1st day out - towed back to dock - stalled / no start

Just bought a 97 bowrider to appease the family. Mercruiser 3.0LX fired right up on the hose and at the ramp. It actually ran great for 30min or so - mix of idling in neutral, running on plane and some no wake zones. After that we were just idling in gear and it stalled. Restarted fine, so jumped up on plane, cruised some more w/o issue. 15min later or so we had to idle for another no wake and it stalled. It would not restart - didn't even try to fire. We were close to the ramp and luckily someone offered to tow us in. Including drive home, total time down 80min or so. Fired right up on the hose when we got home.

When it was cranking and not starting, I couldn't see fuel spraying, but butterfly valves were wet. I couldn't really smell a strong fuel smell either though. I was not able to pull a plug to check for spark. I'll be honest, carbs pre-date me - all vehicles I've owned have been TBI / FI. Since it started at home, it will be difficult for me to diagnose. Any ideas? I have a receipt from PO from Sept 2013 that it received a new fuel pump and carb rebuild. I know he didn't use the boat a lot though - 3 or 4 times since then (kept an hr log). It has Ethanol free fuel in the tank - I don't know about any sort of stabilizer though. I also don't see any mention of fuel or F/W seperator filters in his stack of receipts, so I guess I'll do those for a baseline for myself. I will also pull plugs and cap this week for inspection and verify timing. I need to nail this down though, b/c my crew of 5 wasn't thrilled on their maiden voyage....

ALSO, where is tach signal taken from? I don't know if it's my tach or signal, but at idle it reads 3k or so. Definitely NOT helpful (nor accurate).

i dont know crap about inboard rigs but do yourself a favor and put an inductive timing light in the boat tool box..the decision on any motor is the problem fuel or fire?this will get you working in the right area...the light will cost in the 40 to 50 buck range...

was the fuel tank vent completely open?

First thing, being as the boat is new to you, unscrew the fuel pickup line on the fuel tank, & check the screen on the end. If the tank has any crud in it, that's the 1st place it'll go. Next, check the anti-siphon valve it should be right on the tank line (it looks like a hose barb, but it is a one way "spring &ball" check valve), make sure it's not clogged either (blow thru it both ways). This lets the gas flow out of the tank when vacuum is applied, but stops it from flowing backwards.

Now, check all your fuel lines to the filter, then check the external filter, next check to see if there's filter inside the carburetor where the fuel line goes in, this must be cleaned or replaced if you have one? Finally check your fuel tank vent, by opening up the fill cap while running to make sure there's no vacuum being created in there, to stall your engine.

This is usually the areas where you might find the most problems, so you want to do all this, and check it well!

Once all is well here, we can move to the ignition if need be?

I appreciate the leads - I will start inspecting those areas. It's my first I/O, so not familiar w/ any of this. I didn't even think of a vent - just assumed all internal tanks are always "open" in some way or form. Guess I have a lot to learn!!

I do have a digital timing light that shows RPM, but didn't have it on the boat. I'm gonna grab a water proof box and some more dedicated boat tools too before next time out.

I'll post updates as my inspection continues

Surely if it was running then conked out it is either a fuel problem or you have a bad ignition module. The "marine" module is about $80-odd from our hosts. I would be caught with one in my on board spares kit.
Tach sig comes from coil. Should be grey in color going to main multi plug then forward to panel/gauge.

if you have access to a military surplus store then the ammunition boxes are the most water and moisture proof thing i have found for tools....i have one that fits perfectly under the console..and a smaller one for paperwork etc i dont want to mix with the tools and grease...

UPDATE
I inspected fuel lines and a few other things an old boat mechanic at work suggested. Fuel line is clear from tank to carb. Fuel pump doesn't appear to be bad (no fuel in the clear hose). I pulled plugs and they were black and wet. I threw new ones in today before we headed out. Cruised for several hours and in no wake zones I had to go a little faster than ideal bc I could tell it was gonna stall (tach is wildly off, but I'm guessing 1200-1400rpm or so). Never tried to idle bc I just didn't trust it. It finally stalled at boat ramp due to traffic. Literally only time i tried shifting into neutral. I immediately tried to restart and it wouldn't fire. Drifted off to the shore and sat 10min for some boats to clear. It restarted, popped into gear and lasted about 10sec before it stalled again. Would not restart and still won't at the house.

New plugs are wet. It won't fire if I try starting fluid. So clearly I'm getting fuel. I tried checking for spark, but entire motor is painted, so hard to find a good ground to rest plug on. I am not seeing one though. Anyone know how to test a module? It doesn't make sense to be a coil bc it wouldn't care that I wanted to idle. So I'm thinking module has a bad idle circuit (??) or carb is running rich at idle. It seems to only be an issue when it is hot though bc I have run it several times at home (obviously not in gear and no load)

Just went outside and it fired right up - first try. It truly does run well. It's been sitting for about 90min and I had the cowl open. So either an electrical item cooled off or the flooded engine disapated.

Ayuh,.... Sounds like a carb problem to me, runnin' waayyy to rich,.....

Could be choke related,....

Maybe a carb kit, 'n rebuild it,....

I'm thinking it is carb too. It was rebuilt a year ago (previous owner had a receipt in records), but I don't know the quality of parts or work. Has anyone swapped in a Rochester? I heard these MerCarbs aren't as good.

the problem with running at home is the temps do not come up the same as when you are on the water, wet plugs= not enough ignition to fire the plugs. having a hard time finding a ground, find a convenient bolt, remove and clean it put a short wire on it so you always have a ground point to check with

Is it possible I could be getting so much fuel that the plugs won't light it? I just don't know why it will run for hours wo missing a beat and then once I try to idle, it would lose spark. Does the coil alter its output? Since these don't have ECU's, the module controls everything right? So does it have different circuits? Idle, run, full throttle, etc? I'll throw some wire on a bolt, but until I can heatsoak it again, I won't be able to test anything

The motor should be able to idle for hours with no issues.....

in the water or on a hose it does not matter.......and the motor will come up to full temp either way.

The thermostat controls temp of motor not the water source. Water source will determine if it runs a bit cooler or hotter overall.

What running on a hose vs in the water under load is, with a hose it can cover up water restrictions or weak/bad impeller as the water is being forced into the motor vs when in water under load it is being sucked in by the impeller.........that is it!


The motor itself should be able to run on its own until it runs out of gas whether it is on a hose or in the water if all is right.

So something is NOT right with your motor set up.

Wet plugs are either bad/weak ignition or over rich fuel and or weak compression.


You need to do some process of elimination here.


Answer to your question about so much fuel the plugs wont ignite.........YES at idle if there is too much fuel it will stall and possibly not reignite until the fuel has evaporated.

Dont start over thinking your issue. For now stick with basics until no other answer can be found.

If it starts and runs then the ignition is working.........period..........

when it stalls is when you need to start the process of elimination.

Basic questions and answers..........

1. what is the spark plug gap?
2. what are ther spark plugs number?
3. when idling is choke open when motor is warmed up?
4. when idling remove flame arrestor and look into carb.......if choke is open is there excess fuel flowing into carb?
4a. have you adjusted the air fuel mixture screw? If not maybe try. With motor running and idle rpm in neutral at about 600 to 700 rpms turn the idle screw in slowly until motor starts to miss or stumble ever so slightly, then back it out or unscrew the mixture screw until motor reaches max rpms, may not be more than what idle was already but do this a few times so you can hear the subtle changes and see if you can get the max rpms and smoothest idle quality. If the idle mixture screw needs a lot of change from where you start this may be part of your issue. If adjusting the idle mixture screw makes no changes then the carb has issues and/or there is a vacuum leak
5. Is motor over heating? If not are you sure?
6.when you look into carb can you see droplets and if so can you tell if they may be water?
7. there are two differnt elctronic ignitons, one has a distributor cap with spark plug wire wires the other does not it has a coil with spark plug wires , which do you have?

Your tach signal is from the GREY wire and depending which ignition you have is where is comes from but it is a grey wire for either.


So if you had a ignition component fialure most lekely it would not run at all or when it fails it would not restart.

Also if you have the igniton with no distributor cap the spark at the plugs is very difficult to see. it will only jump a gap no bigger than 1/4 inch so it must be somewhat dark and the spark checker must be set to the corect gap. If using a spark plug to check for spark, use a wire and wrap it around the threads of the spark plug and connect the other end to the Battery Neg (-). stick the plug back into the spark plug wire and crank the engine over........If you see a blue spark jump the electrode gap you are good to go! If it is yellow or orange then there may be an issue or bad spark plug so try anothe plug first.

NOTE:****** when perfoming the above spark test make sure all spark plugs are removed so motor will not start*****!!!!!!!!!!!!!