Bad news...Piston cracked.

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bnt5

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Noticed the jetjon running a little rough and had a hard time getting any throttle response....hmmmm....Got home and did a compression test, rear cylinder was way low. Pulled the top end off and found a chipped piston. The cylinder walls are still in great shape, one has very slight scoring that I can feel with my fingernail barely and the other is perfect, a good honing should take care of them in preparation for the new pistons. I don't know what caused the rear piston to go, I am running premix gas at 50:1, possibly the carb was lean or fuel pump issue? I am going to rebuild the carbs and do a leak down test once everything is put back together. Any other suggestions to finding the reason of a blown piston? Also the cylinder head has some pitting and damage from the piston would it still be ok to use if cleaned up?
 

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They are pretty black piston wash looks to have plenty of oil maybe too much. I found on a jetski forum the piston should look kind of like mine not completely black but not completely metal either. Also how old and how many hours on it? Probably due for a good top end rebuild
 

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no clue to hours, but it hadnt had a top end done since 1995 as everything was factory. I'm just glad it didnt jack up the cylinder walls. Amk, youre right about the color of the pistons, I guess if there was a lean condition that caused the piston to overheat it would show in the color on top the piston, I guess its a possibility that the piston was just old and brittle from running since 1995...who knows. :?:
 
when I look at the too it looks kind of different around the sealing surface and why does the one have what appears to be water droplets maybe on the intake stroke some water got in? Are you planning to rebuild yourself?
 
Maybe a piece of carbon broke loose, and got between the piston and wall? Last one i saw like that had scarred the cylinder pretty good, not so with your though
 
Looks like it over heated to me. Best I can tell the exhaust side of the pistons the one that's missing a chunk. That's the first part of a piston to go when they over heat because it's exposed to more heat then the other half of the piston that's being cooled by incoming air/fuel mix. When the exhaust port starts to open hot gases shower the side of the piston that still exposed in the exhaust port window. That combined with the fact that part of the piston is only in full contact with the cylinder wall when at bottom dead center it doesn't have much time to transfer heat out of the piston into the cylinder wall. You can take a look at the bottom side of the piston and it will probably have some carbon build up under it which is another sign the piston was getting over heated. I'd say that cylinder went lean over heated and just started eating away at the piston. Luck it didn't break the ring and eat he cylinder up.
 
Personally I'd swap it out with an SBT reman. You've done beat the snot out of the bottom end bearing and dinged the head as well. Each of those marks is pre-ignition waiting to happen. To me it's just not worth missing with. www.shopsbt.com

Get the carb kits, set the pop off pressures correctly and get the factory oil injection working again. Mine has been ridiculously consistent since I put the reman in a couple years ago.
 
Looks to me like she needed a decarb treatment using Seafoam. I do them every 50-hours of operation via Dunk's 'shock treatment' method, on all motors including lawn/snow blowers et al.

Sorry to hear she went kaboom ...
 
Looks like part of the ring broke and I think took the edge of the piston. Not sure how that cylinder overheated, it has always pumped water just fine. I rebuilt the carbs last fall and probably only have 20 hours on the boat since I built it, if that. I pulled the oil pump when I got the engine to reduce the chance of burning up a cylinder, and I always premix at 50:1. Could that mix be too thin? Maybe go to 40:1? Of course you have to remember that this ski sat for a couple of years in a field while mice made nests in the carb and destroyed the wiring, no telling how it was taken care of that before I got it. I did check compression prior to putting it in the boat and all seemed good, however there might have been a seal leak that I couldn't see??? #-o

I checked the connecting rod tolerance as best I can and it seems to be ok, at this point a remanufactured cylinder at $475 is out of the question, and the cylinders are in really good shape and mic out within tolerance. I am going to order a rebuild kit with new pistons, needle bearings and seals then check out the carbs including a pop off test, once everything is back together on the topend I am going to do a leakdown test to verify that my seals are holding, once that is done I am not sure what else I can do to prevent further issues like this?
 
bnt5 said:
... not sure what else I can do to prevent further issues like this?
Prop it right for the load YOU carry on your specific rig (see link to a prop calculator below) and make sure for the typical load you turn no more than 200 RPMs off the max. Not that you need to run way up there (every one in a while is fine ... plus at least a few times on each trip for a bit, like on the way back in)

https://www.tinboats.net/forum/viewtopic.php?f=4&t=37433

Then decarb every 50-hours using Dunk's (Bob Dunkleberger, from NJ, a marine surveyor and OB guru of some reknown) 'shock treatment' method below. Meaning, it is NOT that same as adding Ring-Free or Startron with every fill up and falsely believe that will decarbon an engine ... Your OB blew up as a ring carboned up and you may have been over-pitched or the motor saw hard duty.

Dunk's Decarb Procedure

"This works for Carbed, EFI, Ficht, HPDI, Optimax and even 4 strokes ... First you need a separate small fuel tank. One of those 3-gal red Tempos works great or an empty gal milk jug will also work, but might be a bit messier.

I use Seafoam over the OEM stuff like OMC Engine Tuner or Merc Power Tune because in the last few years they changed the formula and you have to let them sit up 12 hours. Who's got time for that?? Seafoam you can buy from NAPA, CarQuest or other auto stores. Seafoam works in 15-minutes.

You'll need 3/4 gal of gasoline and one 16oz can of Seafoam for each engine. Don't forget to add 3oz of oil if you are premixing in a carbed engine. Use about 3 ft piece of fuel hose off the little tank. You connect this tank to your engine by pulling off the main tank fuel hose off the intake side of your water separating fuel filter and plug the hose off the small tank on to that fitting. Or you can separate the fuel line on the tank side primer ball, so you can still use your primer. If you have an engine that has fuel plug then you need a fuel plug on the little tanks hose.

Start the engine, let it warm up and start pulling the mix into the engine. You may have to increase the idle to keep it running once she get loaded with the Seafoam. Run the engine 15 mins in the dock or just cruising around under 2500. Then shut it down and let it sit for 15 mins. Restart the engine, the smoke you see is the carbon burning off. Do the whole thing again and let her sit again for 15 mins. If she smokes after the second time do it again, but I've never seen one still smoke after three doses. The gallon mix should be enough to do this 3 times. You don't need any wide open throttle, you don't need to change the plugs. If it's cleaning the combustion chambers it's also cleaning the plugs, but every 50-60hrs is good time to change plugs in most engines.

I cleaned a antique Evinrude one time that had a 1/4" of solid carbon on the exhaust chamber walls by running a 1/2 gal of the mix through it. Seafoam has been around since the 30's and it's what they used when they were burning straight 4 stroke 40SAE oils in outboards.

You guys with the 4 strokes think you are immune from this? Those engines work 10 times as hard as any auto engine ever will and they will carbon up. I bought a Bronco two years ago that had 95,000 miles on it. When I used Seafoam on it I had the neighbors hanging out of their front doors looking for where the fire was after I started it the first time there so much smoke.

Too many are under the assumption that it's totally the 2 stroke oil that causes the carbon, Wrong... it's also the additives they put in the fuels today. The carbon inhibitors in 2 stroke oil are there for this reason also. Remember when gasoline used to smell like gasoline, today it smells more like bad cologne.

For those guys that like to do the carbon treatment by spraying it down the carbs Seafoam also comes in spray can called Deep Creep. It's the same stuff under pressure. Says right on the can Oxygen Sensor Safe, for you Yam guys.

After that if your engine manf recommends a daily additive treatment then do that in the mean time, but all 2-stroke outboards need to be decarboned every 50-60 hours. If I owned a 4-stroke, I would do it the same. Once you are set up with the tank and hose the Seafoam is only 5-6 bucks can. It's too easy not to do it!"
 
bnt5 said:
Looks like part of the ring broke and I think took the edge of the piston. Not sure how that cylinder overheated, it has always pumped water just fine. I rebuilt the carbs last fall and probably only have 20 hours on the boat since I built it, if that. I pulled the oil pump when I got the engine to reduce the chance of burning up a cylinder, and I always premix at 50:1. Could that mix be too thin? Maybe go to 40:1? Of course you have to remember that this ski sat for a couple of years in a field while mice made nests in the carb and destroyed the wiring, no telling how it was taken care of that before I got it. I did check compression prior to putting it in the boat and all seemed good, however there might have been a seal leak that I couldn't see??? #-o

I checked the connecting rod tolerance as best I can and it seems to be ok, at this point a remanufactured cylinder at $475 is out of the question, and the cylinders are in really good shape and mic out within tolerance. I am going to order a rebuild kit with new pistons, needle bearings and seals then check out the carbs including a pop off test, once everything is back together on the topend I am going to do a leakdown test to verify that my seals are holding, once that is done I am not sure what else I can do to prevent further issues like this?


Before you spend any money on this thing pop over to PWCtoday and ask those guys on the Seadoo sub forum what they would do. The Seadoo oil injection system works really well and is simple to set up. Guys do away with it, bugger the mixture and grenade the engine when it overheats. Running premix is going to lean your mixture out right away (because the oil lessens the amount of fuel flowing through the jets).

You did up your jets and do all the ignition chops at different RPM / loads so you could check the plugs to see how the carbs are fueling right?

What I see happening with your build is you'll toss money at it and it'll grenade again because the bottom end is wounded. Read about it many times while I was working through the bugs on mine.

https://www.shopsbt.com/sea-1995-xp/sea-doo-engine-717-720-xp-spx-hx-gti-gsi-gs-sp-gts.html

It really isn't expensive until you do it twice...
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