50hp Merc 2-Stroke - KNOCK KNOCK! :(

[MEllis]

Hey!
So I was driving my Spectrum 1606 with the 50hp Oil Injected Merc (1991?) and it was cruising along when all of a sudden the motor started a very consistant knocking sound. I immediately powered down and went to neutral. The knock is still there.
The engine doesn't smoke or anything, still starts up and runs fine (not that I've run it except to test in idle since), but this darn knocking sound is there and quite loud!
Anyone have any idea what this could be?? PS I noticed when I started it back up that there wasn't any water being expelled from the back of the engine...could the water pump be making this sound?

PLEASE HELP! THANK YOUUUU!

 

[fishingdan]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

You have reasons to be concerned. The water pump will not make a noise like that and the water pump may be the least of your concerns. The water pump is easy to fix and you can start you tab with $50 to $80 in parts for that fix.

You need to find out what that knock is. Unfortunately, it can be any number of things. Did you notice any loss in power when the noise started?

 

[rickdb1boat]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

I'd run a compression test on it. Could also be a rod bearing failure...

 

[MEllis]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

Hmm...crap! hehe...no there was absolutely no change in power or anything...just started seamlessly...took all 3 plugs out and rotated motor by hand...you could hear compression coming from all 3, but it is always one particular spot (seems TOC of one cylinder) that you get the metallic "rub" that is causing the loud knock...

 

[MEllis]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

Seeing as how I just bought this thing...and that was the first time out with it (even though the guy who sold it to us actually took us out in it ironically enough the day before with no issues)...do you think this engine is worth fixing or should I look into getting like an 80hp or something? any trade in value on this as-is?

 

[Scaaty]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

One cylinder at a time, by hand, rotate each piston up to top dead center, and as you pass TDC, go about another 2 inches or so (enough to still get at and push on the piston). Stick a screwdriver in there, and while someone is holding the flywheel, give it a push on the piston. Takes a little finesse to get the feel, but you will notice it. You can rock it back and forth in this position, and get a second chance on checking. You just want the crank pulling down on the piston, then you give it a shove, looking for slack. Get my drift?
ANY movement will tell you either the big or little end rod bearings went south

 

[diaric]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

question about your oil injection. is the oil tank on the motor, or a seperate resevior down with your gas tank. the external ones were bad for not getting oil up to the motor as it was such a long distance. these were safer to disconnect and use premix.

 

[MEllis]

Re: 50hp Merc 2-Stroke - KNOCK KNOCK!

Thank you all for your input!

The oil injection tank is on the motor, under the hood.