Challenger 787 single engine PTO piston seizure

[Blademan]

Hi folks, newbie from New Zealand.
I purchased boat and completely rebuilt the motor including carbs.
After run in and about 10 Hrs, I had a PTO piston seizure.
PTO piston is toast, with a hole in the side of it.
When it seized, I was pulling a tube and had 3 adults in the boat, so was loading the motor. Found out that the pto rave valve piston cap was broken, so it wasn't opening.
Do you think this would cause the seizure?
Pulled motor, did leak down, all good.

Rebuilt carb specs - does this look ok- should I run richer on anything?
Pop off 36 psi
Mag main 142.5
PTO main 145
Pilots 65
HSA 0
LSA 1 1/8

 

[Dr Honda]

The broken cap won't cause a piston to melt by it self.

Unfortunately, without pictures... it's hard to say what happened. Here's the normal issues........

1) bad crank seal causes a lean run.
2) Plugged up carb, or feed hose causes a lean run.
3) High ignition timing (with bad/low quality fuel) can melt the crown.
4) The oil system was plugged, and you lost oil.
5) Did you have the right oil??? Seadoo engines need an API-TC oil. TC-w3 oils CAN NOT support the power of a Rotax !!! (especially when new/rebuilt)

Since it was a rebuild:

6) Improper index on the crank causes high timing.
7) Crank slipped, causing high timing.
8) When the engine was put back together, was the timing set properly? (I see all the time where people just bolt the system back together, and don't check it)
9) The squash band is too tight. (I see this WAY too much)

10) Or it's simply a bad piston.


Post pics of the melted parts, and we can try to help.

 

[Blademan]

I did a leak down test on the engine, cooling system and rotary valve. They all held pressure.
New pistons were pro-x, and cylinders bored and honed to match.
Squish was good, probably on the higher gap side, but definitely in range.
Checked crank by using 2 DTI's down spark plug holes. Both traveling in opposite directions at all times, so 180 degree.
Timing in flywheel and rotary valve done by the manual.
Oil is Spectro PWC Extreme API-TC for oil injection, and I have used for years on my other ski.
Checked oil injection volume at 1500 rpm as per manual, all in spec.
When I rebuild the carbs, I kept all the same adjustments, as had been used in this boat for the last 18 years, and the pistons that came out looked good.

The only thing I can think of is heavily loaded motor (couldn't get it up on the plane with 3 adults and towing a large biscuit with kids on), and the fact that the rave valve wasn't opening has caused excessive heat to start melting the piston. In hindsight I would not do that again.

Can you guys give me your tried and tested jetting specs for 97 challenger 787 single engine.
Might run a little richer on the PTO by opening HSA a little.

Hows this for a melt down.

 

[Dr Honda]

Sounds like you checked over everything during the build. BUT, I'm still not believing that the RAVE was the issue. Over the years, I've seen tons of melted RAVE caps, and all that happens is you loose RPM.


But... from the picture... that piston/cyl was HOT !!!
1) The crown edges (that aren't facing the exhaust port) are still sharp.
2) The skirts aren't scared. They show melted, and re-deposited aluminum. (tight, and hot)


So... ummmmm... I think your clearance on the piston was a little tight to begin with... and then you were working it hard while it was still fresh.

When I still had my shop open... I rebuilt a lot of air cooled motorcycle engines, and you could tell when they started to break-in, because the temps would drop considerably. (That was normally 1000 mi, or about 8~9 tanks of fuel)


Now... you can call me crazy... but DO NOT run premium (high octane) fuel in a fresh engine. it just makes extra heat. Personally, I would run "Regular" fuel, and drop the ignition timing a couple degrees. But on the 800, that takes a BUDS system. And also... it's VERY important to run extra oil. so add oil to the main fuel tank. (2~3 oz/gal)

 

[Blademan]

Ran extra oil in the first tank of gas when running in.
What do you think of my jetting set up?
Just don't want this to happen again.
Will do some plug chops, but now will have to wait until its run in before holding WOT for a few minutes.

 

[Dr Honda]

Personally.... for jetting.......

If you have the accelerator pumps... get rid of them, and jet the carbs to "1996 XP 800" spec. I think that's the most stable, and safe set-up.

40iBN
main: 142.5
pilot: 70
Low: 1
High: 0
N/S: 1.5
Spring: Black
pop: 23~43 psi (I like it around 35~40)
OEM/FACTORY AIR BOX !!!!!!!!!!!!
OEM/MIKUNI PARTS ONLY !!!!!!!!!!!! (aftermarket suck, and cause issues)



I've rebuilt (literally) hundreds of seadoo carbs, and if you use factory parts, that engine will run spot-on with the above specs.

 

[Dr Honda]

Just as an FYI....

On a 2-stroke, running rich, can help cool the crown. BUT... if you are too rich, then fuel is still burning when the exhaust port is exposed. at that point... the heat builds on that edge of the piston, and can melt it. (kind of like in your pic)

From the pic you posted... I don't see any "Wash" on the piston crown (even the one that is still in the engine) and that's screaming "Lean run" to me. OR... it got crazy hot.


Have you checked the temp sender, and buzzer in your boat? You could have had a plugged up inlet, and you just overheated.