Raceman, here's whats up, this engine started acting up after recent usage, My test checks are: checking fuel delivery from tank to pump for leaks or air intake, checks good, I even substituted a portable tank for testing, then hooked an outlet hose to the pump, primed with squeeze bulb, cranked engine, pump won't even splutter. I've tested compression on all 4 cyls. @ 140 down the line, that should give me enough pulse, Yes I even rechecked my check valves, they are correct, and will open and close with slight mouth air pressure. Diaphragm should work, this has me stumped, this thing should work.
Any more ideas? Please? thanks, Darton

[STVmod]

pull all the plugs out, hook up a fuel psi guage to your fuel supply to the carbs and spin the motor over. it should spin fast enough to build some fuel psi.

take the fuel line off ans spin the motor over, see if any fuel volum is being delivered.

this will eliminate carb problem, to a certain extent.

if you don't have psi, i would start again with the diapharams in the pump. make sure the plates are in the right place and not flipped over...

make sure if it is spring operated that the spring is on the correct side of the diaphram...

how about a defective pump ball??? pickup tube, vent???

i think you said you tried a alt tank, so if you rule out carbs, rule out the tank, it has got to be a problem with the fuel pump.

make sure the vent hole on the fuel pump is not clogged, if it is the diaphram will not work properly.




just some randum thoughts, you have already thought about, but i would start all over and recheck everything again if it were me.

good luck!

[gotmerc]

How old is this engine??