82' 350 260hp mercruiser clatters/knocks around 2000 rpm

[Witt]

Just bought boat had shop check it out before buying it! Said compression was good boat ran great but could use tune up and maybe carb rebuild. Pit boat in started up sounded great tried to get up on plane and around 2000 rpm it knocks or clatters!!when in nuetral it revs up and sounds good no loss in oil pressur or high temp.put cap roter and condenser in as well as plugs, fuel filter, water fuel filter all oils. It really sounds good at idle or nuetral what could this be? We did notice one plug was white in color! Any idea??? Sorry only under load

 

[WoodieMan]

What came first, the knocking or tune-up? You can also try hooking up a portable gas tank to your fuel pump and go for a ride. If it runs good, the problem is from the engine to the boat's fuel tank; line, anti-siphon valve at top of tank, pick up tube, fuel.

 

[Witt]

Knock came first. We hooked up separate fuel tank today same noise!

 

[Witt]

Knock came first. Tried seperate fuel tank today same noise

 

[chiefalen]

Thats why you never buy a boat for more then 500 dollars that you did not run on the water first.

Do a compression test again on a warm motor, all the plugs out and the throttle fully down.


Maybe do a vacuum test before the compression test.

Post back the result.

 

[bobbyc]

What is the timing set at???? What is the point dwell??? Put some high test gas in that separate fuel tank to see if the knock is pre-ignition...if only one plug is white, check for a manifold vacuum leak...check the firing order to be sure the wires are correctly attatched