Detonation - Hole in the top of the piston!

[BoLongo]

Yes it happened again, I am one of the most persistent guys you will meet; take a look at the photo below. Detonation is burning a hole in the middle of the piston and the top ring has caught the edge of the RAVE Valve Port and chipped the top of the piston. The piston is not white so I can rule out a lean condition. This looks like a classic case of timing being too far advanced.

This will be the fifth time I have rebuilt the top end of the starboard engine because of a ring catching an edge on a port. This is the first time I have seen detonation, but this is a stock piston which have softer crowns than aftermarket pistons.

This is a new used motor with the same result so it has nothing to do with the engine.

I have switched carburetor sets between motors and I have bought a used set which were all rebuilt with OEM carburetor kits. So it has nothing to do with the fuel system

The only thing still in the equation that has not been replaced is the ignition system. I have been talking with Jess and Nick at Westside Power Sports. They have been very helpful. I just bought a used coil junction box from them.

I will be setting up timing marks on the PTO and I will use a timing light to verify timing after the new top-end is installed. If the timing is still faulty I will have to replace the MPEM.

I wish I would have checked the timing after the first motor failure. I had that feeling but I did not follow through:facepalm:

 

[BoLongo]

It is strange that the timing is too far advanced on one cylinder not both,
but you can see the pattern of the fire ball hitting the piston during compression.

 

[racerxxx]

To change the timing you need a programmer. If that is a stock bottom (never opened) end there should be a number stamped near the engine tag basically on the centerline of the PTO. You would need to verify what number is set in the MPEM. If it is timing I would think you would need to be at the far extreme ends of the timing range to possibly have issues.

Your piston is not hitting the rave valve. There is enough clearance in there to run the first over size piston without having to rework the rave valve. If you were hitting, the rave housing would have to be so worn (like severely) or the rave valve would have to be un-threading from the rave cup, still wouldn't hit it's meant to mechanically stop before hand. You're melting the exhaust side of the piston. There is not enough fuel in there to cool the exhaust side of the piston, it could be timing but I doubt it. I have replaced/rebuilt enough engines with different cranks and have NEVER checked the timing with a programmer, there should be enough fudge in there. I can barely get a tank thru a new engine until the throttle is pasted to the handle bars. Look at the MAG piston wash then look at the little lines of wash on the PTO piston, looks like too little fuel to me. I get that you changed the carbs to rule it out but what does the rest of your fuel system comprised of? How are the lines connected to the carbs? Which carbs are on it? Are they boat carbs or ski carbs?

Can you post good clear full size pics of the fuel system and connections?



Borrowed this from the interwebs

 

[BoLongo]

Fuel System on Starboard Engine

 

[BoLongo]

I added high volume Mikuni pumps with separate feeds to each carburetor.
I blocked off the line that shared incoming fuel but kept the shared return line.

 

[racerxxx]

Has the same fuel pump been on that engine the entire time? If the diaphragm that pumps fuel to the pto carb isn't flowing the correct amount the pto carb will lean out. I don't agree with running separate feeds to each carb like that and sharing the return. The stock fuel system is a loop, you don't have a loop. I see you have different pre filters/separators, the factory just used a standard plastic inline filter. We had a member here a few years ago that did a total restore/upgrade to his boat and had fancy fuel water separators. I talked with him offline many times as he was having fuel related issues. I told him to take off the fancy crap and get back to a standard fuel system, his problems went away. He was using the same fuel pumps as you but always ran them as a single feed. I know we all want the best for our toys but honestly sometimes simplicity is the best. I don't run anything extra on my fuel systems. Iirc, the factory fuel filters were lower than where your top separator is, which is that the engine having issues? Are you running factory air boxes on these engines? I'm not chastising you, just trying to get some info is all. I keep reading this before I post and can't change the verbiage to sound better, sorry.

 

[BoLongo]

Thank you racerxxx,

You do not have to sugar coat anything. I have no doubt that this could turn out to be something as simple as the fuel pump or filter location.

After I rebuild the top-end I will switch pumps with the other motor and go back to using a single outlet on the fuel pump and block off the 2nd, that way the carbs will share a single fuel source like they did with the stock pump. The filter location on this motor is in the same location as the stock filter.

I am getting closer to the source of the problem, I’m still leaning towards an ignition issue, which a timing light should verify when the motor is running again, but my diagnostic skills have failed me so far.

 

[BoLongo]

Once my engine is running again, I will use an Endoscope Inspection Camera through the spark plug hole to read the burn pattern.
This seems to be the simplest way to verify that the problem is fixed before losing another piston.

 

[Steelmesh]

Unfortunately for advanced issues, in my experience, you can no longer make assumptions (so if you didn't take it apart and check it and understand the check, then you're assuming). We all know this boat can function satisfactorily, so there is a problem or a combo of problems waiting to be solved.

Detonation will commonly occur with over advanced timing for the fuel (bad fuel, too low octane for the compression ratio), lean mixtures under high loads (bad carb tune, too hot air charge, faulty fuel supply), hot spots in the chamber (damage, dings, sharp spots, carbon buildup).

Another thing is, you need to focus on listening for 2-stroke knocking to prevent excessive mechanical damage. From now on, just be sure to tell the brain to listen for weird sounds under high load. Some say it is BBs in a metal coffee can shaken super fast, but sometimes it can sounds like you are breaking 1,000 skewer sticks in a tin can but "tingy", like a fast crackle. I think each engine makes a unique sound, but they have a similar frequency.

 

[BoLongo]

Thank you Steelmesh,

Yes the problem to this point has been my assumptions, they have cost me dearly. This time I will check timing and I will check the burn pattern throughout the day. It’s now a pride thing; I can’t let this boat beat me!

I have had many single engine PWCs and yes you can hear how the engine is running but with two engines running, it’s hard to hear what is going on with a single engine. I can tell how a single engine is running based on the RPMs and the steering wheel. When both engines are firing the steering wheel is straight and the boat is tracking straight. When one engine is struggling I will have to counter steer to keep the boat straight even if the RPMs are the same. So it is more of a feel with two engines.

 

[BoLongo]

Here is the last piston failure on the old engine. We see the same dull burn pattern but we can see the ring caught the intake port. This piston has no detonation marks on the crown like the current piston failure.



If the mixture was running lean we would expect to see the same dull burn pattern on both pistons but this piston has damage on the intake side which counters the lean diagnosis, and we do not see detonation marks on the crown of this piston which counters the timing diagnosis. (BTW the piston failure before this one was also on the intake side)



On the latest piston failure (Different stock motor) we see the same dull burn pattern but this time we see detonation marks which support the advanced timing diagnosis and failure on the exhaust side of the piston which supports the lean diagnosis.

The one thing in common is the dull burn pattern, so it is clear that combustion is the issue but what is causing it?

is it a lean condition or advanced timing?

 

[k_24]

Once my engine is running again, I will use an Endoscope Inspection Camera through the spark plug hole to read the burn pattern.
This seems to be the simplest way to verify that the problem is fixed before losing another piston.

Dont buy the cheap 10$ camera i got one and i cant see sh*t it pretty much useless

 

[BoLongo]

I love Amazon. I only buy things with positive feedback and if I’m not happy, Amazon makes it so easy to return a product if you are not satisfied.

This is the scope I bought 4 ½ stars from 29 people.

http://www.amazon.com/LEADNOVO-Endoscope-Inspection-Megapixels-Semi-rigid/dp/B06XYGWXJR

 

[racerxxx]

Wow, $30? They're$9.99 shipped from China with good feedback on eBay. A guy I ride with bought one. But, they're android only, no iPhone. You have to load a sketchy app on your phone. That's the only reason I haven't bought one.

 

[racerxxx]

Here you go, U.S.shipper

http://www.ebay.com/itm/152018440308

 

[BoLongo]

Now you are starting to sound like the old me.
If there is one thing owning this boat and being on this forum has taught me is that buying the lowest price thing is not always the best thing.
You were one of the guys that taught me that!

I’m iPhone so I had to spend a little more for one compatible with the iPhone.

I think we would agree there is value in this tool with the ability to read the burn pattern without removing the head. I will post images from the scope once I have the engine running.

The potential of this scope to evaluate engine performance and cylinder health could be amazing!

 

[nickj718]

I would buy brand new OEM fuel pumps and put the system back to the way it came from seadoo

Id also redo the carbs so ultrasonic clean and genuine mikuni kits. Make sure you have the small jet inside the accelerator feed line and you don't need check valves. These carbs have internal check valves. Just my $.02 I did that to my boat and its running good

 

[BoLongo]

Where do you take your carbs to have them ultrasonic cleaned?

I’m still not convinced it’s a fuel problem but at this point I need to go back to the basics.
This go round I will take the time to set up the ability to check the timing.

How hard can it be? You just need fuel and a spark right (I’m being sarcastic)

 

[nickj718]

If you want to send me them with new carb kits ill ultrasonic clean them and assemble

 

[BoLongo]

My new top-end kit is coming today and the jugs are heading to the machine shop tomorrow. I am switching out the coil box with a used one from Westside Motorsports.

Once I get it back together and if I’m still having problems with the burn pattern, I might take you up on your offer.

 

[snikwad003]

That sucks! No possibility of air sucking any where?

If you want to send me them with new carb kits ill ultrasonic clean them and assemble

I want this done after I buy some new used carbs one of my adjusters is corroded in place.

 

[BoLongo]

The 787 has a history of seizing the PTO Cylinder. Rotax Marine upped the jet size on the PTO carburetor to get more fuel in the PTO cylinder. They also have an issue with the accelerator pump where the check valves on the carburetors will allow air to come back through the accelerator pump and introduce air in the fuel making just the PTO carburetor lean. So they added a second check valve to prevent that. Group K has a great write-up on PTO cylinder seizure. I have the over-sized stock jet and I have the secondary check valve so those issues are covered.

I’m confident that my fuel delivery and carburetors are functioning properly. This is the third set of carburetors on the starboard side and all three had a piston failure. The fuel pump has been the same from the beginning but it shares the same feed as the Mag cylinder which is working properly. I’m going to make some changes to bring the fuel pump set up closer to stock but I don’t believe it will solve the problem. I have the exact same set up on the port engine and it is working without issues.

I will be looking at the burn pattern on the top of the piston. The 787 makes a very distinct pattern when it is running correctly. The PTO cylinder on this and the old engine were creating a dull pattern all one color. I will be using an inspection camera to monitor the burn pattern. I will also be setting up timing marks so I can use a timing light on the PTO cylinder to verify that the timing is correct.

I will solve the problem this time. I will not put it back together and assume all is OK like I did the last two times. Actually I was reading the plugs, monitoring the exhaust gas temperature and I was using a laser thermometer to check the surface temperatures of the motor but none of these gave me any indication that something was wrong.

 

[racerxxx]

I would disassemble that fuel pump and have a look see, but that's just me.

 

[BoLongo]

Yes, I will do that and let you know what I find.

 

[racerxxx]

Yes, I will do that and let you know what I find.

Pics please