Blown Merc 150 BlackMax

[MathewsMan]

Dropped the top piston ont he left bank last weekend. All other cylinders are dead even at 110psi compression. Piston in question doesn't even show up on compression gage.<br /><br />Questions is: Replace one piston or all at the same time? It would seem the best thing to do is replace them all with complete power head rebuild. Just wondering if anyone has has any experience with just changing one piston.<br /><br />Have not torn motor down yet. But I suspect I have burned a hole in the piston based on the compression readings I took.<br /><br />First time outboard rebuild, however I have done automobile engines in the past.<br /><br />MathewsMan

 

[KCLOST]

Re: Blown Merc 150 BlackMax

$$$'s answer that question.<br /><br />Take a look and that will tell you which way to go...

 

[LubeDude]

Re: Blown Merc 150 BlackMax

Sorry to hear of your engines demise.<br /><br />What year engine are we talking about?<br /><br />First thing you need to do is figure out why it blew. Otherwise if you rebuild it and use the same carbs and all you may have it happen again. Not a good thought.<br /><br />You need to pull the head on that side and see if<br />you have a hole in a piston or a scored cylinder.<br /><br /> If its just a burned piston, you just have to hone and change that one piston, if its scored badly you will have to bore that cylinder which requires a complete tear down. I have heard of people replacing just one piston without pulling the powerhead but not sure if it can be done with this engine.

 

[MathewsMan]

Re: Blown Merc 150 BlackMax

This is a 1995 Merc Black Max 150hp. <br /><br />Not sure why it blew. I went out fishing all day long...fired everytime I hit the switch with no problems. Ran smooth all day ect. Headed back to the landing, on plane, 5000rpms trimmed and she just powered down on me and quit. Refired and seemed OK. I reasoned it may have just been a bubble in the fuel line or something. Went a little ways and same thing happened. Refired and thats when i heard what sounded like a dead cylinder. Got towed in by a friend from there so as not to cause further damage. <br /><br />Haven't had a chance to tear it down and see exactly what the problem is. I strongly suspect its a hole in the piston. It doesn't knock like its a bearing or connecting rod.<br /><br />Honestly, I have no idea what even causes a hole to be burned in a piston?? I guess that would be a good thing to know. So if anyone knows what causes that, please let me know.<br /><br />MathewsMan

 

[LubeDude]

Re: Blown Merc 150 BlackMax

When you say that you were at 5,000 RPM trimmed, do you mean that WOT is just 5,000???

 

[gss036]

Re: Blown Merc 150 BlackMax

Holing a piston is normally caused by an extra-lean running condition. I have never holes an outboard, I have done it to 175-250cc Yamaha 2 stroke dirt bikes. Back then they would put on a thicker head gasket to lower the compression a little.

 

[MathewsMan]

Re: Blown Merc 150 BlackMax

I wasn't at WOT. I usually try to pull the RPM's back a little when I am on plane and the water is slick. No sense max'ing the motor out when its not necessary. So I put the boat on plane using WOT....trimmed motor up, then eased back on the throttle.<br /><br />Thanks for the info on Piston holes!<br /><br />Reckon its possible that theres a broken valve or stuck valve preventing the piston from reaching any kind of compression?<br /><br />MM

 

[KCLOST]

Re: Blown Merc 150 BlackMax

"Reckon its possible that theres a broken valve or stuck valve preventing the piston from reaching any kind of compression?"<br /><br />Nope! 2-strokes don't have valves...

 

[MathewsMan]

Re: Blown Merc 150 BlackMax

Yeah....guess that makes sense.<br /><br />Fingers working faster than brain....Sorry.<br /><br /><br />MM