2003 mag 350 clunking noise at startup no power

"2003 mag 350 - fresh water co

"2003 mag 350 - fresh water cooled - bravo 3
150 hours - starbord engine

When I crank the engine - after 5 seconds of crankning the engine makes a loud "clunking noise" (like a sudden stop to the engine) - like a cylinder is filled with fluid and cant compress - 1/2 the time the boat will start immediately after this sound, 1/2 the time it wont. When it does start it idles at 750 rpm but seems to have a missfire (the idle is smooth on the tac but you can feel the misfire if you hold the engine). The boat was scanned and no codes were found. All the fluids were sent to a lab for diag (oil, drive lube, etc.) and no problems found (no water in oil, etc..) Oil level is good, drive lube level is good. I tried running the boat on different gas hoping it was water in the gas - no change - the port engine runs good. The water pump impellor was recently changed - When it does run - you can rev it with no problems (over 3000rpm easily) other then the misfire to about 2000 rpm - after that you dont notice it - when it is in gear you can go about 5mph with no problems - if you try to accelerate you get the enging warning noise and no power from the one engine (engine automatically reduces rpm)
any help is greatly appreciated
keith
brick nj [email protected]"

"Sounds to me like a down cyli

"Sounds to me like a down cylinder for some reason.At higher rpm it is not as noticable.Check your plug wires closely,spin the engine a few times and pull the plugs looking for one (or more)loaded with fuel from not firing.Might of bent a pushrod creating valvetrain noise....Todd"

"If you get an engine alarm th

"If you get an engine alarm there should be a record of why in the Fault History (Freezframe Memory).

As suggested by Todd, pull the plugs and check for signs of water (rust). Do a compression test while the plugs are out. Does it have dry joint exhaust? If it doesn't, you need to pull the elbows for an inspection.

Also, check the distributor cap for moisture and corrosion. The HVS distributor is known for corrosion issues."

"...engine makes a loud &#

"...engine makes a loud "clunking noise" (like a sudden stop to the engine) - like a cylinder is filled with fluid and cant compress - 1/2 the time the boat will start immediately after this sound"

If you find 7 dirty plugs and one clean plug; suspect water since it will clean the carbon off a plug when running.

update - replace cap = rotor =

update - replace cap = rotor =
rotor looked good
cap has corosion on terminals
fixed clunking problem and start up problem
no more misfire
thanks to all for the help - now new problem - see new post