1972 Johnson 9.5 Fuel Pump - insufficient compression to drive ?

[dcremer]

Hi,
I have one of the above engines, and the fuel pump is not pumping. I rebuilt the pump and all seems OK, however I noticed that the tube leading into the crank case which is used to drive the fuel pump does not generate much pressure (or suction) when cranking the engine by hand. It is a fraction of the pressure of a spark-plug hole on the engine and I'm thinking this might be the problem.

The engine runs well (when you keep pumping the fuel line bulb to fill the carburettor), and compression on both cylinders is 60psi.

The cylinder head gasket is new, but I was wondering whether I might have a leak in one of the other gaskets, or perhaps a problem with a leaf valve.

Any suggestions much appreciated !
Regards

David

[Will Bark]

Sounds like a fuel restriction between the tank and the bulb; could be air leak at tank connection or a tank vent thats not open. Check for any air leak at the line connection to the motor. What shape are the o-rings in the connector in? My 15hp also does not have near the pressure that a piston generates at the spark plug hole.

[F_R]

dcremer, that is an interesting theory, but just plain impossible. If it runs, it has enough crankcase pressure to operate the fuel pump. The running crankcase pressure on the downstroke is less than 5psi, less on the upstroke. That is why you can't detect it by slapping your finger over the hole. But if it were so low that the pump wouldn't work, it would be way too low to scavenge the cylinders and it would not run.

So.o.o.o.o.o.o., the answer is check for air leaks and if there are none, and if there is no restrictions between the tank and pump (filter??), then your fuel pump is no good. It's as simple as that.

[kenmyfam]

I'm with FR on this one.
disconnect the fuel out side of the pump and attach a temporary line to a container.Crank the motor and see what (if anything) comes out. Quick and easy check of the pump.

[BoggyBoy1995]

I had a 1972 9.5 Johnson that was doing the same thing as yours. My problem was not the actual water pump. What i did was i bought 3/8 plastic tubing and clamped it to the copper tubing running to the block. Then on the other side of the hose I put a hose fitting on there and pumped water through the block. Works fine now. Because the impeller in the water pump spins by the driveshaft running through it. Hope it helps.

[JB]

He is concerned aboout his fuel pump, not water pump, BoggyBoy.

[dcremer]

Thanks guys - good to know that the low pressure at the pump inlet is normal. I have replaced all hoses from tank to carb, so it sounds like I need to look at the fuel pump components more closely or replace the pump.
Much appreciated.

[F_R]

Is this the pump you have? If you are messing with it without thoroughly reading and understanding these instructions, you are lost before you even begin.

Are you aware that Johnson said those pumps are unrepairable? Yeah, I know, people are doing it anyway.

http://www.sschapterpsa.com/rambling...g_fuelpump.htm

[dcremer]

That is indeed the pump, and I did use those excellent instructions. I took it apart again (this time properly). The problem turned out to be the bottom membrane (that takes the pressure feed from the crankcase). The engine had previously had a blown head gasket, and the water had got into the fuel pump area. A lot of scale had built up between the membrane and the metal pump base, and this was preventing an airtight seal. I scrubbed the membrane clean, and used a whetstone to flatten and polish the pump base. This gave it a great seal, and once reassembled it worked fine.
Looking forward to planing on my rib tomorrow.
Cheers.

David