5.7 Mercruiser stalls out

[Boatbuyingloser]

So I've been having this issue for quite some time, i've checked everything I can think of - new carb, new fuel pump, sparkplugs, plug wires, distributor cap + rotor, checked all the fuel lines, pick up tube screen, fuel filter, anti siphon valve, fuel tank vent, but nothing seems to actually solve the issue.
Any advice on what to check next?
It ran fine for the first ~10 hours this season, until I felt the RPMs drop one time, since then its been getting more frequent as time goes on. Had a similar problem last season but was temporarily "solved" after I cleaned the fuel vent on the carb (may be wrong on what its called, but it was the 1/4in line running from the fuel pump to the carb, NOT the main fuel line)

https://imgur.com/MGgufcR

https://imgur.com/a/WHy3qkp

 

[stresspoint]

i would say fuel pump , however you have changed that , possibly a connection or something the like to do with the fuel pump continuing to run.

 

[Scott Danforth]

Had a similar problem last season but was temporarily "solved" after I cleaned the fuel vent on the carb (may be wrong on what its called, but it was the 1/4in line running from the fuel pump to the carb, NOT the main fuel line)

define what you mean by cleaned

the tygon fuel line is there from your fuel line to the carb in the event your fuel pump fails it will pump the fuel into the carb and not the bilge.

because if it pumps into the bilge, you blow up and die.

if you have fuel in that line, you have a ruptured fuel pump diaphragm

 

[Boatbuyingloser]

define what you mean by cleaned

the tygon fuel line is there from your fuel line to the carb in the event your fuel pump fails it will pump the fuel into the carb and not the bilge.

because if it pumps into the bilge, you blow up and die.

if you have fuel in that line, you have a ruptured fuel pump diaphragm

Well I took the fitting out of the (old) carb and it was completely blocked, so I cleaned it out thinking it was a fuel vent, and after that it ran fine for another ~5-10 hours until it started happening again. I don’t remember ever seeing fuel in the line though.
Could this have something to do with ignition? I’m wondering if the mechanic who worked on it last season hooked up the wires in the wrong order. Do you know if any diagrams I can use to check this?
Also the distributor itself hasn’t been replaced but the coil is an aftermarket Pertronix flamethrower. Maybe that could be part of the issue?

 

[Boatbuyingloser]

i would say fuel pump , however you have changed that , possibly a connection or something the like to do with the fuel pump continuing to run.

That’s what my thought was, but I’ve run through and tested everything in the fuel system I can think of. Im thinking ignition now but I don’t even know where to start with that

 

[Scott Danforth]

Not wiring or it would not come and go

Measure fuel pressure while under load

I don't trust anything pertronix

 

[Boatbuyingloser]

Not wiring or it would not come and go

Measure fuel pressure while under load

I don't trust anything pertronix

I come armed with new information once again
I was out at the boat yesterday and I think I may have found the issue, or at least part of it. The distributor cap and rotor don’t seem like the correct part for my motor, it has a vacuum fitting behind it with nothing connected, also the plug wires seems like they’re excessively long but I didn’t catch them in the video.

https://imgur.com/a/Da7u4dI

I’m also hearing a humming sound at random, any ideas on what this could be or if it’s related?

https://imgur.com/a/TPQ7UGG

 

[Boatbuyingloser]

Not wiring or it would not come and go

Measure fuel pressure while under load

I don't trust anything pertronix

I’ve got my fuel pressure tester, I just need some adapter to fit on my 3/8ths line. Problem is I can’t really test it “under load” without getting stranded on the lake.. once it stalls out, it’s pretty hard to get it started again (most of the time)

 

[Scott Danforth]

bring a paddle, or keep it strapped to the trailer on a deep cement ramp

 

[dubs283]

Cap and rotor appear correct, there is no vacuum line/port on the distributor or cap. It's a vent and is supposed to he there. Ignition wires are long to route properly around the exhaust manifolds

Only issue I see is old style ignition sensor in the distributor. I've seen those act up occasionally

You need to verify if your no start issue is fuel or ignition. Too big a margin for the guessing game, unless your rich with lots of free time

 

[Boatbuyingloser]

Cap and rotor appear correct, there is no vacuum line/port on the distributor or cap. It's a vent and is supposed to he there. Ignition wires are long to route properly around the exhaust manifolds

Only issue I see is old style ignition sensor in the distributor. I've seen those act up occasionally

You need to verify if your no start issue is fuel or ignition. Too big a margin for the guessing game, unless your rich with lots of free time

Ahh I see, thanks for the info. It’s not a no start issue, it runs for about 5 minutes at idle with no load, then stalls out

I’ll clean the ignition sensor today and reset the timing

 

[Scott Danforth]

Ahh I see, thanks for the info. It’s not a no start issue, it runs for about 5 minutes at idle with no load, then stalls out

I’ll clean the ignition sensor today and reset the timing

You mean replace the ignition sensor with the new 3-wire version

 

[Boatbuyingloser]

You mean replace the ignition sensor with the new 3-wire version

Yes, waiting on parts from the marine store, I hope it’s not the ignition coil because that’s on back order till mid September..

 

[Scott Danforth]

Yes, waiting on parts from the marine store, I hope it’s not the ignition coil because that’s on back order till mid September..

its never the coil

nothing special about the coil, you either have a 1.5 ohm coil or a 3 ohm coil

 

[Boatbuyingloser]

its never the coil

nothing special about the coil, you either have a 1.5 ohm coil or a 3 ohm coil

Ok thanks again for the help Scott
I’ve got a couple stupid questions, I’m having a hard time setting the timing. With the thunder bolt iv, do I need to have cylinder 1 at TDC, and is there an easier way to do this with just one person? Turning a wrench and trying to feel air through the spark plug hole is not as easy as I anticipated.. so I’ve given up for now and just left it the way it was, as it was running alright before this fiasco.

Also do you happen to know if the spark plugs for the mercruiser 5.7 are the same as automotive ones? My marine store is closed on the weekends and I wanted to get new plugs in but ran out of time Friday..

Last question, can you point me in the direction of a guide or something on how to tune a carb without a vacuum system? Is it just by feel, or do I need to pull the plugs out every couple minutes till they aren’t excessively sooty or ash white?

 

[Scott Danforth]

no, marine plugs are not same as auto. marine plugs are nickle plated. spark arrestor cover has the AC delco numbers on it.

if running NGKs The BR6FS is for non-votec heads, the BPR6EFS is for votec heads

if running AC delcos, AC-MR43T for non-vortec heads AC - MR43LTS for vortec heads

use a timing light to set timing to 8 degrees BTDC

regarding carb. use an unlit propane torch and a . simply wave the unlit torch over the carb when running at idle. if the RPM's go up, you are lean, if the RPMS go down, you are rich. if the RPMs do not change, your just right.

 

[nola mike]

Ok thanks again for the help Scott
I’ve got a couple stupid questions, I’m having a hard time setting the timing. With the thunder bolt iv, do I need to have cylinder 1 at TDC, and is there an easier way to do this with just one person? Turning a wrench and trying to feel air through the spark plug hole is not as easy as I anticipated.. so I’ve given up for now and just left it the way it was, as it was running alright before this fiasco.
?

it doesn't need to be at tdc to time it. it does need a timing light

no, marine plugs are not same as auto. marine plugs are nickle plated. spark arrestor cover has the AC delco numbers on it.

if running NGKs The BR6FS is for non-votec heads, the BPR6EFS is for votec heads

I think only the delcos are plated, right? you can get plugs at the auto part store.

Last question, can you point me in the direction of a guide or something on how to tune a carb without a vacuum system? Is it just by feel, or do I need to pull the plugs out every couple minutes till they aren’t excessively sooty or ash white?

you aren't really tuning the carb. you're setting the idle mixture. many ways to do it, easiest (though not best) way is turn screw in until it stumbles, turn out until it stumbles, and then put it between those 2 positions