Bent ground electrode

[Gritzlor]

Hi All, I'm rying to troubleshoot a very strange condition with a 2000 GTX RFI.
Did a top end rebuild on it not too long ago, actually on two of them, identical parts and process on both. Dialed squish to spec, great compression, elaborate break-in, all good. Then all of a sudden the other day it died out of nowhere on the river. Ripped the plugs out and found one to be as if the gap on one was bent shut. Replaced plugs and all good. Wrote it on possibly having dropped the plug before installing and not checked the gap.
Now this happened again. Lost power out of the blue, wouldn't run right, but we were close to the ramp so limped in. Just pulled the plugs, same thing again. Mag side plug almost closed, a little carbon and heat and it probably doesnt spark at all.
I am really at loss with this, maybe it's leaning out in certain conditions, but wouldn't the piston be complete toast before this happens?
After the first time I checked the piston with a scope, nothing unusual there, nice black coat of carbon for the most part, no nicking out of it near the exhaust port or anything. Tiny wash spots near the transfers.
Any ideas guys?
By the way this is my wifes ski but she has a remarkable ear for unusual noise and such, neither of us have found or heard unusual rattle or pinging which would indicate bearings or whatever mechanical breaking down.

 

[Tiggo]

I'm no expert but are you using the correct spark plug? Size/length/crush washer, etc.? It sounds like the top of the piston is hitting the plug.

 

[Gritzlor]

Yeah correct plugs indeed. BR8ES.
The piston physically doesn't come remotely close to it, about 1/2" space at TDC.
I ruled out the obvious stuff already, now looking for the experts to chime in with these edge cases...I clipped the plug wires not too long ago and they might be a little too short, upon impact they might have pulled some out of the coil pack. Can a compromised spark cause the electrode to overheat? I ordered a replacement coil pack and am planning to fab new wires and all, just to eliminate that as a factor.

 

[Tiggo]

Weird... Another thread on the same subject.
Seadoo GTS bent spark plug

 

[Gritzlor]

Thank you Tiggo, I read that one too already.
Here's what I added there, just speculating:

Having basically same thing happening to a 787 RFI in a GTX 2000. Run Premium gas but it does contain ethanol. The best non ethanol I can get my hands on here is 89.
Wonder if a good compression plus gas loaded with too much ethanol can heat the electrodes and simultaneously some shock wave bend them. I know that theory doesnt make a lot of sense but I am at loss here...

As soon as I have the ignition parts ruled out I'm going to bump the plugs up to BR9ES.

Also, I'm contemplating an EGT sensor. Got one set up on my other ski since I wanted to get a good baseline for temps, now I got that and could maybe catch a lean condition...

Dirt in injector maybe....not so sure. Injectors are 2years old and all filters in check. A speck of dirt though..can always do damage. It kinda feels like these injectors are pretty solid big pieces though.
Didn't change them because they were bad, just precautionary OCD kind of deal.

I guess I'll rip out the exhaust and tap the darn thing for an EGT. Can't deal with not knowing WTF is going on....

 

[Gritzlor]

Well. Thank you guys for pitching in. Now I think I found the culprit, and embarassing it is indeed. Very embarassing.
I kept on searching the forum and found this:
97 SPX 787 Mag cylinder is hot?

Checked my hoses, lo and behold: I had switched the two at the rear of the head.
This makes the MAG cyl run hot, cools the sensor so no overheat alert...I was about to fry the MAG completely. Guess the 93 octane saved what could be saved after all.

The good news is this is no mystery anymore, and in a weird way I'm happy to contribute a symptom of this easy to make error: MAG side spark plug overheats and bends shut.

Still embarassing.

Have a nice day Y'All.

Grimur the Icelandic Floridian...

 

[Tiggo]

Wow...glad you figured it out. That could of been a real stumper and sent you down a deep rabbit hole. Awesome you can trace the hoses and figure it out. I am going to replace my cylinder head cover with a newly painted one. The original has corrosion growing (bubbling) on it. Just superficial but a replacement was cheap. Painted it Bronze metallic (engine high temp) and going to swap them out. Got the gasket set from OSD. Will also give me a chance to see inside the engine. Anyway, I could see me getting the hoses installed backwards and then dealing with the same issue...lol. Thanks for the update.
Alan

 

[Gritzlor]

Taking an occasional stab at this as it's cold outside and still recovering from that Covid thing...
Decided to check compression rather than tear everything apart. Checks out OK I think, 165/168 or so. A few psi lower on the mag but still within reason I think, they were maybe hitting 170 when brand new. Maybe. I didn't write it down.
This is a bit above the commonly mentioned 155+ for 787's, but I rebuilt these motors with new sleeves, nice bore & hone from a local shop, new stock size pistons, radiused and polished the sleeves in some key spots prior to assembly and dialed the squish to the narrow edge of the spec (1.2mm)
Original heads.
IDK why exactly I'm hitting almost 170psi, the gage isn't a name brand but I've checked it against another one and it's all spot on.

Couple of pictures where I'm touching up the sleeves:


Didnt touch the ports in terms of size, just removed some sharp edges. RFI's are allegedly not happy with port mods. I just wanted them to breathe freely, and I think they do.

I will post something on my water injection mods too as soon as I get to it, some interesting fails and successes there...

 

[mikidymac]

If your gauge is correct that’s much higher than a stock 787 should ever be.

Messing with the ports like that will not increase compression but if you don’t know what you are doing and change the port angle you can really mess up the power.

If you have an actual 170 psi compression you need to be running 91 octane.

 

[Gritzlor]

I never run anything below 91 if I can help it. Since the rebuild I've filled with Premium, don't like the ethanol but out of two poor options I rather give up some slight power and detonation with it.
The exhaust is pretty consistently at 930F @ WOT. Max temp is at around 5300RPM, about 1060F in good cool air. It seems to be filling the cylinders really well at that rpm, or there is a slight lean spot in the gas curve.
Since this rebuild, and a little dial-in on the water injection, I've been getting better performance than ever and very smooth running motors, they're just purring away (except when a hot plug shuts itself).

Port angles are mainly governed by the roofs, which I didnt mess with for that very reason. I beveled the outer bottom edges to reduce vortex resistance on that step to the piston wall. Also, beveling the sleeve skirts helps with air making that 180° turn under them when the piston is coming down.
It wouldn't surprise me if these mods somewhat helped compression since the cylinders get filled ever so slightly better, but I agree, 10psi over spec is like 7.5% increase which is a bit much to be explained with this. Maybe the new rave valves are closing in well. It's all brand new and to spec.
I just dont have much else, same cranks, same heads, same gage. I just rechecked it and it's spot on matching two other gages.

I'm by no means a 2 stroke expert, but I've read everything I've found, studied the engines I've rebuilt, fixed every breakdown happened and so on. 20 years of prosthetic leg R&D doesn't harm either.
That all being said, I'm still a newbie and always learning something new, that's really what makes this fun.