Scissors-like abnormal noise from 5.7

[Lou C]

No idea of hours I’m third owner- (for sure the one adding the least hours to it)
looking back on my previous posts I realize I’ve done nothing but chasing various issues last few seasons
I did a compression test last year which to my knowledge showed the engine to be in ok condition 175 lowest- 190 highest.
The blue smoke thing has been seen last few seasons.. usually as the boat’s been sitting for awhile.. so the valve guide seals sounds to be a likely cause!

Along with the replacement of exhaust system last year also the carb was rebuilt by the Merc shop ( they replaced impeller as well since there was this overheating of elbow (though I doubt it was the culprit as It was only one season on it / very few hours)

So if engine dying on low rpm is related to carb that would mean their expert on carbs did a substandard job ?!

As there still is an intermittent issue with fuel delivery I cannot avoid suspecting the mech. fuelpump - Yesterday before pulling the belt, Engine started without any hesitations - idling fine a couple minutes only to die..
I need to put a clear hose in the fuelline watch it as a hawk

Another funny thing is that the buzzer/alarm doesn’t sound even though key is in run position ..
Oil pressure switch was replaced last season which then seemed to solve that issue but obviously not..
couple of days ago I went through the harness under the helm - found the crimped threeway splicing of the purple wire -soldered it as an extra measure and got the alarm to go off lounder than ever..
until Yesterday when I again find it to be silent on Key on.. Oh I have tried to reseat and clean the pins in the 10 pin multi contact .. what’s next?

If it's stalling at idle, it's best to rule out other causes before condemning a carb.
Start with ignition, are the cap and rotor in good shape, these can cause problems due to age and corrosion
Same with plug wires they can be checked for resistance
Spark plugs, for wear and fouling
Once your ignition system is proven good
Move on to the boat's fuel system, fuel tank vent, anti siphon valve and pick up tube the last 2 especially on older boats can cause fuel restrictions that will limit power and max rpm
If they are good, then test the fuel pump (pressure, volume, there is also a vacuum test that can be done). This is more work because you have rig up fittings and hoses to connect a pressure gauge due to most OEMs using a steel fuel line from the pump to the carb.
After all that, then suspect the carb!

 

[IslandExplorer]

Good points! There are definitely a whole lot of simple things to rule out 1st like Lou said. Verify healthy blue ignition sparks all around.

It is definitely very common to have crap in your fuel system. No matter how careful you are it just gets funked up over time. Doesnt help that gas quality is such crap these days. So I suspect the fuel delivery flow rate also.
With restrictions often after sitting they'll start fine but then die later. This is the debris clogging and uncloging the last few pathways thru your filter, anti siphon valve, internally swelled fuel line, etc. Look thoroughly. Ive found crazy things like upside down installed floats (outboard I bought, 2 of 3 floats upside down lmao, actually ran but starved for fuel under any strain), as well as super simple stupid things like water intrusion through cap, etc., etc. Just try not to assume anything and systematically / painstakingly go through it step by step.

 

[IslandExplorer]

Did you try wiggling key to see if it effects the buzzer? If it does that thing is toast and just waiting to leave you stranded! I had a terribly intermittent ignition key switch once. It gas lighted me into thinking all kinds of other stuff was wrong. Replaced the dumb switch and it was completely reliable suddenly.

 

[Happygolucky]

Its not the mechanical fuel pump. they either work, or they dont - its pretty much that simple. do your troubleshooting vs firing the parts cannon



my guess is you have crud in your fuel system, which is now in the carb or fuel pump or anti-siphon valve, or pickup tube strainer if its dying at idle. that has nothing to do with the guy that rebuilt your carb, and more to do with the guy maintaining the boat

regarding electrical, remember, these boats were slapped together to make it thru the warranty period. look for bad connections starting at the battery, and work your way thru every connection to the bow light. specifically look at the grounds.

back to your noise. when was the last time the drive was pulled and the u-joints and gimbal and alignment checked. remember, this is an every 50 hours, or yearly thing

Totally agree re troubleshooting rather than fireing any cannon!
Ok so not the pump then most likely, Though it seemed as prime suspect as I ’ve had problems with engine not fireing up - solved by putting the primer bulb in the loop and manually pumping gas.
If there were some airleak due to poorly secured hoseclamps I would assume you’d see some traces of fuel leaking out ?! So what else could cause this lack of fuel in carb?

But on the other hand, wouldn’t a fuel shortage get the engine to stumble /Surge before dying ??rather than as in this case, just die as if key was turned off..
I last pulled the drive end of season 2024, u-joints and gimbal bearing found to be in good shape.. however no tool to check alignment ..
When I recently replaced fuelfilter I found the gasoline Ok with no sign of water however did see some particles floating around. Possible blocking the idle circuit in the carb..?!

Re electrical issue,
If the purple wire via the multicontact that supplies 12v to coil somehow falls out leading to engine dying then adding an extra 12v wire to the coil directly from battery would keep engine running .. right?!

 

[Scott Danforth]

Totally agree re troubleshooting rather than fireing any cannon!
Ok so not the pump then most likely, Though it seemed as prime suspect as I ’ve had problems with engine not fireing up - solved by putting the primer bulb in the loop and manually pumping gas.
If there were some airleak due to poorly secured hoseclamps I would assume you’d see some traces of fuel leaking out ?! So what else could cause this lack of fuel in carb?

But on the other hand, wouldn’t a fuel shortage get the engine to stumble /Surge before dying ??rather than as in this case, just die as if key was turned off..
I last pulled the drive end of season 2024, u-joints and gimbal bearing found to be in good shape.. however no tool to check alignment ..
When I recently replaced fuelfilter I found the gasoline Ok with no sign of water however did see some particles floating around. Possible blocking the idle circuit in the carb..?!

Re electrical issue,
If the purple wire via the multicontact that supplies 12v to coil somehow falls out leading to engine dying then adding an extra 12v wire to the coil directly from battery would keep engine running .. right?!

the primer bulb is illegal on an I/O. thats how you meet st peter

your issue there is most likely the anti siphon valve

no, air leaks do not necessarily mean fuel leaks

your dying is most likely ignition related to a bad connection, or if you have the original 2-wire distributor trigger

anything floating in the fuel filter may make it to the carb. hope you didnt re-use the filter. thats a guaranteed clogged carb

if your ignition (purple wire) is dropping out. there is your issue. most likely a bad connection.

 

[Lou C]

the anti-siphon valve is screwed into the top of the fuel tank pick up tube, there is a spring loaded ball inside that will shut off fuel flow if the fuel line from the tank to the pump/filter leaks in the bilge. The ball can corrode or get stuck from crud getting caught in it. Be careful if you are going to remove both the valve and the pick up tube. Some pick up tubes have a screen on the end of it, mine did not.
Pick up tube is the aluminum tube in the pic below, anti siphon valve is the brass colored fitting next to it. Excuse the dirty old gas tank. It is original (1988).

 

[Happygolucky]

Good points! There are definitely a whole lot of simple things to rule out 1st like Lou said. Verify healthy blue ignition sparks all around.

It is definitely very common to have crap in your fuel system. No matter how careful you are it just gets funked up over time. Doesnt help that gas quality is such crap these days. So I suspect the fuel delivery flow rate also.
With restrictions often after sitting they'll start fine but then die later. This is the debris clogging and uncloging the last few pathways thru your filter, anti siphon valve, internally swelled fuel line, etc. Look thoroughly. Ive found crazy things like upside down installed floats (outboard I bought, 2 of 3 floats upside down lmao, actually ran but starved for fuel under any strain), as well as super simple stupid things like water intrusion through cap, etc., etc. Just try not to assume anything and systematically / painstakingly go through it step by step.

plugs , Cap & rotor replaced end of season 24, plugwires + coil replaced last season
Carb rebuilt end of last season, but of course if there’s some debris in the fuel system the fine passages in the carb might be clogged on the very first run back home from the shop..

 

[Happygolucky]

the primer bulb is illegal on an I/O. thats how you meet st peter

your issue there is most likely the anti siphon valve

no, air leaks do not necessarily mean fuel leaks

your dying is most likely ignition related to a bad connection, or if you have the original 2-wire distributor trigger

anything floating in the fuel filter may make it to the carb. hope you didnt re-use the filter. thats a guaranteed clogged carb

if your ignition (purple wire) is dropping out. there is your issue. most likely a bad

the primer bulb is illegal on an I/O. thats how you meet st peter

your issue there is most likely the anti siphon valve

no, air leaks do not necessarily mean fuel leaks

your dying is most likely ignition related to a bad connection, or if you have the original 2-wire distributor trigger

anything floating in the fuel filter may make it to the carb. hope you didnt re-use the filter. thats a guaranteed clogged carb

if your ignition (purple wire) is dropping out. there is your issue. most likely a bad connection.

Bulb will be removed !
anti siphon valve already replaced..!
So an airleak would be a possible cause of the fuel issue then despite no sign of leaking gas
Pick up tube not yet replaced

Not sure if the trigger is the 2-Wire type but
Why would it fail only at low rpm’s ?

if purple wire drops just momentarily - Is that enough to kill engine ? Wouldn’t it be the same as the shift interrupt switch function which just briefly stalls engine but not killing it?

 

[Happygolucky]

the anti-siphon valve is screwed into the top of the fuel tank pick up tube, there is a spring loaded ball inside that will shut off fuel flow if the fuel line from the tank to the pump/filter leaks in the bilge. The ball can corrode or get stuck from crud getting caught in it. Be careful if you are going to remove both the valve and the pick up tube. Some pick up tubes have a screen on the end of it, mine did not.
Pick up tube is the aluminum tube in the pic below, anti siphon valve is the brass colored fitting next to it. Excuse the dirty old gas tank. It is original (1988).
View attachment 418606
View attachment 418602

ASV was replaced already..will have to check the pick up tube

 

[Scott Danforth]

if purple wire drops just momentarily - Is that enough to kill engine ?

If it drops for a very very very short time, the motor will stumble

If it drops more than .2-.5 seconds while in gear, the motor would most likely die