OMC 3.0 misfiring when hot/after being under a load

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kbibeau1

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I have a 1987 omc 3.0 with an omc cobra drive.
The problem I am having is after about 40-50 mins of runing the boat under a load it will start to misfire intermittently. The boat has always charged about 15Volts, which I had thought to be a bit high.
If i adjust the tilt while under a load it makes the skip much more noticable which leads me to believe that it is something electrical. I have already replaced the following:
-coil (from local auto parts store,not a marine coil, does it matter?)
-plugs
-wires
-distribuor cap
-rotor
-condensor

Could this be a bad resistor/regulator?
If anyone has any ideas please help and thank you in advance.
 
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Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

kbibeau1 said:
I have a 1987 omc 3.0 with an omc cobra drive.
The problem I am having is after about 40-50 mins of runing the boat under a load it will start to misfire intermittently. The boat has always charged about 15Volts, which I had thought to be a bit high.
If i adjust the tilt while under a load it makes the skip much more noticable which leads me to believe that it is something electrical. I have already replaced the following:
-coil (from local auto parts store,not a marine coil, does it matter?)
-plugs
-wires
-distribuor cap
-rotor
-condensor

Could this be a bad resistor/regulator?
If anyone has any ideas please help and thank you in advance.
Click to expand...

Points...Change the points.
 
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bryancurit

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Re: OMC 3.0 misfiring when hot/after being under a load

I have the OMC 2.5 and am having the same problem. I have changed every ignition part and have had the Carb rebuilt. Mine always does it after twenty minutes and after I come down from running fast. Mine also got worst when I trimmed the motor. I went back and checked it to find it was number 4 cylinder skipping. Eventually it goes from just 4 to running so bad I can't keep it idling. I let mine sit for 20 minutes and it started right up and ran fine again for another 20 minutes. I should also mention that I had found a cracked plug on number 4, I replaced that but still have the same problem. I was told to check the exhaust manifold and riser elbow to see if they were rusted out or cracked and letting water into the cylinders through the exhaust valves. That would make sense since number 4 cylinder is the back cylinder and is the lowest in the especially when you tilt the motor. I haven't been able to find any cracks or water leaks though to prove this. Any suggestions would be appreciated.
 
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kbibeau1

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Re: OMC 3.0 misfiring when hot/after being under a load

bryancurit said:
I have the OMC 2.5 and am having the same problem. I have changed every ignition part and have had the Carb rebuilt. Mine always does it after twenty minutes and after I come down from running fast. Mine also got worst when I trimmed the motor. I went back and checked it to find it was number 4 cylinder skipping. Eventually it goes from just 4 to running so bad I can't keep it idling. I let mine sit for 20 minutes and it started right up and ran fine again for another 20 minutes. I should also mention that I had found a cracked plug on number 4, I replaced that but still have the same problem. I was told to check the exhaust manifold and riser elbow to see if they were rusted out or cracked and letting water into the cylinders through the exhaust valves. That would make sense since number 4 cylinder is the back cylinder and is the lowest in the especially when you tilt the motor. I haven't been able to find any cracks or water leaks though to prove this. Any suggestions would be appreciated.
Click to expand...

I will let you know if I can find any solutions to this. I plan on checking/cleaning all my connections today and testing voltage at the coil. Hopefully I can find something to help the both of us.
 
Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

Also check the voltage on the + side of the coil while the engine is running. Should be 7-9V. If it is getting more V's that may be overheating your coil and causing it to cut out.
 
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kbibeau1

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Re: OMC 3.0 misfiring when hot/after being under a load

Levinz11 said:
Also check the voltage on the + side of the coil while the engine is running. Should be 7-9V. If it is getting more V's that may be overheating your coil and causing it to cut out.
Click to expand...

Mine is getting 12-13V! I am guessing that is more than likely te cause of my problem. I am guessing that is due to a bad resistor? Where would I find a replacement?

Thank you!
 
Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

You have a couple options:

-Get a coil with an internal resistor OR one that runs on 12V constant
-Make your own resistor wire
-Wire in a resistor

Do a quick search in this forum for more specifics as I have never done any of these suggestions.
 
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bryancurit

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Re: OMC 3.0 misfiring when hot/after being under a load

You need to be careful about just wiring in a resistor, as far I as I know these ignition systems use the full 12V to the coil during starting for a hotter spark and then switch to across the resistor to drop it to around 8-9V while running. As another note, I'm not sure how trimming the motor would cause a misfire if the problem was just too much voltage to the coil.
 
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kbibeau1

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Re: OMC 3.0 misfiring when hot/after being under a load

Do any marine stores/websites carry the OEM resistor?
 
Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

bryancurit said:
You need to be careful about just wiring in a resistor, as far I as I know these ignition systems use the full 12V to the coil during starting for a hotter spark and then switch to across the resistor to drop it to around 8-9V while running. As another note, I'm not sure how trimming the motor would cause a misfire if the problem was just too much voltage to the coil.
Click to expand...

There are 2 wires to the + coil term. One (forget the color) supplies 12V while cranking only. The other (purple) is a ballast/resistor wire coming from the alternator.

If some one "rewired" his ignition and he has an older alternator supplying those volts, I can see how hitting the trim might cause the symptoms he describes. Old wiring can cause all kinds of weird issues.
 
Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

kbibeau1 said:
Do any marine stores/websites carry the OEM resistor?
Click to expand...

The resistor wire is in the harness and would be tough to change. Coils are pretty cheap. Get one that runs on 12V constant or has internal resistor and wire it directly to the alternator.
 
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kbibeau1

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Re: OMC 3.0 misfiring when hot/after being under a load

bruceb58 said:
With engine running?
Click to expand...

Yes it is actually closer to 14V with the engine running. I am going to try a coil with an internal resistor. Will let you all know how I make out.

Thank you all!
 
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kbibeau1

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Re: OMC 3.0 misfiring when hot/after being under a load

Replace with internal resistor coil and still have the problem. I'm at a loss :mad:
 
Levinz11

Levinz11

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Re: OMC 3.0 misfiring when hot/after being under a load

Well if you changed the points and everything else on the ignition....time to start looking at the fuel system.
 
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bryancurit

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Re: OMC 3.0 misfiring when hot/after being under a load

Same here, I'm still having the problem as well after replacing the coil with a 12V version after finding almost 13V at the + side of the coil. I have already rebuilt the carb and fuel pump, I am starting to lean toward vapor lock, just not sure what is causing it.
 
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bryancurit

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Re: OMC 3.0 misfiring when hot/after being under a load

Kbibeau1 did you ever come up with anything?
 
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58bang

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Re: OMC 3.0 misfiring when hot/after being under a load

I have the same problem with my 1977 3.0 OMC Sterndrive motor. I have replaced points,coil,condenser,resister,cap,plugs and also replaced the ignition switch and fuel lines. Could a misplaced wire cause this misfiring??
I disassembled and rebuilt the motor so I may have mixed up a wire connection?
 
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