Mercruiser 3.0 rough running

detrlions10 · May 26, 2011

Well, I have tried several attempts to get this boat running correctly, but I have failed up to this point. Basically, after running good for about 30 minutes, the motor feels like it is starting to miss. It will not pick up speed and is very sluggish. If I shut the boat off for awhile, I can restart it and it will run good for about another 20-30 minutes.

I originally thought it was fuel related, so here is what I have done so far -

Emptied the fuel tank, filled with premium.
Added Sea Foam to the gas
Replaced the fuel pump filter
Replaced the carb filter (and fuel line)
Added a fuel/water separator

What else could be wrong? I need some help here. The boat is a 1988 Sea Ray Seville. The motor is a 3.0 Mercruiser 1988

Bondo · May 27, 2011

Re: Mercruiser 3.0 rough running

detrlions10 said:
Well, I have tried several attempts to get this boat running correctly, but I have failed up to this point. Basically, after running good for about 30 minutes, the motor feels like it is starting to miss. It will not pick up speed and is very sluggish. If I shut the boat off for awhile, I can restart it and it will run good for about another 20-30 minutes.

I originally thought it was fuel related, so here is what I have done so far -

Emptied the fuel tank, filled with premium.
Added Sea Foam to the gas
Replaced the fuel pump filter
Replaced the carb filter (and fuel line)
Added a fuel/water separator

What else could be wrong? I need some help here. The boat is a 1988 Sea Ray Seville. The motor is a 3.0 Mercruiser 1988

Ayuh,... Plugged tank vent, Bad anti-siphon valve, Bad carb,...

detrlions10 · May 27, 2011

Re: Mercruiser 3.0 rough running

Thanks for the reply. I have taken the gas cap off while it is running rough, but it makes no difference. I think that can rule out a plugged tank vennt. If it was a bad anti-siphon valve, would it run good for 30 minutes or so? I don't know the symptoms for that. As for the carb, I am guessing this is the issue. Would it be worth it to try and do a rebuild kit? I also failed to mention that I changed the plugs and wires. Would the distibutor make it act this way?

Thanks

detrlions10 · May 27, 2011

Re: Mercruiser 3.0 rough running

Any more thoughts on this? The part that throws me off is that is runs so good for about 30 minutes. It has done this the last three times I have taken the boat out. Any suggestions would be welcome.

Don S · May 27, 2011

Re: Mercruiser 3.0 rough running

Could be dirt in the tank. May take 30 minutes to plug up the screen in the fuel pick up tube in the tank. When you shut it off, the vacuum on the debris is broken and it falls off. then when you restart it plugs up again.
IF you has a lot of very old varnished gas in that tank before, then drained to put in the premium the ethanol in the new gas will clean the varnish off the tank.

detrlions10 · May 27, 2011

Re: Mercruiser 3.0 rough running

Don S said:
Could be dirt in the tank. May take 30 minutes to plug up the screen in the fuel pick up tube in the tank. When you shut it off, the vacuum on the debris is broken and it falls off. then when you restart it plugs up again.
IF you has a lot of very old varnished gas in that tank before, then drained to put in the premium the ethanol in the new gas will clean the varnish off the tank.

Do you think I should pull the tank out and clean it? A siphon is most likely not getting it all. If there is dirt, it will be in the very bottom.

alilley · May 27, 2011

Re: Mercruiser 3.0 rough running

if your tank is ease to get to y not pu;; the sending unit out and inspect and go from there.

Howard Sterndrive · May 27, 2011

Re: Mercruiser 3.0 rough running

get an outboard fuel can, connect it to the supply to your fuel pump and go for a run and see if it acts up

did it ever run right for you? or is the boat new to you?

detrlions10 · May 30, 2011

Re: Mercruiser 3.0 rough running

Howard Sterndrive said:
get an outboard fuel can, connect it to the supply to your fuel pump and go for a run and see if it acts up

did it ever run right for you? or is the boat new to you?

The boat is new to me. Unfortunately, I did not read this prior to getting at the tank. Now I am forced to remove the tank due to a bolt snapping off at the tank. This boat is becoming a nightmare...

makinc80 · May 30, 2011

Re: Mercruiser 3.0 rough running

i had a similar problem and went through the same steps as you. a rebuild on the carb took care of it! i did it myself over the past winter and it only took me a few hours. i am pretty handy with a wrench. just take your time and follow the directions carefully. my float was way out of spec. runs great now!

detrlions10 · May 30, 2011

Re: Mercruiser 3.0 rough running

makinc80 said:
i had a similar problem and went through the same steps as you. a rebuild on the carb took care of it! i did it myself over the past winter and it only took me a few hours. i am pretty handy with a wrench. just take your time and follow the directions carefully. my float was way out of spec. runs great now!

Good to hear, thanks for the encouragement. I need some at this point.

Mercruiser 3.0 rough running

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