OMC 3.0 Skipping (Misfiring) After being under a load

[kbibeau1]

I have a 1987 Starcraft 3.0 that I am pulling my hair out with.:mad-new: My issue is that once the boat is run under a load for about 20-30 mins it begins to misfire. For the first 20-30 mins the boat runs terrific with no issues. I have done a lot of reading on this forum and many others about this problem. I found that many people have had similar problems but none of their solutions have helped me unfortunately. I have tried many different things to solve this problem, I will list them below.

What I have tried:
-Cleaned all connections and replaced positive and negative battery cables.
-Replaced coil
-Switched to a internal ballast resistor coil and ran power to it from the battery when the ignition was in "on" position. The reason I did this was because I thought my resistor wire from the alternator was no good and was supplying too much power to the coil therefore overheating it.
-New Plugs.
-New plug wires
-New cap, condenser, rotor and points.
-When the problem would occur I would run a jump lead from the (+) battery terminal to the (+) post of the coil and nothing would change. I figured if it was an issue with the coil not getting enough power that would temporarily solve the problem.

I was wondering if I should just convert to an electronic ignition system. I do not see why that would not fix the problem. Although I would still like to know what in the world could be causing this too happen. Thank you in advance to anyone who could shed some light on this topic!

 

[Howard Sterndrive]

Key switch?
Try a run with the tachometer disconnected.
Maybe a voltmeter at the coil when it's missing would help.
Have you looked at the fuel system? Is the miss really obviously ignition/electrical?

 

[kbibeau1]

I have tried disconnecting the tach. That is the two grey wires on the (-) post of the coil. Correct? And yes it is obvious to be an electrical issue because when the problem is occurring if you adjust the out drive it gets worse.

Also I was wondering if I was to convert to electronic ignition if it would be best to get the one where you convert your current distributor or the package that comes with a whole new distributor and coil.

 

[kbibeau1]

If this were to be caused by a fuel issue where should I begin my diagnostics?

 

[Howard Sterndrive]

And yes it is obvious to be an electrical issue because when the problem is occurring if you adjust the out drive it gets worse.

that's good info - don't let me distract you with fuel system questions.

I would look at your ignition switch at the helm, safety lanyard switch - that sort of stuff - A voltmeter between coil + and batt neg when it's acting up will verify the issue and then start looking at everything between batt + and your coil + for voltage drops along the way.

 

[kbibeau1]

When it began to act up I attached a jump lead from the (+) on the battery to the (+) on the coil to ensure it was getting full power and it didn't resolve the issue. So I do not believe that the problem has to do with it not getting enough power. By the way I am using a coil with an internal resistor that is designed to be given a full 12V.

Also my voltage gauge at the helm reads that it is charging between 15-16V. I know that is too much but could that cause the issue? I believe the gauge is inaccurate but I will check what kind of voltage I have at the battery with the engine running and see if that gauge is accurate or not. But if the system was in fact charging that much would that be somewhere to look? Do these alternator/generators have diode resistors in them?

 

[jerryjerry05]

Fuel.
Try running on a second tank.
A 6 gal outboard tank.
Missing after 20-30 min. Could be the vent line clogged with wasp nest?
Change the filters. One in the carb, possible one in the fuel pump.
Possible the pickup in the tank is clogging?
Ethanol can delaminate the inside of older hoses and they can collapse?

 

[john cole]

I'm thinking it is a electrical issue too, from the sounds of it, and yes it is over charging which could lead to some electrical components over heating (battery) and building up a high resistance which would lead to various electrical issues. I would look at your alt and regulator and go from there. if you have dual batteries run just the one and once it starts skipping switch it to the other and see what happens for the heck of it, it may help solve your problem. but regardless that is to many volts for the system.

 

[MikDee]

Okay, lets get down to the nitty gritty, Exactly what spark plugs are in the engine now, brand, & number?

 

[kbibeau1]

Okay, lets get down to the nitty gritty, Exactly what spark plugs are in the engine now, brand, & number?

I am going to check that and get back to you. I am also going to check how many Volts it is charging with the engine running. Thanks for everyone's input. I will update soon!

 

[kbibeau1]

Okay, lets get down to the nitty gritty, Exactly what spark plugs are in the engine now, brand, & number?

Spark plugs are Autolite #26.
The plugs are a little less than a year old.
Are there some brands of plugs that cause this issue?

I have not had the chance to run it and check how many volts it was charging yet.

 

[MikDee]

I prefer AC Delco MR43T spark plugs, they are an alloy spark plug (never rust) with a beefy center electrode (for a strong spark) they're the recommended replacement spark plug in most Mercruiser applications.They come in different heat ranges, & length of thread: MR44T,MR45T, MR44TS, MR45TS the (S) are long thread plugs. I was never impressed by Autolite (take them out, and see how they look?).

 

[kbibeau1]

I prefer AC Delco MR43T spark plugs, they are an alloy spark plug (never rust) with a beefy center electrode (for a strong spark) they're the recommended replacement spark plug in most Mercruiser applications.They come in different heat ranges, & length of thread: MR44T,MR45T, MR44TS, MR45TS the (S) are long thread plugs. I was never impressed by Autolite (take them out, and see how they look?).

Which variation do you think I should use? I have a 3.0 OMC Cobra 140 hp.

 

[MikDee]

There should be crossover chart from the Autolite, to AC online, or check your service manual.

 

[kbibeau1]

Replaced the plugs with MR43T AC Delco

Ran the boat today and the alternator is charging 15.3 Volts. Is this too much for the system? Do you think this could be where my problem lies?

 

[kbibeau1]

Replaced the plugs with MR43T AC Delco

Ran the boat today and the alternator is charging 15.3 Volts. Is this too much for the system? Do you think this could be where my problem lies?

 

[MikDee]

No, overcharging really wouldn't cause this. I hope you have the right sparkplugs? I know there was some controversy over that, because you said you had an H.O. engine, if so you may need longer thread plugs like MR43LTS,,, The only way to really know, is pull out a plug, and put a narrow metal measuring tape into the hole, and hook the edge of the tape on the bottom of the thread, then read how deep the thread is to the outside surface.

I know you did a lot of electrical trouble shooting, but could there be water in your fuel? That would create havoc.

 

[bruceb58]

15.3V is way too high. Did you measure that with a meter at the battery? As far as the alternator high voltage causing an issue, that would be easy to check. Unplug the field wire going to the alternator so it doesn't put out a charge and run like that.

What is your dwell set to? Did you set it with a dwell meter? If you have your dwell set too high, your coil would saturate which could cause it to overheat.

 

[kbibeau1]

I am going to test it on a dwell meter over the weekend and see what I come up with. What should the dwell be set at? As far as water being in the fuel I am pretty sure that is not the case. I thought that was a possibility so I drained the tank and refilled and nothing changed. I am going to set the dwell right on the money and clean all of my fuel filters and try her out again. <br /> I was thinking possibly a valve problem once the engine was warm so if the problem comes back im going to do a compression test when the problem occurs and see if I have any weak cylinders.

 

[bruceb58]

I believe the dwell is 31°-34°. I am sure it has it in the service manual.