Johnson 90 2-stroke knocking

just got a quick question, looking for some things to think about with my buddy's engine. not looking to drive this one to solution.

don't have the model number which I know is something you guys really like. basically his engine has developed a knock. you probably wouldn't know it at idle if you hadn't been around the engine. it is more perceptible at higher rpms. it really stands out when coming out of WOT.

the knock is not a pinging noise like from bad gas. i cannot exactly tell if it is coming from the lower unit or powerhead. there is no lack of engine performance. it is not a distinct (hammer to anvil) knock but rather a little muffled. the best thing i know of that sounds similar is a bearing that is wearing out.

i will be out on the boat tomorrow again. just curious what sort of things could be delivering this knock? i understand that list could be rather long but thought i would give it a shot.

Thanks for any pointers.

Re: Johnson 90 2-stroke knocking

Problem is the knock could be coming from the p/h or the g/c. You may want to check the g/c oil and see what it looks like. You should post a model #.

Re: Johnson 90 2-stroke knocking

You can check for play in the block.
Remove the spark plugs, rotate the flywheel by hand. Use a long thin screwdriver and insert it into the cylinder through the plug hole. Rotate the flywheel slowly and let the piston push the screwdriver to the top, rotate the flywheel about 1/4 turn and the piston will start going down. Use the screwdriver and gently push on the piston and see if there is any slack in the rod/crank bearings. It sounds a little crude, but if you have slack in there you will hear/feel it.

Re: Johnson 90 2-stroke knocking

Make sure you have the screw driver dead nuts in the center of the piston. If not you may mistake piston slap for bearing probs.

Re: Johnson 90 2-stroke knocking

I had an other wise great running 90 hp evinrude, but since I take it out in the ocean in winter, i didnt trust it. once I took it apart I found the piston skirt was slapping on one cylinder. it must have over heated at some point....had to buy new pistons( I just went and machined all 4 holes oversize...) and bought all new parts.

i had asked about what could cause it like you...thought and hoped it would be something under the flywheel loose.....no go

good luck

bob

Re: Johnson 90 2-stroke knocking

Two strokes are not prone to knocking. Those of you that know the internals of a two stroke know that. Most likely it's the gear case. It probably starts becomming pronounce around 2800 rpm and sounds more like a few marbles knocking around. It's probably the upper gear case bearing. You may or may not have water in the lower unit and may or may not find metal in the gear lube.

Of course, it could be the one problem that nobody guesses. If the gearcase lube was changed recently and filled all the way to the top it will expand when operating and cause sort of a floating action to take place that sounds like a knock or a hand full of marbles. Let an ounce of it out and try it.

Forgot to mention that it will happen when in neutral also. The drive shaft is still turning.

Re: Johnson 90 2-stroke knocking

can somebody give me a quick one liner on what piston slapping is?

the knock is not random. it's frequency matches rpms so i can understand how it could be some of the items above.

also random question; how hot will the water be coming from the engine once it has been running for a while, enough to make you pull your hand away? not talking about water from the pee stream as i know that most of the time that is just an indicator.

he will be having a mechanic check it out in the near future. once i know more details i will update the forum for the archives. Thanks for the help.

Re: Johnson 90 2-stroke knocking

Piston slap is when the piston skirt is worn in the cyl for some reason and the clearences are not correct.