OMC 3.0L missfire/ fuel starvation!?

[ghostgixxer]

Hi folks !! First I want to apologize for the long text but I want to explain the issue I’m having in full details !! Bought a new boat 2 days ago , the guy said in the add : boat is running great used it about 40 times last season , since he has moved about 1 hour and a half away from the marina recently he had the marina guys to do a quick summer tune up for him (oil , oil filter and fuel/water separator filter, new plugs and they saw a cracked ignition wire so they’ve changed all 4 , then parked it at his dock in the water . Met the guy at the marina , first did a compression test (all good on all 4) and quick visual inspection, started the engine (fired up right away and idle was top notch) let it idled for 5-8 mins (temp was ok) shut it down and looked the oil to see any signs of water getting in there (cracked block) everything was good so far . Restarted the engine and took it out the marina bay in gear at idle speed (no wake zone is about 4-5 minutes ride) , went for 1/4 throttle for 2 mins to slowly gain speed while still be able to listen for any weird noises coming out the engine (everything ok so far) , pinned it full throttle, pulled strong for about 2-3 seconds then lost most of its power within another 2-3 seconds !! Put it in neutral , bugged slightly then idled fine again , since then I could only throttle it between 1/4 and 1/2 throttle about 2800-3000 rpm , anything over that range it runned like **** !! Felt like both misfiring and fuel starvation it was really weird , usually missfire you can pin it and even with lack of power it will slowly gain rpm / speed still, and fuel starvation usually happens after a mid/heavy load in the engine then after let it idle a few seconds you can pin it again and it’ll work fine for a few seconds until the lack of fuel happen again, but my issues feels different!! Maybe both fuel and ignition problems ? While coming back at the docks **** got worse, couldn’t go past 2000-2200 rpm , over this range the engine was bugging big time , even below 2200rpm it often sounded like it was running on 3 cylinders, but again didn’t sound like only 1 cylinder was misfiring, like all cylinders randomly misfired, even at idle speeds it often and randomly ran on 3 cylinders. Took the plugs out at the docks, all 4 were pretty identical looking slightly lean but not that bad , and none of em had signs of water intrusion so I don’t think the gasket is leaking water into the cylinders, if it does it must be very minimal so pretty much untraceable . Long story short , the guy was going through divorce and needed money right away so I offered about 60% of the price we first agreed on and he accepted!! Took the boat back home and tried to start it this morning and fired right up and idled like a champ ! Gave it a bit of throttle with some short full throttle hits and never ran like crap (I know I can’t replicate loads out of the water ) sometimes I could hear missfire .
You guys have any clue what I should diagnose first ?
Thanks
Chris

 

[itsathepete]

Well, you're not giving us much info about the powerplant other than it's an OMC 3.0. Engine serial number would be great but year is a good start. Go through the ignition and fuel systems. Put some seafoam or other cleaner in the fuel and see if it helps

 

[southkogs]

Have you looked at the carburetor?

I'd work backwards from the plugs at this point, and make sure I'm getting the spark I'm supposed to. But I think I'd also do some basic checks on the carb, and make sure it's working correctly.

 

[ghostgixxer]

Have you looked at the carburetor?

I'd work backwards from the plugs at this point, and make sure I'm getting the spark I'm supposed to. But I think I'd also do some basic checks on the carb, and make sure it's working correctly.

I’ll disassemble the carb today

 

[southkogs]

Might want to check fuel flow and filters before you tear it all apart.

 

[ghostgixxer]

Might want to check fuel flow and filters before you tear it all apart.

Oil,oil filter,fuel/water separator filter was done about a week before I’ve met the guy (shop bill as proof) and the guy filled the boat up before we went for a test ride ( tank was almost empty and he filled it , 300$ of fresh fuel) still this morning I’ve took a fuel sample both in tank and at the mechanical fuel pump outlet… no signs of water !! I’ve also inspected the rotor cap and ignition wires connections both at the cap side and plug connectors side , tested each wire ends to plugs with a spark tester tool and everything at cranking speeds seems fine !! Also took the valve cover off and checked for sticking valves (all fine ) . Even inspected pistons and cylinder walls with a telescopic camera (no signs of major wear and all 4 looked identical so I guess the missfires doesn’t happen on 1 cylinder only) . Tomorrow I will do another compression test with 2 good batteries and run te tests at equal cranking speeds , if one on em takes longer to reach its maximum pressure then I’ll look for valves seal issues. Also I didn’t have my multimeter at home today so I couldn’t test the coil resistance I’ll do that tomorrow!!
Thanks for your input !!
Chris

 

[ghostgixxer]

Well, you're not giving us much info about the powerplant other than it's an OMC 3.0. Engine serial number would be great but year is a good start. Go through the ignition and fuel systems. Put some seafoam or other cleaner in the fuel and see if it helps

Sorry I didn’t provided those infos the top engine cover with these informations is missing , it’s a 1989 omc 3.0 140hp , as far as I know those 3.0 OMC’s aren’t much different from year to year, they all used same ignition/fuel components pretty much if I’m correct!? When you suggest using seafoam do you believe carbon deposits could obstruct valves seats bad enough to create missfires ? Is it common on these engines ? Thanks
Chris

 

[ghostgixxer]

Btw folks : I didn’t have enough time to disassemble the carb today !! I’ll try to do it tomorrow!!

 

[itsathepete]

When you suggest using seafoam do you believe carbon deposits could obstruct valves seats bad enough to create missfires ? Is it common on these engines

Seafoam will help clean deposits out of the carb and intake tract. Other fuel treatments can be used, though. Should be points ignition. I'll wager they haven't been cleaned or replaced. Do that and check timing

 

[ghostgixxer]

Seafoam will help clean deposits out of the carb and intake tract. Other fuel treatments can be used, though. Should be points ignition. I'll wager they haven't been cleaned or replaced. Do that and check timing

Thanks !! Timing should be -8 degrees right ?

 

[ghostgixxer]

Seafoam will help clean deposits out of the carb and intake tract. Other fuel treatments can be used, though. Should be points ignition. I'll wager they haven't been cleaned or replaced. Do that and check timing

Also , do you think a small leak from the intake/exhaust gasket could throw just enough water into the cylinders to create missfires without leaving much water intrusion signs ?

 

[Serf27]

My OMC 3.8 is having a similar issue as to the bog down under load at higher rpm. No misfires, 3k rpm and below, it runs well, soon as I hit 3400, it bogs and I either drop the throttle and catch the idle or it dies.

I’ve done, plugs, cap/rotor, fuel pump and lines.
I’ve read and heard of other items I haven’t changed yet, maybe it’s the same issue with yours. Last parts (I think) to swap is the ignition coil and accelerator pump in the carb.

Things I checked off that you may want to look at, fuel tank vent and fuel pick up tube.

 

[itsathepete]

Also , do you think a small leak from the intake/exhaust gasket could throw just enough water into the cylinders to create missfires without leaving much water intrusion signs ?

I doubt it. That would likely be one cylinder and would be pretty constant I suspect