PTO cyl low comp again, lean or water entering??

[1983]

Ski: 96 XP, all stock. 787 engine.

Previous symptom: Lost compression in PTO cylinder at the lake Norman meet. Found cylinder and piston skirt vertical scoring only on exhaust side of piston. Lots of gray dust on spark plug and cylinder walls and combustion chamber. 130 psi on comp test.

Repair a few weeks ago: new STD. bore pto piston, rings, and hone of cylinder netted me 148 psi on pto cylinder after break in. Replaced base gasket, carb gaskets and exhaust manifold to jugs gasket. All torqued to factory specs.

What's up guys I got my ski back together two weekends ago and put 5 to 6 hours on it and got it all broken in.

148 psi on the PTO cylinder afternoon rings and new piston and honing of cylinder stock bore. Have been on multiple trips to lake and it runs great spark plugs match in color, although both kinda rich I felt it was still on the premix in fuel tank for break in. ( ski still has factory oil inj)

Went to ride it yesterday rode for an hour or so. Still curious why only getting 6600 rpms max I put a fresh set of spark plugs and it got about a mile from the dock in and just shuts off during wot run for a few secs. Clunks during try to restart. Waited 15 secs starts right back up doesn't have a lot of power stops around 5000 RPM nursed it back to the dock check the plugs they don't match at all now rear plug his gray.

I got home, still have great 164 psi in front cyl but rear pto is low again at 134 :-/

Pulled head, still has no pitting, top of piston looks fine with no pitting or ringland damage but the piston skirt on exhaust side has even worse vertical scoring than the last time I lost compression at the lake meet. Walls have some vertical scoring but not super bad.

What concerns me is that there is this gray oily almost watery substance all over the cylinder walls piston skirts and combustion chamber. Is water entering combustion chamber via the head o-rings? I didn't change as they looked ok.

I did rebuild the carbs with oem mikuni parts pop off at 35-36 psi per carb and bleed down psi check passes. All set to factory specs, proper jet sizes, 1.5 needle and seat replaced new gaskets diaphragms.

I'm almost leaning towards water into cylinder being my issue. What do you guys think? Pics coming I'm in garage now.

Thanks!

Rob

 

[1983]

PTO on right.

Scoring on exhaust side.

 

[1983]

Pic of plugs taken with flash on. PTO is pretty gray. Note: these plugs had only 4 mins of runtime so that is why mag plug is white.

 

[1983]

 

[racerxxx]

Unfortunately you tore it down before leak testing the cooling system. That would have taken all the guessing out of the equation. On the 787 I have seen more vertical scoring on the exhaust side than any where else. All the one's I have seen like that you can feel the ridges and required being bored. Your's looks on par with the ones I have seen. Could be water, could be carbs. I know, it runs great.............they all run their best right before they grenade,LOL.

 

[1983]

Is the only difference in a PTO to MAG cylinder the cylinder drain port on bottoms direction it points?

The little brass rights angle fitting.

 

[racerxxx]

That should matter.

 

[1983]

I'm just checking other jugs I have.
I know it's a no-no to mix jugs from other engines but just looking around see what I have I just checked the part numbers from A set of jugs and they have the same part number .

 

[racerxxx]

You can swap jugs, you can't swap case halves

 

[1983]

Gotcha




Rob

 

[1983]

Almost dont even want to use that rear carb again. Considering rebuilding an extra carb and using it. Ive got multiple sets laying around.

Could the front carb have a restriction at the fuel exit nipple that feeds the rear carb and under WOT situations the front carb is just flat out not delivering enough fuel to rear?

Still considering water issue. Going to replace o-rings at top of cylinders after going through carbs again.


Rob

 

[snikwad003]

The gray color makes me think water and there is a little spot on the top of the jug at about 2 o'clock that could have been a leak but ive never seen a piston have that grayness so that is a guess

 

[1983]

I yanked the engine last night, going to tinker in carb this evening.
[MENTION=57920]racerxxx[/MENTION] what kind of grease or sealant can i source locally to use on the o-rings that go on top of jugs? I recall someone saying there is as sealant that can go on the o-rings and still allow you to remove head if necessary.

molykote or 0511 or something....?

 

[racerxxx]

I put 518 or 515 on them.

 

[1983]

To end this thread, I ended up yanking the engine and putting another in. It was water ingress into rear jug causing a steam cleaning and heat expansion of that piston/jug scoring and low compression, whether the o-ring failed, or a crack. I'm leaning towards a hairline crack in the jug.

It was not the PTO carb leaning out the cylinder, so that's good.

Now for the missing RPMs, the engine I put in had the same issue of maxing out at 6600 rpms. Ended up being RAVES bellows were missing the coiled spring on the lower side that holds them tight to rave. The larger diameter spring that holds it to the plastic housing was there, but lower missing. Ended up using tie wraps, and upgraded to slotted RAVEs I had laying around. Top end RPMs are back.

Now fighting a hot start issue. Carbs have been rebuilt with OEM parts and of course needles and seats as well. Passed pop-off test of 34 psi in each carb. Bleed down test passed, carbs held 20 psi for 1 minute. Now it starts fine cold ( don't they all ) but when its hot i have to nearly have 3/4 throttle and clear it out once it hits. But many times im in no wake so cant clear it out, I idle out of no wake and give it gas, it dies. Then its real hard to get going. Clearly its flooded. Going to open carbs again tonight and see if adjusting the needle and seat lever down 1 mm below body to keep it away from the red button on the diaphragm helps. Possibly with fuel in there in a hot engine compartment it is pushing on it.

Its running really great so almost don't wanna mess with it. Lows are 1 out and highs closed as per fact specs. I did the 1/4 throttle for 1 minute test and nailed it, it took off fine, no bog or hesitation. Before removing carbs im going to crank it up with airbox off and take a mirror and flashlight and see if fuel is dribbling into throat at idle. That will be tell tale needles/seats are being held open.


Rob

 

[snikwad003]

It feels good to figure it out after banging your head on some wrenches for a while doesnt it? :cheers:

 

[1983]

It does haha,

Many hours we dump in these things, I just wanna ride at the lake!

Rob

It feels good to figure it out after banging your head on some wrenches for a while doesnt it? :cheers: