Mercruiser Knock.

[Jed Brown]

I have a 1988 4 Winns 245 vista with a Jasper rebuild.

Motor will run great with no load.

Under load starts to knock at about 1800 RPM.

Motor does not jump or shake with Knock.

It will run for hours trolling with no issues. (at about 1000 RPM}

When boat is in neutral you can bring it up to higher RPM's and there is no knock.

Only knocks under load.

Any ideas????

 

[gasman2438]

SOUNDS LIKE A SPARK KNOCK TO ME CHECK THE TIMING.

[HB2]

Low,octane fuel ? 

[1 more drift]

Could also be carbon build up on pistons,or a piece of carbon off a valve Yyou mentioned hours of trolling.If after checking the timing and fuel octane and you still have the knock get a can of GM top engine cleaner and follow the directions on the can, or you can also use  Sea Foam to clean the valves and decarbon the cylinders. 

[Jed Brown]

Found it,

Compression check 3 And 5 are dead.

I pulled the  head and there is a channel burned between the cylinders.

Started as blown head gasket and eventually ruined the head.

Replacing head today. 

Thanks LOU.

For the input.