3.0 merc bogs when cold

[novicetech]

I have this recent problem, with a bog down under full throttle operation. When you push the throttle all the way down motor will run at about 2000 - 3000 rpms for maybe two to three minutes before it sputters and misses and slowly climbs up to about 4800 rpms. This problem only occurs when the boat is first started for the day. The rest of the time the boat runs great. I have given the motor a complete tuneup. New points dwell 30 degrees, new plugs gapped 0.035,new condenser, timing set at 6 degrees BTDC, I even installed a fuel water separator, but when I pulled if off and dumped the fuel expecting to find water, there is no evidence of water.<br /><br />The motor is a 1987 3.0 mercruiser model ob738125. It has a delco distributor ignition with a mercarb # 1389-5962. Oh, one other thing it seems that the crankcase is slightly overfilled, maybe a pint with a slight fuel smell to it.<br /><br />So what do you guys think my next step should be.

 

[tommays]

Re: 3.0 merc bogs when cold

2 to 3 minutes is a pretty normal amount of time to allow a motor to warm up <br /><br />most people would be happy if it ran good all day after a warm up<br /> <br />i would not want to run mine up at that rpm any sooner <br /><br />if your oil has a fuel smell thats bad ,how long has it been in a ? <br /><br />the fuel pump leaking is the most likely way for his to happen<br /><br />there should be a small clear or yellow line running from the pump up to the carb if its got fuel in it the pump is leakng<br /><br />tommays

 

[novicetech]

Re: 3.0 merc bogs when cold

The clear yellow line is empty, no evidence of leakage, Im thinking a flooding problem. The engine will usually idle at least five minutes while wife parks truck. The motor has maybe 25 to 30 hours since oil change during winterization. And I am going to change it before next run.

 

[novicetech]

Re: 3.0 merc bogs when cold

bump^

 

[novicetech]

Re: 3.0 merc bogs when cold

And, i dont really mean cold, the temperature guage reads about 150 - 160 degrees as always.

 

[rattana]

Re: 3.0 merc bogs when cold

You could pull your plugs and see what color they are. If they are fouled the engine could stutter like you describe.

 

[novicetech]

Re: 3.0 merc bogs when cold

The old ones were black, and gapped wrong and champion. I will try that along with new fuel filters for the ppump and carb. Funny thing is it did not start this until I started buying gasoline at this marathon station that is close to the lake that I frequent.

 

[Don S]

Re: 3.0 merc bogs when cold

Put in some real spark plugs instead of those champions, and try getting an outboard tank with some gas in it and hook it to the fuel pump and take it for a run and see how it does. If it runs fine, hook the regular fuel tank back up and try it.

 

[novicetech]

Re: 3.0 merc bogs when cold

I had already changed to AC MR43T, gapped to 0.035. If I can get a hold of a small outboard tank, I will try it. At least it will tell me if the problem is in the tank or the delivery system on the motor.

 

[searay_89]

Re: 3.0 merc bogs when cold

Is the choke operating properly?<br /><br />Agree that fuel smell in oil is bad - but could be caused by slight blow-by, epecially if choke is causing too rich mix?<br /><br />I agree with tommays - if my boat ran great all day after a questionable warm up I'd be pretty happy.<br /><br />Regarding your Marathon comment ...... could be something to it ...... read my recent posts and comments ... I can't seem to get any of the pros to agree (yet)<br /><br />Good Luck

 

[novicetech]

Re: 3.0 merc bogs when cold

Problem solved, It was the fuel filter in the pump, I was really surprised to see so much gunk in there. There cant be much of a pickup screen in the tank. I wanted to say thanks to all who posted suggestions.