1985 305 Mercruiser Engine Noise

[kingmike]

I have a 1985 Mercruiser 305 that is making a "thumping" sound at idle. When the motor is revved up to about 1500-2000 rpm the sound stops. I recently had a mechanic install a new carberator and new spark plugs. I found that he had crossed the no. 4 wire on distributor with the no. 7 wire and also had unplugged the no. 3 wire because he thought the thumping was in no. 3 cylinder. I have not run the boat in the water yet - just backed in and let it run on trailer because I don't want to tear up anything. The boat has been sitting up for over a year and I wonder if a valve might just be sticking and if it might unstick if I run it? I also wonder if engine could just be a little out of time - it seems to idle OK.

 

[Bondo]

Re: 1985 305 Mercruiser Engine Noise

The boat has been sitting up for over a year and I wonder if a valve might just be sticking and if it might unstick if I run it?

Ayuh,..... Welcome Aboard Mike,...

A Compression,+ Leak-down Test will give you That answer...
And,...
Running it til it get's Better,..??..?? Usually Don't happen.....

Doesn't a Complete Tune-up come with the New Carb,+ Sparkplugs,..??
The Timing is part of the process....

 

[kingmike]

Re: 1985 305 Mercruiser Engine Noise

I am wondering if the mechanic has screwed something up inside the engine by having the no.7 & no.4 wires switched around on the distributor? Could this have resulted in the "thumping" sound that I now hear?

 

[bomar76]

Re: 1985 305 Mercruiser Engine Noise

not really.

 

[kingmike]

Re: 1985 305 Mercruiser Engine Noise

I appreciate the input, I'll see about getting a compression test and leak-down test done. Just wondering - could the "thumping" sound that I hear be the valve hitting the piston?

 

[bomar76]

Re: 1985 305 Mercruiser Engine Noise

They usually only hit once, and that's to put a nice clearance hole in the piston.

 

[Maclin]

Re: 1985 305 Mercruiser Engine Noise

....and that sounds more like an aluminum bat cranking out a good line drive, kind of like "PINGK".

 

[kingmike]

Re: 1985 305 Mercruiser Engine Noise

So the "thumping" sound might not be a valve? - I don't know much about engines really and especially not the odd sounds that might indicate a problem. I wonder if this sounds like a real major problem or can the boat be run as is?

 

[bomar76]

Re: 1985 305 Mercruiser Engine Noise

As Bondo said above:

A Compression,+ Leak-down Test will give you That answer...
And,...
Running it til it get's Better,..??..?? Usually Don't happen.....

Doesn't a Complete Tune-up come with the New Carb,+ Sparkplugs,..??
The Timing is part of the process....

And never had a boat heal itself....

 

[fishjumped]

Re: 1985 305 Mercruiser Engine Noise

help how do I properly send out questions re my 260 mercruiser and recieve answers back to my account nam fishjumped...???? help

As Bondo said above:



And never had a boat heal itself....