Hey Jeff, I work on my own motors, auto and boat. Start by first checking the vulcanized rubber between the prop and the prop shaft receiver hole in the propeller where the prop shaft goes through. This is a piece of metal with fluting on it to fit over the prop shaft. This rubber breaks if a prop hits a rock. Other motors, johnson, evinrude, yamaha have a shear pin that shears if you hit a rock. Alot of merc props have this vulcanizing to hold the prop to this piece of metal with the fluting. To me its one of the down falls on some merc props. It looks like a one piece prop until you look close and see this thin rubber vulcanizing strip. If its taken a hit just once or a very few times it won’t knock until you put the prop under a heavier load like higher rpm’s. It might be this because its common for merc props to do this, check this first. It will jump like heck if this vulcanizing is broken. If the vulcanizing in the prop is ok, for shure, check and see if the clutch dog in the lower unit is worn. It will do this when its put under a load but not as much in a merc but more in evinrudes and johnsons. Try these two things first. If its neigther one of these things It its a bearing or oil pump problem. It could be a lower bearing in the power head and the drive shaft is wobbeling out of round but i doub’t that it is this on this new of a motor and its rare that this happens. If it has an oil filter see if its clogged. Usually at higher rpm’s the oil pump works better so knocking is less likely to happen. If the oil pumps worn too much its more likely to knock at lower rpm’s. Outboards unlike auto motors have needle bearings between the rod and crankshaft and between the rod and piston. If your going to run it, run it slow and have it checked as soon as possible. Its not pre ignition at those rpm’s or carburation, its mechanical and it sounds like its in the power head{motor]. Does it do it on every stroke or just every other stroke. If it is a bearing going out don’t rev it much, a rod could go through the crankcase. Most of the time its the lower piston from having to much fuel in the bottom of the crankcase from flooding over a period of time and its been wearing excessivly on the lower cylinder between the piston and cylinder wall. The lower cylinders are the ones to usually go out from excessive gas mixture, thin oil, not enough lube over a period of time. I worked on a motor for some people that had the same problem, replaced the piston and it blew anyway because the clearences were’nt tight enough between the piston and cylinder wall. I put a new piston and rings in it but there was still too much clearance thus it blew. It was eigther that or another power head. On alot of outboard motors you can’t buy over sized pistons like you can when rebuilding a car motor. Its one of those things that if you put a new piston in it, it might run for awhile but because theres too much clearence it’ll happen again. Take it too a marina and have them check the compression on the cylinders, if its low on the lower cylinder you can bet its worn too much. If its unrepairable try finding a new powerhead, the rest of the motors ok. Get ready for a price shock while you get the news on your motor. If you do the shopping for your own power head you’ll save money. A knocking sound usually is prop vulcanizing first from the prop hitting something, clutch dog wear second from shifting at too high of rpm’s, rod and bearing related third. The first two are the cheapest to fix, try these first. Put on another prop and if it still does it, go to the second, cluth dog. You can do this or a marina, its a little more involved. Get a schematic and take the lower unit apart and reassemble in order of schematic, new rubber o ring seals and prop shaft seal and clutch dog, remember to put the very thin metal o ring spacers in the way they came out. They are machining spacers for gear clearances. If in doubt take it too a marina. If your mehanicaly inclined you might give it a try on the lower unit but you’ll need a spanner wrench to take out the rear hub around the prop shaft. All this sounds hard but it can be done by taking your time and keeping things in order when disassembling and reassembling the lower unit. Try the prop first. Hope this helps and its a cheap fix.
