Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Well, it certainly is possible that the flying rod debris initiated the reed problems. Looks like you/we were right about the reeds, but for the wrong reason! In any event, I would certainly change that reed block with a used one that you have ordered. That ebay reed block/manifold assembly looks a little rusty, but I see the rub marks on the screw. It is hard to say whether reed manufacturing problems caused the rod to hit the reed block, or whether that engine came apart like yours did slapping against the reed block. I’m sure the used rod you are using is OK.
I would probably use the reed block that was not hit along with a new gasket. I don’t know about Yamaha, but I am usually fearful of disassembling reed blocks. Altering leaf valve positioning, or flipping the leaf plate over usually causes the leaf valves to break. I would probably check all the screws to ensure they are tight, and make sure the leaf valves look OK and are seated properly/not rusted. If the screws seem loose, you could remove them one at a time, then carefully reinstall the screws with a drop or two of Loctite. I would probably use a few drops of Loctite on the two screws that hold the reed block to the manifold plate with a new gasket as well.
Unfortunately, I don’t know if this was a known Yamaha problem, hopefully someone will chime in.
Do you know what caused the original rod failure on your engine? It doesn’t seem like rubbing on the reed screws would cause the rod to fail, I’m thinking the messed up reed block was collateral damage.
I don’t know just how much clearance there is between those reed blocks and the rods. You could try assembling the new reed block to the manifold plate without the gasket first, then snugging the manfold in place. Now, slowly turn the crank listening for rubbing. If the rods hit the reeds without the gasket in place, I’m thinking there isn’t enough clearance, and you should investigate with Yamaha for known problems/fixes for this situation.
I’m glad you found the problem, sorry you had to rip into this thing after rebuilding it, but happy the powerhead is not damaged….