Yamaha 700 engine rebuild

[Crisprieto140]

Hi all, picked up a wave venture 700 and 1100 which have compression problems on one cylinder each ski. Took apart the 700 yesterday and this is what I'm looking at. I'm wondering if it's possible to only replace the bad piston and have that cylinder bored or do I have to do the whole thing. I'm also concerned that I'll have to resurface the head because of the dings. I added some pics below. I'm new to this and would appreciate any input

[grumpy_steven]

You need to address the cause of that piston damage. The top of the piston looks like damage from a lean mixture, often caused by a bad crank seal. The damage on the piston sides looks like faulty oil injection. Whatever the cause, you need to address the problem before putting the motor back together, or you'll have the same problem after one or two rides.

[Crisprieto140]

Thanks for the quick reply. A few things I forgot to mention. The guy I bought the skis from claimed he was a mechanic and that they had good compression but needed fuel pumps. So to start them for me he ran them on straight starter fluid no oil in the head in the tanks. He ran them very briefly but I'm thinking this was the cause? Both skis have the same problem and they were otherwise well cared for. I already went through the carbs and they were pretty dirty but I don't think they were awful. I'm trying to avoid tearing into the lower case to replace those seals if I can avoid it.

[Crisprieto140]

Forgot to mention we did shoot a ton of WD-40 in there before resorting to starter fluid. I'm gonna run premix to rule out the oil issue.

[arctic388]

on older bottom ends like these i just replace with a SBT engine and make sure carbs are clean and set right and then ride on. the thing about just replacing piston is the bottom end bearings are questionable and seals could be bad. if you just refresh top end it will not last long if bottom end is not 100%.

[Crisprieto140]

I thought about a new bottom end but I'm really trying to rebuild it myself if I can. It would be around half the price and the motor only has 150 hours on it

[Dave in DE]

If your going to rebuild it be sure to at least flush out the lower end if you don't pull apart and reseal. That piston left debris all over inside. Pull the intake and clean the reeds as well.
Beware of sbt motors. I use there parts but never a motor.

[Myself]

Actually the lower end looks very clean, no heat discoloration, thrust washers intact, no aluminum residue sitting in the port channels. Is one locating pin for the top ring missing? Looks like the ring caught the exhaust port, broke that little piece of the piston, then the excessive heat scored up the exhaust side. If it was oil loss the whole piston would look like that. The head is minor damage, make sure no hard metal is stuck in it and run it. With the rod to one side you should have a good view of the reeds but I'm betting they look good. Wash on that rear piston also looks ok. I'd probably hose that lower down with brake cleaner. Then shove a paper towel right at the lower rod bearing and rotate the crank slowly. It will smash the paper towel against the case and wipe it clean and come back up. Do it a few times until clean. If at any time you see more than 2 or 3 metal particles on the paper towels the lower end needs split and cleaned. I really don't think it's the case here. I can't see the cylinder wall that good but spray it with wd40 and hone it for about 60 seconds. Then clean it and see what it looks like. If you're lucky it will hone out. At that point you can rob a piston from the 1100, drill a hole on the intake side like the 701, put it together with a few new gaskets, and ride it while you give the 1100 a proper rebuild!!

[Crisprieto140]

So I decided to just bored both cylinders and do the full top end wsm rebuild. My idea was to pull the good piston from the 700 into the 1100 and call it a day but I guess they aren't identical? That's a good suggestion I didn't thinking of flushing the bottom end. Is brake cleaner the best for that though, I feel like that would be hard on the seals.

[Myself]

I don't think the 1100 has the cooling hole in the piston. It also will work fine like it is. You can use the good 701 piston if you want. I use the non-flammable brake cleaner in a red can from Wal-mart. Works great and won't hurt the rubber.