Need opinion on Sport Jet 90

[Redbarron%%]

The other day we went out in our freshly overhauled Hobie Jet Skiff with the Force 90 engine.
We started out and the engine ran really well.
We fished for a while and while running ~ 4500 rpm or so we noticed a little blubber but nothing alarming.
We ran up to 5000 rpm (full throttle to see if it ran OK) 33 mph.
On the way back the engine started to bog and finally quit near the dock. I noticed a little milky blue crud in the filter going to the carbs, but was not too worried as I have a fuel/water separator.
I figured I would flush out the tank and empty the filter canister.
I found a lot of water in the filter, almost full, When I poured it into a container there was only a little gas on top and mostly dirty water.
I pulled the bottom carb bowl off and found it was full of milky blue mixture.
All of that leads to the real story. A quick compression check showed 30 pis compression on the #3 bottom cylinder.
Attached is a picture of the piston and what was drained from the carb bowls and filter.
I assume that there was just enough water in the fuel that got to the bottom carb to cause the detonation that burned the piston.
Luckily the cylinder walls are OK and the #1 and #2 pistons look good and have the same compression as when the engine was built.
Well I would have posted photos, but have been unable due to PHP Error or file too large

 

[roscoe]

so whats your question?

 

[jerryjerry05]

You have to resize the photos or use a photo sharing site.

Milky blue, is water mixed with the oil you use TCW3

 

[racerone]

To me it is classic.----Rebuilt engine , but no maintenance on the fuel system.-----Time to rebuild it again I say.

 

[Redbarron%%]

So true, but not entirely accurate. I thought I had gotten all of the water out of the tank, but undoubtedly I failed to tilt the tank as far back as it would be underway in the water due to the jet hitting the ground on the trailer with the front of the trailer jacked up.
I think that there was water up front.
To clean the tank this time I put the wheels on blocks to gain some angle.
By the way there was a fuel / water separator installed which probably worked pretty good until that cup or so of water filled it and it acted instead like a water injector...
Live and learn.
My mistake.
Also this was a New Tank installed because the original Hobie tank corroded out. actually the entire fuel system was new and this was my error in not being careful enough in checking that all of the Florida condensation had been removed.
That won't happen again. Also a larger water trap is going in as well.

 

[racerone]

Look at the deck fittings.---Not uncommon for them to let water in the tank.------Rain water has ways.

 

[jerryjerry05]

One way to stop the water, keep the tank full when the boats setting.
With Ethanol and even the leaded fuels, condensation forms really quick.

On the way home, stop for gas.

 

[Redbarron%%]

I was amazed as to how much water had collected, enough to overwhelm the water separation filter.
Expensive lesson, well a PITA lesson. Pulled the engine and have ordered a new piston.
I was surprised by the burned piston on #3 (bottom). I would have expected rough running and loss of power, but not a burned piston. The other pistons had no sign of distress and the edges of the pistons were perfect, showing no sign of detonation.
I wish I could post a picture, but when I resize to get below 2 MB I get an error every time

 

[jerryjerry05]

Use photoshop or any photo sharing site.

 

[Redbarron%%]

https://www.facebook.com/photo.php?f...UYgekyPCF4Xljm

[url]https://www.facebook.com/photo.php?fbid=1911829485614525&set=pcb.10157383885472894&type=3&__tn__=HH-R&eid=ARAFl2Cf4f_oqV6J3civVJYcID3AHCNjDHcCRUJdhU7s080HC2iwiupdBdWR-_yoj5rlPw7moAAJGMFI

https://www.facebook.com/photo.php?fbid=1911887882275352&set=pcb.10157383885472894&type=3&__tn__=HH-R&eid=ARA5oxOtdG_GzfS0B8OAOxwlK8QYJRYNSeeo1ckNcbKaM-lj2YGPXVG_hHC7frA5icJdzJZfOwMxNaQL[/URL]

 

[jerryjerry05]

You sure the head gasket wasn't bad too?

 

[Redbarron%%]

Head gasket looked perfect.
If I didn't have a new one already I would be tempted to reuse it.
All of the gaskets were installed with Hylomar blue and can be reused to reassemble the engine.
To be able to easily remove it from the boat I removed the exhaust manifold and I already had removed the carbs to inspect them originally thinking that water in the bottom carb had cause the engine to quit.
Luckily my inner brain told me that Jerry-Jerry would say to check the compression...
I have a 'scope that works with my phone and after the 30 PSI reading (and seeing a little flaky aluminum on the plug) I looked in and saw the burned piston.
I have tried a number of modifications on this engine hoping for greater reliability and this may have contributed to failure!
The main difference is in fitting the late model Mercury CDM modules for the ignition. (with the original flywheel) One issue there was figuring out the bias circuit to keep the timing from advancing too much at high rpm. My concern was that this had happened, but the edges of the other two pistons are sharp and show no signs of distress. When building the ignition I checked the timing on each cylinder, but not a 5000 rpm.
Something to keep an old retired guy busy.
Thanks for looking at the issue and offering suggestions.
JD.

 

[Faztbullet]

Just cant figure how just one carb had water but not the others???? Piston shows sign of classic preignition due to lean or timing problem. Need to index flywheel and check timing on each cylinder at 3500 RPM and up.

 

[jerryjerry05]

That water in the #3 ??
From the looks?? I'd say gasket ?? It only needs to be a pin hole leak, or a bolt not tight enough ???
Cracked head??
Cracked cylinder walls?
Check for rust in the cyl.?

 

[Redbarron%%]

All carbs had water to some extend, blue fuel with emulsified oil no visible clear water anywhere. Cylinder had no water visible. What I saw was mostly the emulsified oil in the fuel. I was surprised that one cylinder burned a piston with no visible problem or compression problems on the top two.
Perhaps there is a blockage in the bottom carb that caused a lean condition in addition to the contaminated fuel. I plan to clean and inspect the carbs before returning to service.
One issue I have seen is that the engine wants to bog getting up on a plane, perhaps the idle mixture screws need to be opened up a little to give a little more fuel in the lower midrange of the throttle opening.
As I understand it the carb runs on the main jet at full throttle and the idle circuit carries the fuel flow for a greater part of the throttle opening than a car carb.
I ran the boat up to full throttle and 5000 rpm to start and then throttles back to 44-4500 since the engine is still fairly fresh on overhaul. Looking at the spark advance the spark is more advanced just off full throttle than at full throttle.
It seems to me that the carbs open up very little initially and the engine increases power with timing and then the cam opens up the carbs more as the throttle is advanced towards wide open.
There was no evidence of any issue from the spark plugs (good edges etc.) on any cylinder, but for some aluminum flecks in the gap of the surface gap on #3.
I have replaced the #3 piston and I am putting the engine back together and rechecking everything.
The entire problem could be contaminated fuel and perhaps a clogged high idle port in the #3 carb.

 

[jerryjerry05]

The air screw is for fuel/air mix at low speed.
The main jet takes over, about 2k

 

[Redbarron%%]

Thanks, my concern was mid-range leaning at cruise where the timing is advanced more then WOT

 

[jerryjerry05]

Once it's running, do the starter fluid test.
Start motor, spray SF around the intake side, base of the carbs, under the electronics.
If it makes the motor run ANY different, cough, sneeze, stumble or stall??
Then it's sucking air somewhere??

One tiny cough, backfire, can blow the port cover gaskets and cause poor running.

 

[Redbarron%%]

Thanks, I will do that when I finish getting the engine back together in the boat. I have the block reinstalled without the exhaust and a reed carrier forward. I am carefully checking all of those things for possible leaks etc.
I feel that stupidity on my part is the problem in getting water in the fuel while running.
Since the amount of water that breathes in and out of the fuel tank each temp cycle every day with our humidity put more in the tank than the water separator could handle and it just acted as a water injector instead.
I bought a new separator that has a clear bottom with a drain to be able to see better what is happening there and also adding a H2Out fuel vent desiccant drier to help prevent future contamination.
My only real question is why only the bottom cylinder burned a piston. I can only guess that it was related to less fuel getting to the lower cylinder and more water, raising the compression ratio, but I would have thought that the water would have cooled the mixture and not detonated.
Well theory meets the real world and I am confused and humbled.
Thanks again, Jerry