Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[joesnuffy]

Got the carbs off and reed valve assy. The screws on the reed block did not get damaged by spinning crankshaft so that’s a good thing the rod is not sloppy. I feel like I am missing something simple so I checked a few things. Both cylinders have over 1/4 inch of spark on each coil so that part is ok. I re-checked both high and low jets in carbs which also were clean. The high speed jets have pretty large holes and go straight up to the carb throat so it should get to WOT if everything else is working OK. The reeds looked good no light was coming through them so I don’t think they are the problem may be that one of the screws on the reed valve assy came loose after rebuild and went bouncing around on top of top cylinder dome and might have cracked cylinder dome or possibly cracked the piston sleeve. I re-checked compression it was 175lbs on the top and 180lbs on bottom with reeds and carbs off.

I am going to order some new gaskets for intake manifold, carbs, etc. I will also order a different reed valve assy. I am thinking about putting 2 gaskets under the read valve assy where it mounts to motor to give it a little more clearance unless someone thinks that’s a bad idea. I don’t think it could hurt and I could always remove it later if it did make motor not run right.

I already have a good used head and new head gasket on order so I am going to just slow down a tad on this project and think about it while waiting on parts. When I pull the head I will take a look at the top piston head to see if it has been damaged by a screw head bouncing around in there. It was a new piston so if it is damaged it happened after rebuild and it could have damaged the head possibly causing a small leak in dome. I will also take a good look at the bore to try and see if it might be cracked might not be able to see it with naked eye would probably need a pressure test.

The top spark plug looked to be washed I could even see the white cermaic part of plug like it is being washed by fuel or water. I think I will replace fuel pump also.

Thanks for the help,
Joe

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