Waverunner 760-rebuild or replace motor

[ronmvdb]

I have a 97 wave venture that the motor was replaced in 2002. It was a replacement from SBT and only had about 40 hours on it. The rear cylinder lost compression and seized the rings. The head is pitted and cylinder scarred. I see cylinder kits for sale but nothing about the head. What can be done to smooth the head out or does it need to be replaced. Am I better off replacing the motor again? Not too happy about it lasting such a short time. Is there anyway to determine the cause?

[torquenut]

Re: Waverunner 760-rebuild or replace motor

If you rear cylinder lost compression was it due to lubrication? how does the piston look is it a dull dry grey ? if it is you were running lean.

[ronmvdb]

Re: Waverunner 760-rebuild or replace motor

Both cylinders look to be a dark grey. They are dry but it's been setting since last Sept. It looks like I could get by with a top end rebuild but I'm not sure about the head being pitted. Is that a concern or should it be replaced? Thanks.

[torquenut]

Re: Waverunner 760-rebuild or replace motor

dark grey is lean they should have a nice chocolate to blackish color.
If there is a lot of pitting you may want to get a new head or else if a sand disk can work do that but only to take the burrs off not to mill the head.

[ronmvdb]

Re: Waverunner 760-rebuild or replace motor

Thanks for the feedback. I did rebuild the carbs awhile back and replaced the after market flame arrestors. I guess I must not have gotten the carbs adjusted right before turning it over to the kids to ride. Live and learn.

[ronmvdb]

Re: Waverunner 760-rebuild or replace motor

I've added some photos of the damage. I'm a little nervous not knowing what exactly caused the meltdown. Comparing these pistons to the "SBT Tech piston guide" pictures, it seems like a combo of 3 different reasons. The carbs were rebuilt and the ski was running great and had gone thru 1.5 tanks of gas before this happened. The low/high speed adjust screws are in specs according to stock settings. I replaced the Ocean Pro F/A's (rotted out) with some Tau Ceti F/A's at the same time. The crankcase was about 1/4 to 1/2 full of gas/oil mixture when I pulled the cylinders off (I premix). I don't know if that's a sign of anything or just a result of trying to start it. Still debating to rebuild top end or replace with SBT exchange.

[dsolie]

Re: Waverunner 760-rebuild or replace motor

Looks like you were too lean to me.. Did you ever perform a plug chop?

Since material has now gone through your cases (looks like pieces of ring), I would do a complete tear down, and then assess what you need from there.

[torquenut]

Re: Waverunner 760-rebuild or replace motor

Dsolie is right, lean, now you need to see if it was from carb or case leak.
How does the Mag seal look? Does it have tell tale sign of air leak?

[ronmvdb]

Re: Waverunner 760-rebuild or replace motor

More pics. I'm concerned about the carb gasket on the pto side. I don't know why the blue seal is gone vs. the other side. Also, I thought the rods were discolored but after cleaning the front one a little bit, they look o.k. to me. I haven't pulled off the mag case yet but no evidence of air leak from the outside. I really suspect the carbs or possibly they didn't premix properly. This is more the family ski. I don't let anyone ride my 96 XP. Splitting the cases is more than I want to take on and it's a major pain pulling this thing so I may just bite the bullet and do a swap again. Thoughts on the gasket? Thanks.

[dsolie]

Re: Waverunner 760-rebuild or replace motor

All that carb gasket tell me is that it wasn't lined up perfectly with the carb throat and it got some turbulence wash from the normal operation of the ski. Common when the gasket has a slightly smaller diameter than the carb or intake.

Splitting those cases is easy... if you have gone this far, another hour and you are done!

[torquenut]

Re: Waverunner 760-rebuild or replace motor

From the picture it looks as the rear piston was the one that lean out as the connecting rod looks a bite discolor. If this is the case most probably it was a carb problem. But I am guessing or using my experience. When it is the forward piston, and when I open the flywheel cover is there is a trail of oil running down at six oclock position is most commom a front seal gone bad.

[d.ginther]

Re: Waverunner 760-rebuild or replace motor

If there is a lot of pitting you may want to get a new head or else if a sand disk can work do that but only to take the burrs off not to mill the head.

How would that work? You'd have one combustion chamber with a larger CC volume than the other - you'd have a compression difference...

[dsolie]

Re: Waverunner 760-rebuild or replace motor

A very small difference, but nothing that would affect motor performance.

The procedure Torquenut mentioned was to remove any "burrs" or raised edges of the foreign material that was smacked / melted to the domes during meltdown. This is, of course, dependent upon the condition of the dome and head. Massive damage cannot be repaired without a specialty shop re-boring the dome, then decking the head to keep the compression in spec.

[d.ginther]

Re: Waverunner 760-rebuild or replace motor

A very small difference, but nothing that would affect motor performance.

Seriously? I don't regularly do compression calculations on 2-stroke, so maybe there is a lot available head volume - but on the head that I've got in similar condition, cleaning it up would add at least 10cc in volume, I'd think that the result would be a significant difference in compression.

[dsolie]

Re: Waverunner 760-rebuild or replace motor

read the rest... It's all dependent upon the actual damage done to the head and other factors (how much foreign material is there, deto, is the squish band damaged too) etc.

And yes, 10cc would give you a significant change in compression regardless of the stock dome size...