140 Engine tapping noise when hot

[bmiszuk]

We had the boat (1974 trihull Merc 140 I/O) on the lake for the weekend and had a great time. However after the engine gets hot there's a tapping noise that seems to be coming from the top half of the engine. It's a metallic tap rather than a heavy clunk, gets louder under heavier load (more people on the tube or in the boat), and is most noticable above 2,000rpm.<br /><br />What's the best way to diagnose this? I figured I'd pull the valve covers and take a look, but am not sure what I'm looking for. A recent compression test looked pretty good.

 

[Richard Petersen]

Re: 140 Engine tapping noise when hot

Does the engine sometimes like to keep running a few seconds after the key is turned off? If yes, it is probably carbon buildup in the engine. Let us know.

 

[bmiszuk]

Re: 140 Engine tapping noise when hot

Yep, that has been a problem. Now I tend to leave it idling (if it's safe to do so) to avoid the run-on. It did it once this weekend when I finished a run and turned it off quickly.

 

[Bt Doctur]

Re: 140 Engine tapping noise when hot

Could be some carbon, poor engine cooling,incorrect timing,too hot sparkplugs,lean carb.

 

[bmiszuk]

Re: 140 Engine tapping noise when hot

Could be some carbon, poor engine cooling,incorrect timing,too hot sparkplugs,lean carb.

Timing is correct, engine runs cool, correct spark plugs are in, and I recently adjusted the carb. This sounds like metal beating on metal, like perhaps a valve that's too tight or something like that. But now we're at the end of my diagnosis knowledge -- I don't know what to look at next. I assume pull the valve cover but then what?

 

[Mahoney]

Re: 140 Engine tapping noise when hot

try grabbing a can of seafoam and following the instructions on the can next time you take it out for pouring it directly into the carb.<br /><br />I had the same noise and this solved it in a half hour. Hopefully thats all it is, is some carbon build up on some valve seats or something.<br /><br />Did you recently get this running again after a long sit or engine work?

 

[bmiszuk]

Re: 140 Engine tapping noise when hot

This is our third year with the boat. The previous owner probably had it parked for a while but I don't know how long. I'll try the seafoam and see what happens.<br /><br />Thanks for all the replies.

 

[cneste]

Re: 140 Engine tapping noise when hot

If the sound is coming from the top half, It could be a loose pushrod/ rocker arm or a sticking lifter. I would try some marvel mystery oil to see if that quiets the engine down some.Also use some good high quality/ high octane gas<br /><br />Chuck

 

[Richard Petersen]

Re: 140 Engine tapping noise when hot

Your engine sounds like "pinging" has advanced to "detonation" which does sound in some engines like metal is being hit. A shock wave of early firing fuel in a cylinder is smacking everything in the cylinder as the piston comes up. It will ruin that cylinder if you can't clean it out with the sea Foam. Don't keep running it that way. $$$$$ to someone. Some carbon can get so hard that sea foam takes 2 or 3 loads to clear it. Others need a head pulled and scraped clean. What causes this? sticky chokes, idle mixture too rich, old dried up gas in the carb jets that never dissolved with new fuel etc..

 

[Mikevn1500]

Re: 140 Engine tapping noise when hot

Broken/missing bracket on Alt,Vibrating against the block. Check it out

 

[waterone1@aol.com]

Re: 140 Engine tapping noise when hot

Do you have an oil pressure gauge ? How does the oil pressure look when the engine is hot ? When was the oil changed last ? Does the oil look contaminated (light brown or milky) ?

 

[bmiszuk]

Re: 140 Engine tapping noise when hot

Oil pressure is 40 when hot and running, about 15-20 at idle. It seems okay. Oil never looks milky or contaminated, just darker.<br /><br />Do I need to use the SeaFoam on the water or can I do it in the driveway?

 

[Richard Petersen]

Re: 140 Engine tapping noise when hot

Do you think you can do it in the water? It is a helluv a lot safer to the engine than a garden hose. Plus, your neighbors will still talk to you when all the smoke comes out. A breeze away from other boats or people is nice.