4.3 loss of power/slight vibration

Hello everyone. I have a 1998 Caravelle 188 with a 4.3 mercruiser. The boat was running fine then all of a sudden it has lost power. It takes forever to plain out, has a slight vibration under acceleration, and has lost about 7mph top end and won't turn over 4k rpms. I have checked the choke plate, and replaced the plugs, and checked the distributor cap and rotor which looks fine. I'm lost and don't know where to start next. The wierd thing is it did this about a month ago and did it for about an hour, then ran fine again. Ran fine the next few outings and then started again 2 outings ago and hasn't stopped. Anybody run into this problem?

oh and it is leaving a black film on the back of the boat, almost looks like soot, but doesn't come off easily

Ayuh,.... Check the Contents of the fuel filter for Water, 'n Crud,... then change the filter of course,...

Sounds like a Fuel Delivery Issue,...

bondo said:

Ayuh,.... Check the Contents of the fuel filter for Water, 'n Crud,... then change the filter of course,...

Sounds like a Fuel Delivery Issue,...

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i'll check that, thanks. very well could be. we bought tthe boat from my father in law who bought it new. so i have been filling it up at a different station same 90 octane non-ethanol though.

Try a compression test with all the plugs removed and throttle wide open and report the results

Bt Doctur said:

Try a compression test with all the plugs removed and throttle wide open and report the results

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i'm not a tard when it comes to motor work...... just not familiar with boats.... this is my first one.... but I have had plenty of fast cars.... which were all fuel injected motors. I appreciate your helpful advise though.....

Have you considered that the loss of power and vibration could be due to a bad cylinder...... either form low or no pressure...., or from not receiving spark???


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a motor is a motor , a cylinder dosent care if its injected or normally aspirated , but if the piston or rings or valves are damaged the comp goes low/to nothing and a dead cylinder will cause a vibration and loss of power.

All of you want the correct answer but refuse to do some basic tests to narrow it down. Untill you get a base line all we can do is give you our best guess.
you may; have a clogged lifter,bad lifter,sticking valve from water ingestion, wrong spark plugs, timing module issue , fuel pump, fuel filters, fuel hose, anti-syphon valve
and most likely some things i forgot to mention.
What did the plugs look like when removed, are they the correct ones, mechanical fuel pump or electric, a engine serial number would be helpfull also.