Yamaha 70hp knock after bored cylinder

[Kyle4512]

I have an 89 Yamaha 70hp etlf. I just had the #2 cylinder bored out and I put the oversized piston and rings on. Compression is back up to 119-125 in all cylinders (125 being the bored out cylinder). The engine seems to run great except for a faint knock that I can hear at lower rpm's. I can't hear it at higher rpm's, but it may just be the engine noise is drowning it out. It's definately coming from somewhere in the powerhead. Does anyone have any ideas as to what might be causing this? If you need more info just ask and I will tell. Thank you!

 

[robert graham]

Re: Yamaha 70hp knock after bored cylinder

If new piston/cylinder bore clearances are correct then it shouldn't be #2. Maybe you're listening a bit too closely to the point of imagination? Are you running the double oil mixture for new motor break-in? I've heard things before in my 90HP and my wife says, "noise is in your paranoid head", and you know what, she was dead right! Good Luck!

 

[Kyle4512]

Re: Yamaha 70hp knock after bored cylinder

Thanks for the quick reply. I am running 50:1 in the tank in addition to the oil injection for break in. The knocking isnt very loud and my wife basically said the same thing about it being my imagination, but after listening closely she can now hear it too. I could just ignore it and run it like it is, but I don't want something possibly minor to turn into something major.

 

[robert graham]

Re: Yamaha 70hp knock after bored cylinder

You sure it's in the powerhead and not the drive shaft or lower unit? The cylinder/piston clearance would have to be out a good bit to create a piston slap, otherwise maybe a connecting rod or crankshaft bearing? Good luck and let us know what you find!