1990 Mercruiser 4.3 175hp intermittent knock 4002 4900 RPM

[Kwpeterson@aol.com]

Engine has good static compression 150 lb per cylinder new ignition wires plugs, new rotor cap and pick up sensor. Carburetor is 2 Barrel rebuilt with set up float perhaps on the high side spec.

Im running a 17-pitch prop for out of the whole power for a slalom skier so engine will tack up to 5000 RPM

On misfire or knocking sound feels like an actual misfire... and not just a knocking sound. Knocking sound is somewhat like hammer on engine block case.

The fuel is non-oxygenated and new.



Id appreciate any ideas.

I have all winter now to think this through as I'm in Minnesota and just winterized the engine prior to 27 degree weather.
Thank you for any input or ideas!

Kerry

 

[alldodge]

Being a 1990 it shouldn't have points, but if it does check those.
Points or electronic check the cap and rotor, to look for anything that looks abnormal. All contacts should show the same tracking. Grab hold of the rotor and see if there is any side play in the distributor shaft

 

[Scott Danforth]

check for crud in the fuel

check your fuel pressure prior to the carb

check to see if your wires are arcing

 

[QBhoy]

Sounds a strange one. Good advice above. Agree to maybe be looking for signs of water in fuel. Apart from that, the rpms you are turning won’t help matters in general. Not supposed to be seeing those rpms really.

 

[Kwpeterson@aol.com]

All good ideas above but everything has been checked fuel wise with new mechanical fuel pump new fuel lines the unit has a Sierra fuel filter with large cartridge. The distributor cap and rotor are new including the electronic pickup sensor. The fuel pick up line in the tank has been replaced and fuel tank has been cleaned and vacuumed out. No debris.

The engine specs talk about normal operating
4400 to 4800 RPM at high-speed for my skier relates to about 34 miles per hour that we ski at and need the 17 in 4 power out of the hole with a slalom skier.
Need more ideas

 

[Bondo]

Ayuh,..... Welcome Aboard,..... Inspect the spark plugs for color after a Wot run,......

I'm guessin' yer gonna find white clean plugs from a lean condition, that sets up detonation, the noise yer hearin',.....

 

[Kwpeterson@aol.com]

Guess I'll have to wait on checking the plug color they are new and AC Delco. I've now winterize the boat prior to freeup nights and when I winterize I usually run the engine on a heavy oil gas mix for a few minutes and then run the carb dry for over the winter.

I had done a Quick Check of the timing just after the cap rotor spark plugs and wires but I forgot to use the shorting Wire 4 Baseline timing. My boat does not have points but Thunderbolt electronic ignition. In the back of my mind I was thinking some pre detonation was going on due to timing being Advanced too much.

I did set the float on the high side of the spec in the hope I was not going to face a lean condition at near wide-open throttles.
Keep the ideas coming guys! And gals!

Thank you