5.7 merc engine noise

[dgopetactical]

I have a 88 merc 5.7 last week I changed the plugs and took the boat out over the weekend. the boat ran great all day when we got ready to come into the dock the engine backfired. That was strange so I turned around and planed the boat out I had hardly any power and could not go above 3,500 rpm. at that the time the the engine went boom and sounded like a steamboat but still was running with hardly no power. I found out that it spit out a sparkplug due to improper instalation on my part and the plug touched metal and fouled out bigtime.. We drove back on 7 with no plug in due to lack of proper tools. At home I put a old plug in the engine staterted up fine and runs go but it idals and little rougher than it did before (very little difference). I notice a kind of a noise like a random tap or ping comming from behind the engine where the exhaust goes into the lower unit. It almost sounds like it is in the left exhaust pipe. I want to take the boat out on the 4th to see if I still have HP loss. This is the first time I noiticed this noise and it is not that loud at all and with the engine cover on you cant hear it unless you concentrate. does anybody have a idea what my problem might be if there is one at all?
Thank you,
Dennis

 

[Haut Medoc]

Re: 5.7 merc engine noise

Welcome to iboats.....
I'd pull the riser on that side & look at the condition of the water shutter......
If it is not there, it may be down in the y pipe & you will need to get it out....​

 

[Don S]

Re: 5.7 merc engine noise

The noise is probably the exhaust shutter. If the rubber is worn or burned off the edge of the shutter, it will make a tapping noise as the shutter hits the exhaust pipe. Not a problem (won't hurt the operation), but will be a little noisy, the shutters can be replaced.

 

[dgopetactical]

Re: 5.7 merc engine noise

Thanks guys. I started it up tonight and listened with a scope pretty shure your right about the shutter making the noise. I am going to go for a test run tomorrow to make sure. you guys are great I thought I had big trouble.

 

[Coors]

Re: 5.7 merc engine noise

I found out that it spit out a sparkplug due to improper instalation on my part
What does this mean? the plug shot out; it was cross-threaded? it had water in the cylinder?

 

[dgopetactical]

Re: 5.7 merc engine noise

it must have cross threaded (no water) or it hung up and I thought it was tight. plug was hard to put in to begin with, and the way my engine is I was putting them in blind. I put a new plug in the cylinder and tripple checked everything. I am 100% sure the plug came out do to my carlessness. no thread damage thank goodness.

 

[Coors]

Re: 5.7 merc engine noise

Buy a vacuum hose that fits on the plug tip.
spin it in that way- you can feel it grab for 2 threads
That way, no cross-threading

 

[dgopetactical]

Re: 5.7 merc engine noise

thank you for the tip, I never thought of that.