Stumbling at Cruising Speed - OMC 5.8L

[LakeMichiganBoater]

Been working on my 1989 Bayliner Trophy 24' and it ran great... for the most part... last season, but I need help figuring out an issue.

It starts fine, runs fine, but after about 5 minutes at cruising speed (1/2 to 3/4 throttle) it starts stumbling. If I throttle down and then bring it back up to speed it is fine... for maybe another 5 minutes or so. Engine doesn't die and runs normal when I throttle it down.

This is what I have done to it last season:
-Rebuild outdrive and replaced shifter cable between engine and outdrive
-Cleaned and rebuilt carbs, set idle and idle air/gas adjustment, set choke
-Replaced all plugs
-Replaced distributor cap and points, set points
-Replaced fuel filter

Any thoughts on what I am missing? My guess is fuel delivery, but not sure what I missed. Hard to diagnosed as it only happens at cruising speed after 5 minutes of cruising. Temp seems normal when it happens.

 

[stresspoint]

set the points with a dwell meter then do the timing after that set the carb mixture screws, see how that goes
for me that is standard procedure with point dizzys,
failing that fixing your issue >
.
i used to replace the condenser and points at the same time as the condenser can randomly fail with heat (possibly your issue).

also a new coil , since the one you have is possibly original to the motor ??? they are a neglected piece of the puzzle .

 

[LakeMichiganBoater]

Sorry I forgot, when I replaced the points I replaced the condenser as well.

The coil may be my suspect... Would that cause the intermittent, yet consistent issues I am seeing?

 

[stresspoint]

you need to set the dwell rather than relying on the point gap measurement especially on boat motors , once dwell is set then set the ignition timing.

internal shorting in the coil could be the suspect if it has never been changed..

? what was the reason for doing the work you mention above , was it to correct an issue or just preventive maintenance.

 

[LakeMichiganBoater]

Ok, Good to know. Think I used a feeler gauge to set it the first time.

The coil does look old, I will replace it for sure. Are they hard to come by? Aftermarket ok?

Mostly preventative. I got the boat really cheap in non-running condition (seized from sitting) and I just did all the likely suspects just in case.

 

[stresspoint]

Ok, Good to know. Think I used a feeler gauge to set it the first time.

The coil does look old, I will replace it for sure. Are they hard to come by? Aftermarket ok?

Mostly preventative. I got the boat really cheap in non-running condition (seized from sitting) and I just did all the likely suspects just in case.

that is probably the cause of the problem , its very hard to get correct timing if the point dwell is incorrect hence the reason i always used to use the dwell meter ,
I'm not saying it cant be done ,it can , i just feel more confident knowing the dwell is correct so i do it with the meter when possible. !.
you could try ""slightly advancing ""( like 2-3 degree) the distributor to see if it overcomes the problem.

 

[redneck joe]

plug wires?

 

[Scott Danforth]

It's never the coil

It's nearly always not using the dwell meter

 

[kenny nunez]

Get a 6 gallon outboard tank and and first run the boat on the main tank until it starts running bad. Switch to the 6 gallon tank and see how it does.
Most likely your problem is the suction tube in the tank. There is a fine filter in the tube that will have some debris in it. You need to remove the filter from the tube. Also a good time to inspect or replace the anti siphon valve and the fuel hose to the large fuel filter.

 

[LakeMichiganBoater]

Get a 6 gallon outboard tank and and first run the boat on the main tank until it starts running bad. Switch to the 6 gallon tank and see how it does.
Most likely your problem is the suction tube in the tank. There is a fine filter in the tube that will have some debris in it. You need to remove the filter from the tube. Also a good time to inspect or replace the anti siphon valve and the fuel hose to the large fuel filter.

How hard is this to get at? If there is a chance its an issue I want to check it out anyways.

I plan to get a dwell meter and timing light to set everything.

Coil is cheap, so are plug wires, so I will replace them also.

 

[kenny nunez]

There should be a removable cover for access to the tank sending unit where the fuel hose is attached. If the tank ever has to be removed the deck will have to be cut.
A dwell meter is the best way to get the points set. Then recheck the timing.

 

[LakeMichiganBoater]

There should be a removable cover for access to the tank sending unit where the fuel hose is attached. If the tank ever has to be removed the deck will have to be cut.
A dwell meter is the best way to get the points set. Then recheck the timing.

Ok, I think I will take that out also. I would like to pump out all the gas and take a look in the tank too, especially if the strainer is clogged.

I was debating the best way to drain the gas tank, as it may have 20-40 gallons in it still with stabilizer.

My thoughts was a 12v inline fuel pump like this one, disconnect the input at the fuel filter and attach it to my fuel pump. Then attached the fuel pump to a fuel line with 2-3 fuel filters like these in it and drain that into portable tanks, keeping an eye on what the gas looks like... maybe I just keep that to use in lawn mowers and whatnot.

My concern is once I get all the gas out and I end up finding junk in the bottom of the tank, how do I get that out?

 

[froggy1150]

Here is a coil tidbit. There are external resistor coils and internal resistor coils along with 6volt and 12 volt. Make sure you get the right one

 

[LakeMichiganBoater]

Here is a coil tidbit. There are external resistor coils and internal resistor coils along with 6volt and 12 volt. Make sure you get the right one

Well which is the right one for a OMC 5.8L?

Is this the right one? https://amzn.to/4836C6J

I've used Sierra aftermarket parts on other stuff and they seemed decent.

 

[redneck joe]

My last boat sat for three years after the tank was drained. I knew the leftovers would turned into varnish so i bought a dozen basic inline filters and when the motor started bogging i just popped a new on one. No issues for the rest of its time with me.

 

[FiskandShi]

CHECK THE Shift module and microswitches. A mechanic wired the "overstroke" microswitch wrong and strong/quick movement of the cable would trigger the interrupt switch. They need to be in series - so that the overstroke will ignore the interrupt if the cable jiggles/shifts. It looks correct, hardly can see a difference in the wire position in the "boot", but you'll be cleaning fuel filters and adjusting the carb until the day.. you die. Took me 2 seasons... 2 props...

I won't correct but just add: I think that I have the microswitch names overstroke and interrupt backwards - they look identical but work differently. On second thought, I wanted to include that - either way, one signals the module to scatter the ignition timing (overstroke?) and one prevents that signal from being passed to the module (interrupt?) - it's in the manual.

I'll also add, I'm not 100% sure this is your issue, but I've never found this on the forum and always seems overlooked/misunderstood. It took me a while.

 

[froggy1150]

So here are examples of external resistors. I doubt you have a resistor wire. Note how I says on the pink " resistor wire do not cut". Most likely you will have a ceramic ballast. Just make sure if you buy a coil it says "external resistor".

 

[Lou C]

My OMC definitely has a resistance wire instead of a ballast resistor, and I had the problem of the interrupt switch sticking which caused a similar problem. What I found was the little buttons on the switch wore after 25 years of use and would stick on causing a massive stumble. I replaced both switches about 10 years ago and never had a problem with them after that.
My boat also had a problem stemming from a corroded anti-siphon valve, that was replaced about 20 years ago, caused it to not pick up rpm under load, it would start, and rev in neutral just fine but didn't allow enough fuel flow to plane the boat. Fixed by my old mechanic, great guys now retired!

 

[LakeMichiganBoater]

CHECK THE Shift module and microswitches. A mechanic wired the "overstroke" microswitch wrong and strong/quick movement of the cable would trigger the interrupt switch. They need to be in series - so that the overstroke will ignore the interrupt if the cable jiggles/shifts. It looks correct, hardly can see a difference in the wire position in the "boot", but you'll be cleaning fuel filters and adjusting the carb until the day.. you die. Took me 2 seasons... 2 props...

I won't correct but just add: I think that I have the microswitch names overstroke and interrupt backwards - they look identical but work differently. On second thought, I wanted to include that - either way, one signals the module to scatter the ignition timing (overstroke?) and one prevents that signal from being passed to the module (interrupt?) - it's in the manual.

I'll also add, I'm not 100% sure this is your issue, but I've never found this on the forum and always seems overlooked/misunderstood. It took me a while.

Where are these switches located and what is the best way to confirm they are wired correctly?

 

[LakeMichiganBoater]

OK. Got the dwell meter. What is the best method to set dwell? Can't really adjust the points with it running like other models, due to the distributor cap, unless I'm missing something