Imminent motor rebuild, advice/tech info needed.

[Benwaahh]

Late this season my ski began to run a little funny. It felt out of tune as it lost some bottom end snap. I decided to rebuild the carbs and check out the reeds, but as it turns out I have to pull the motor to do so. Next thing I know, the motor is almost completely torn down and my ski is in more pieces than I'd ever imagined were involved. Needless to say I went overboard. I've never done this before, never done so much as a bolt on upgrade to anything that moves. I've done a good amount of research and I understand the general theory behind what makes these things move. However now I'm in need of the finer details of what I even have in this ski, and how to best move forward on the problems I have.

This was sold to me described as a ported 760. I've ascertained that I have a 62t\62t with 85mm pistons.

Pics soon to come, as they aren't sending over to my tablet at the moment. I'll have to login and upload from my phone.

 

[Benwaahh]

First time uploading on tapatalk. This might take a few tries.

 

[BruceSki]

Mmm bearing chunks. Goes good with a side of bad crankshaft.

 

[Benwaahh]

 

[BruceSki]

Looks like ported cases too. You should be at 771cc's at 85mm bore (I think)....

You'll need a new crank by the looks of those Pistons. Basically complete engine rebuild as long as you didn't grenade it real bad and break a sleeve or cases.

 

[Benwaahh]

So clearly something went terribly wrong at some point. What I noticed is strange is that the gash in the side of the piston doesn't continue through the rings. Makes me think that this was previous damage, someone tore it down, replaced the rings and called it a day. There's a small spot in the transfer port that matches up with the piston ring gash as well.

 

[TheCoz43]

Jetmaniac has got everything you need for good prices. Answers all my questions quickly too.

 

[Benwaahh]

Sent jetmaniac a message, thanks for the advice. Had I been chewing up a bearing would I no doubt see some evidence of that in my crank case? It rotates smoothly by hand and I can't see any metal shavings from where I am in the teardown process. I'm going to split the crank case open tonight and see what's going on anyways. Just wishful thinking has me hoping the crank was replaced when those rings were done and for some insane reason the piston wasn't swapped too. I don't see how it could have scarred that piston and not damaged the ring.

 

[BruceSki]

Check your wrist pin bearing. I bet it's wrecked. Probably ruined the rod too.

 

[TheBuzzard]

that damage is from the cage that holds the main crank ball bearings in place, is coming apart. you need another crank. if you want to be cheap and just toss it back together, you can do that too. just replace the crank and smooth those dents in the dome and piston top. if the rings are free and the cylinder isnt gouged the compression shouldnt be off. another crank should get you back up and running unless there is something going on there that i dont see.

 

[Benwaahh]

Roger that, I'll pull the crank and be on the hunt for a new one then. Thanks for the advice. While I've got it down I don't see any reason not to replace that piston and dome at least. Any suggestions on how to identify what domes I currently have? There seems to be some sort of serial number etched in the top. I'll post shortly.

 

[Benwaahh]

Scratch that, it's a 31 mm dome. I was inputting that 1 as an I :/

 

[Harbor]

76 cc and 31 squish........not sure
760 cylinder 31cc...... Not sure

Yours is marked a little funny, mine has the bore size first then cc then squish. Sorry it's upside down yam 85.27.18

 

[Nuklear6]

Ada YA7631 means: Yamaha 760 (84mm-86mm bore) 31cc combustion chamber volume. WAY to aggressive compression ratio for pump fuel (unless domes have been modified).

 

[Benwaahh]

Wrist pin bearings, what you see is indicative of the condition all around. Salvageable?

I just don't know what condition this stuff needs to be in to require replacement. If they appear even remotely questionable I'm likely to replace them.

 

[Sparkplug]

Download the factory service manual. It can be found easily with Google. It has every single torque spec and has the specs for all the parts that see wear. There is absolutely no reason to replace anything that is still in spec.

It has blown up diagrams and step by step instructions on how to do almost everything. Do not be intimidated by these engines. They are about as simple as an engine can get honestly.

 

[BruceSki]

They look to be in decent shape. I don't take any chances though. It's about $40 for two new oem kawasaki wrist pin bearings that will fit that small end of the rod.

Split the cases and examine the crank next. You've gone this far. May as well be sure you have a good runner come springtime.

 

[JetManiac]

Ada YA7631 means: Yamaha 760 (84mm-86mm bore) 31cc combustion chamber volume. WAY to aggressive compression ratio for pump fuel (unless domes have been modified).

X2. Were you running race gas? Did you ever do a compession check?

My guess is your bore is damaged and would recommend bore and 2 new pistons. Looks like you have wisecos right now. Replace crank and pin bearings and rebuild as a new motor.

 

[Benwaahh]

I was not, the seller told me he was running pump gas. Probably should have checked. You live and you learn!

 

[Benwaahh]

Crankshaft out! I have no idea what I'm looking for here. I don't see any physical damage on the bearing housings, but I don't know of what's going on inside. They all spin freely and don't catch in any way, and I hear a faint whirl from them when I spin. The last one pump side seems to have a touch more noise to it, but also spins the best. Rod bearings look just like that the entire way around. There's about a quarter mm of play in the rods front to back. Also, thanks for your help thus far.