[WRIGHTWARREN]

I Have A 150 Mariner.went To Lake,started Right Up,wot And Started To Plane And Then It Started Making A Knocking Sound And Died.started It Back Up And Still Knocking But Got Back To Ramp.got Home And Checked Compression.1 And 2 Cylinder Have No Compression.the Other 4 Have Around 115.what Would Cause These 2 Top Cylinders To Quit At Same Time? I Was Told There May Be A Compression Relief Valve Keeping It From Having Compression But I`m Not Sure.any Ideas Or Help Would Be Great.thanks

[bully-85]

more than likely you burnt a piston and or broke some rings. inspect the spark plugs for signs or molten aluminum ( dry grayish color). if you have access to a flexible light that you can get into the spark plug hole look toward the center exhaust side and inspect the piston edges for damage.

[WRIGHTWARREN]

All I See Is A Little Bit Of Carbon.nothing Out Of The Ordinary.is It Normal To Lose Both Top Pistons At Once Like That?

[j_martin]

Usually a cooling problem will stick the top two pistons like that. If you borescope it you will find aluminum stuck to the cylinder walls.

sorry
John

[WRIGHTWARREN]

Thanks For The Info.im Going To Pull The Heads And Have A Look.both Pistons Still Move Up And Down When The Engine Turns Over But Im Sure The Walls Are Scored And The Rings Are Shot.thanks

[WRIGHTWARREN]

yep,just like you said.pistons are shot and walls are scored.i think i will find another outboard rather than fix this one.thanks for the help and info.