5.0 Mercruiser sudden rough idle and bad miss

[Wave thumper]

Any ideas on this?

5.0 mercruiser, qjet, electronic ignition

boat is running perfectly until it develops a really bad miss while underway, and also at idle. So bad the motor sounds like it is running on 4 cylinders, and stalls at idle

I changed
the coil with new one
new ignition module
leads are all ok
plugs look ok but some were fouled with fuel and some sort of blackened when I cleaned them with the tissue
sometimes I get a black puff out the carb when I try to start it
oil looks ok - no water mixed with it
Then I replaced the dizzy with a good spare

All these checks/ replacements have made no improvement!

Any ideas? What do I do next?

I wonder if I have blown in the inlet manifold gasket?

 

[alldodge]

Re: 5.0 Mercruiser sudden rough idle and bad miss

I'm thinking you should do a compression test, may have a head gasket issue. You could also try pulling plug wires off one at a time to see if any of them have any effect?

 

[Bondo]

Re: 5.0 Mercruiser sudden rough idle and bad miss

Any ideas on this?

5.0 mercruiser, qjet, electronic ignition

boat is running perfectly until it develops a really bad miss while underway, and also at idle. So bad the motor sounds like it is running on 4 cylinders, and stalls at idle

I changed
the coil with new one
new ignition module
leads are all ok
plugs look ok but some were fouled with fuel and some sort of blackened when I cleaned them with the tissue
sometimes I get a black puff out the carb when I try to start it
oil looks ok - no water mixed with it
Then I replaced the dizzy with a good spare

All these checks/ replacements have made no improvement!

Any ideas? What do I do next?

I wonder if I have blown in the inlet manifold gasket?

Ayuh,.... Sounds like a Carb problem to me,....

 

[harleyman1975]

Re: 5.0 Mercruiser sudden rough idle and bad miss

Do a compression test to rule out a hard part failure (valves, piston, head gasket) and if that is ok then I am with bondo possible chunk of something in the carb.

 

[Wave thumper]

Re: 5.0 Mercruiser sudden rough idle and bad miss

UPDATE

Compression was OK

Removed the inlet manifold to find a small oil leak to 2 of the inlet ports.

Also found 4 inlet ports in the head totally flooded with petrol, those inlet valves were sitting in a pool of fuel

Then I checked the primary metering rod piston in the carb, it was stuck open.

So I believe the primary rods were letting too much fuel into the engine until she finally flooded and caused us to limp home

What do you guys think?

 

[Bondo]

Re: 5.0 Mercruiser sudden rough idle and bad miss

UPDATE

Compression was OK

Removed the inlet manifold to find a small oil leak to 2 of the inlet ports.

Also found 4 inlet ports in the head totally flooded with petrol, those inlet valves were sitting in a pool of fuel

Then I checked the primary metering rod piston in the carb, it was stuck open.

So I believe the primary rods were letting too much fuel into the engine until she finally flooded and caused us to limp home

What do you guys think?

Ayuh,.... Sounds like the carb needs a full, 'n complete Rebuild,...