1996 Seadoo GTX 787 losing a cylinder? Cant find pressure leak!

[Travisgg]

Hey guys. Bought 1996 Seadoo GTX 787 last year from guy who had a brand new BRP motor put in....right from dealership. Anyway his blew the pump side cylinder. I got it and had pump side cylinder and piston redone properly(clearances to spec by machine shop etc), redid fuel lines(he had old ones), complete official mikuni carb kits as he used aftermarket stuff, and she was alive and well again. The new cylinder and new piston was about 15 psi lower than the other side last year but ran great. I assumed it needed time to seat. Thought i would check it again this year and its 40 psi lower than the other side. Keep running or what? The chocolate brown picture of plug is the good untouched cylinder, the other is the cylinder rebuilt last by year.

Other notes...fuel selector switch changed.

 

[tobytd]

Get a flashlight and see what the top of the piston looks like.

 

[mikidymac]

No, at 40 psi difference you have a problem, no question.

Typically this is fuel system related but since you covered everything I would be looking at air leaks and the oil supply for the bad cylinder. When you put the engine back together make sure you pressure test it to check for air leaks and make sure oil is flowing from the pump to the bad cylinder intake fitting.

 

[Travisgg]

Took the power valve out and took a peek at the side of the piston... That's definitely not good what the frick.... Brand new piston. I'll do what suggested in terms of checking the oil flow to that cylinder and pressure testing. Unless this picture gives somebody a for sure idea of what's going on let me know, if not we'll report back after rebuild. Thanks!

 

[mikidymac]

Not good, lack of fuel or oil or the machined tolerances were not correct.

 

[Travisgg]

Not good, lack of fuel or oil or the machined tolerances were not correct.

Yeah. Weird part is this was a new crate motor from BRP. Original owner blew that same piston, but he still had all the old fuel lines and aftermarket carb parts. I put back to original mikuni. I got the tolerances inspected before install by a high quality machine shop, so I'm thinking it's got to be something with the oil feed to that cylinder for some reason or air leak like someone else wrote above. Will pressure test once I get to it. Will report back.

 

[mikidymac]

Two failures on the same cylinder?
Definitely air leak or fuel/oil issue.

 

[Travisgg]

Two failures on the same cylinder?
Definitely air leak or fuel/oil issue.

Frack. I know right.

 

[Travisgg]

Frack. I know right.

Weird one cylinder is getting the oil if that's the issue. Guess ill replace the oil line on that side just to be safe and make sure it's bled once I put it all back together. Haven't started tearing it apart yet...

 

[mikidymac]

The little check balls can get stuck on the oil fitting although very rare.

 

[Travisgg]

Yeah worth a look maybe the previous owner screwed something up on the oil line

 

[Travisgg]

Yeah worth a look maybe the previous owner screwed something up on the oil line

Just started digging into it.. found one of my carburetor to block gaskets laying in the bottom of the Sea-Doo must have fell off when I did the carbs last year... To clarify, the side that burnt down DIDN'T have a carb base gasket on it. Could that be enough to lean it out? Just about to start checking oil pump but found that and thought it could be the culprit...

 

[Travisgg]

Here a link to oil pump being tested..... Looks good?

 

[Travisgg]

 

[Travisgg]

 

[burtshaver2021]

So the previous owner had a new crate motor put in and rebuilt the carbs at the same time with jobber carb kit? Did he then loose a cylinder again after putting the crate motor in? You put 1 new piston in when you got the machine and rebuilt the carburetor with Mikuni parts? Sorry just having trouble following the chain of events, is that the first or second time since the new crate motor that the pto piston has melted down?. I would think the missing gasket in the carb would cause a lean condition but very odd that it’s the piston that had the gasket that burned down, . Was a new cylinder sleeve put in and bored to stock size when you had the one piston replaced? Or was it just a new piston put into the existing cylinder with no machine? Maybe Mikidymac can advise but I would think it would be a bad idea to replace only 1 piston? Being as how it’s only the 1 cylinder that’s melting down, I would do a leak down test and see if maybe the engine is sucking air in the oil seal, also maybe try rebuilding that carburetor again and make sure high and low speed screws are set to factory specs. How does the crank feel? I’m not sure but I would think if it was a lack of oil the crank would have suffered damage as well, and not just the piston. More like a lean condition than lack of oil.

 

[Travisgg]

Yeah I think you summed it up pretty accurate lol. I don't know the original problem why the previous owner put in a crate motor but he did. From there he burnt down the pump side piston (I think that's PTO)... within a few hours of running in from new. Sold it to me.

I had the bad cylinder cleaned up and measured and it was still within factory specs and no damage so a new standard size piston was fitted as well and measured by anal machining people so it was in spec. At that same time I replaced all the fuel lines and put in OEM mikuni parts to the carb. Did all the factory settings to carb as per BRP manual.

I'm really starting to think I'm just an idiot here and leaned it out by not having that carb base gasket in there? But that still begs the question why the cylinder went on the original owner in the first place.. which could be related to the original fuel lines and crappy carb parts that he put in there. Too weird.

 

[burtshaver2021]

The carb gasket makes sense except that your saying it’s the side that had a gasket that melted down so that doesn’t make sense. normally it’s mentioned to check fuel valve and fuel strainer o ring but I don’t think it’s that because it just 1 piston, I would honk it would be both if it were sucking air into fuel system but I guess it could t hurt to pressurize the fuel system and check for leaks. Right now my money would be on a leaking crank seal or possibly air leak past the carb gaskets. Did you use blue loctite to hold the bolts for carb in place, where they tight when you took them off? I would think if you cover your bases, leak down test, fuel system check for leaks, and it’s just me but I wouldn’t trust anyone, without checking their work ( mic cylinders and check clearances yourself but tools are a little pricy for a one time thing )

 

[burtshaver2021]

What were your rpms when it was running for you?

 

[Travisgg]

The carb gasket makes sense except that your saying it’s the side that had a gasket that melted down so that doesn’t make sense. normally it’s mentioned to check fuel valve and fuel strainer o ring but I don’t think it’s that because it just 1 piston, I would honk it would be both if it were sucking air into fuel system but I guess it could t hurt to pressurize the fuel system and check for leaks. Right now my money would be on a leaking crank seal or possibly air leak past the carb gaskets. Did you use blue loctite to hold the bolts for carb in place, where they tight when you took them off? I would think if you cover your bases, leak down test, fuel system check for leaks, and it’s just me but I wouldn’t trust anyone, without checking their work ( mic cylinders and check clearances yourself but tools are a little pricy for a one time thing )

Sorry I might have to correct that it's the side that didn't have a gasket... Was running awesome at all RPM from what I could tell.

 

[Travisgg]

So the previous owner had a new crate motor put in and rebuilt the carbs at the same time with jobber carb kit? Did he then loose a cylinder again after putting the crate motor in? You put 1 new piston in when you got the machine and rebuilt the carburetor with Mikuni parts? Sorry just having trouble following the chain of events, is that the first or second time since the new crate motor that the pto piston has melted down?. I would think the missing gasket in the carb would cause a lean condition but very odd that it’s the piston that had the gasket that burned down, . Was a new cylinder sleeve put in and bored to stock size when you had the one piston replaced? Or was it just a new piston put into the existing cylinder with no machine? Maybe Mikidymac can advise but I would think it would be a bad idea to replace only 1 piston? Being as how it’s only the 1 cylinder that’s melting down, I would do a leak down test and see if maybe the engine is sucking air in the oil seal, also maybe try rebuilding that carburetor again and make sure high and low speed screws are set to factory specs. How does the crank feel? I’m not sure but I would think if it was a lack of oil the crank would have suffered damage as well, and not just the piston. More like a lean condition than lack of oil.

Sorry correction the side that melted down did NOT have the gasket. I MISSED THAT MY APOLOGIES

 

[burtshaver2021]

Ahhh, that sucks, but makes sense. That was the problem then. The reason I was asking about rpm’s is because if it is sucking air the rpms would be high and you can lower them by choking the air from it. If that’s the case stop driving. Think you’re going to fix it?

 

[Travisgg]

Yeah she'll be fixed just trying to figure out the cause.

 

[Travisgg]

Yeah she'll be fixed just trying to figure out the cause.

Thanks for all the input. Just got to get the cylinder cleaned up if it's possible... and new piston, and get clearances checked by an OCD guy, and remember to put all my gaskets back where they're supposed to be LOL.

 

[mikidymac]

Yes, you 100% for sure found the cause. Not having the gasket, that cylinder was running lean from the air leak.

It’s a funny joke we had when I worked at a PWC shop, “ It ran the best it ever did, right before it blew up”. Yeah, they do make the most power when they are lean, right up until the point they lock up.