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1972 Mercury 650 65 hp 3 cyl thunderbolt fuel pump issue. A couple questions before proceeding.

S

saeeur

New member
Hi again all,

My goal with this post is mainly to get a better understanding of how these old fuel pumps work.

I believe I have issues in the fuel pump on my 650.
Compression is 150. It starts, runs for ~15 sec mid-idle, slows down, sputters, dies.
Rinse repeat, I have to pump the primer bulb a whole ton til' it gets hard. The carbs are clearly emptied out.

It's not the gas tank; I tried submerging the entire fuel intake in the mixture and nothing changed.
It is clear that there is not even the tiniest bit of suction on the intake hose (even if there was a hole somewhere, there would be some suction if the pump was working at all).
Pulled off the fuel pump, diaphragm has no visible holes (will replace later anyway), but didn't take it apart yet (hoses are clamped on with a very hard to remove metal ring...)
I see water coming out of the back when it runs, so looks like water is pumped properly.
What confuses me is that it seems fuel can clearly run through the system when I pump the bulb, so there is no complete blockage.

Am I on the right track here? Is this clearly a pump issue? Could something else cause there to be no suction?
It looks like there is some diaphragm on the bottom piston and it uses the piston compression to drive the pump. Could the diaphragm just be worn out even if there are no visible holes in it? Could internal mechanism in the pump cause it to do nothing when running even if there is no blockage stopping me from manually pumping fuel?

Lastly just wanted to understand, it looks like something is still pumping water through, and it's clearly tied to the same mechanism. I assume the fuel pump also provides additional pressure to the cooling system in tandem with the impeller.
 
What confuses me the most is that it looks like water from the cooling system is what's on the other side of the diaphragm. Not the actual fuel.
 
Does it have the water cooled fuel pump? Mercury dabbled with running the tell tale cooling water outlet hose through a "jacket" in the fuel pump to eliminate fuel vapor lock bubbles from forming in the pump after a hard run shutdown when the engine temp rose. I had a 402 with that set up. Maybe corrosion has breached that cooling chamber in the fuel pump and now is introducing cooling water into the fuel stream.
 
Water pump circulates water through the fuel pump.----Fuel pump operates with CRANKCASE compression PRESSURE.----Sounds like you need to replace check valves in the fuel pump.-----Or there is an issue with crankcase compression.
 
racerone said:
Water pump circulates water through the fuel pump.----Fuel pump operates with CRANKCASE compression PRESSURE.----Sounds like you need to replace check valves in the fuel pump.-----Or there is an issue with crankcase compression.
Click to expand...
Thanks, I will look into replacing the check valves. Does checking the compression from the spark plug holes not rule out bad crankcase compression?
Seems like the fuel pump must also be helping circulate water through the cooling system?
 
fitz73222 said:
Does it have the water cooled fuel pump? Mercury dabbled with running the tell tale cooling water outlet hose through a "jacket" in the fuel pump to eliminate fuel vapor lock bubbles from forming in the pump after a hard run shutdown when the engine temp rose. I had a 402 with that set up. Maybe corrosion has breached that cooling chamber in the fuel pump and now is introducing cooling water into the fuel stream.
Click to expand...
seems unlikely since it runs as long as i pump fuel manually, but man that would suck hard. The cooling system does run through the fuel pump and had a buttload of sand in it. No corrosion on the water side far as I can tell but I suppose it's time to stop beating around the bush and just crack the fuel pump open.
 
Sorry----You can have ZERO crankcase compression and still show excellent cylinder compression.-----It is one of those concepts that confuses a 2 stroke novice !
 
Thanks!

So as I mentioned before (and in my confusion thought it was the part water runs through) there was a ton of debris in the pump itself. I thought it was sand but in hindsight it was the color of rust. I tested the valves with an ungodly method and I believe they're both botched in unique ways. One is very hard and inconsistent to blow through, harder to suck through, and the other lets air flow freely in/out. Neither behavior seems right for a valve. I think the tiny bits of debris clobbered the valves. They might work fine with just a cleaning but I'll replace them anyway since they're cheap.

It would make sense that maybe manually pumping fuel I can overpower it but the small repeat pumps from the crankcase do not produce enough pressure to overpower the broken valve.

I think what happened here is the gas tank that came with the boat is old as hell, and there is probably some rust sediment at the bottom that got sucked into the fuel line over the 8 times I took the boat out. Finally the valves gave up.
Another lesson learned... don't trust the 60 year old gas tank.
 
racerone said:
These valves are check valves.----If one is stuck open the pump does not work.---See post #4
Click to expand...
Yup, thank you. I replaced these and it works. Sadly I think I best clean the carbs out now because as it turns out there was a TON of sediment sitting in the old metal gas tank.
 
On the tops of the carbs there should be filter screens to trap sediment if they're plugged they hurt flow.
850carbs.jpg
remove them and check/clean.
 
Awesome glad to hear it, thank you so much. I'll do this right away and I'm definitely adding a filter to the line before the pump...
 
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