Pistons Upside Down?

[DomsX20]

Full disclosure... I think I screwed up...

About 7-8 years ago, I bought my boat with a blown engine. It was a good winter project and I something I was interested in. The boat is a 2002 Seadoo X20 with the Mercury 240 EFI Sport Jet. The boat has been good but never really run "right". The boat starts great and with near zero effort, but my RPMs were limited to ~4700 and top end was only about 37mph. I have done significant research, replaced multiple sensors, tuned it exactly IAW the service manual and even bought a DDT. Compression check is consistently ~120psi on all 6. I simply have not been able to figure it out... I finally decided to pull the engine and start to break the motor down. Thats when I saw it....

I think the pistons are upside down... In my defense, most pistons have an arrow which point to the exhaust side. Our pistons do not have an arrow but rather the word "up". I assumed "up" was the same as an arrow, pointing to the exhaust, but that does not appear to be the case. The manual states the word UP should be towards the top of the engine. I assume the top of the engine is once it is installed in the boat (pointing towards the sky).

In summary, I am hoping for confirmation that I did indeed install these upside down and I may have finally identified the culprit of my low power. If yes, any recommendations on what should be replaced as part of this effort? Wrist pins, rings, pistons???

 

[Shortrun]

We are at the same place you were 7 years ago. Engine came to us in pieces, just putting the pistons in now. So I hope you get an answer to your question.
Your pistons are opposite to where we currently have ours.

Did you check the spark plugs color at 1/4 and 1/2 and 3/4 and full throttle at any point of all your testing?
Just looking at your pistons I don't see serious problem, but I am no experienced mechanic.

Then there is the ignition timing, were you able to check that ?

 

[DomsX20]

We are at the same place you were 7 years ago. Engine came to us in pieces, just putting the pistons in now. So I hope you get an answer to your question.
Your pistons are opposite to where we currently have ours.

Did you check the spark plugs color at 1/4 and 1/2 and 3/4 and full throttle at any point of all your testing?
Just looking at your pistons I don't see serious problem, but I am no experienced mechanic.

Then there is the ignition timing, were you able to check that ?

I have run tests on my spark throughout the power cycle and confirmed I have consistent good spark on all of my cylinders.

The spark plug color has been interesting. Typically, half of insulator is a nice brown color while the other half is almost white. Now I am wonder if this is a result of my pistons installed upside down.

I have also checked the ignition timing several times. At idle and WOT. I have made a few tweaks but I am always pretty close to spec.

 

[Shortrun]

On the spark plugs, see the attached.
I have read way to much on spark plug reading/diagnosing , but have limited real life experience. What I believe is that the perfect plug porcelain can be very white in some places, as long as it is not all white.
I am not sure of the attached picture or how these Mercury M2 engines should look. Just saying yours may be Ok.
Looking at your pistons they seem Ok. To my untrained eyes.

 

[abnwayne]

up should be on the flywheel side

 

[Tim75]

On the spark plugs, see the attached.
I have read way to much on spark plug reading/diagnosing , but have limited real life experience. What I believe is that the perfect plug porcelain can be very white in some places, as long as it is not all white.
I am not sure of the attached picture or how these Mercury M2 engines should look. Just saying yours may be Ok.
Looking at your pistons they seem Ok. To my untrained eyes.

I am familiar with your "mark on top toward the exhaust port". Snowmobile,
motorcycle, etc.

The reason for a mark is to align the pins in the piston grooves that align the rings such that the end of a ring will not catch in a port and destroy a cylinder bore.

Yes, your pistons are upside down. Up means toward open sky. However, I don't think that is all of your problems. Your compression numbers are a little low. In a perfect world, compression would be 140psi. One common reason for low numbers in a fresh rebuild is a broken ring. Our rings are thin and somewhat delicate. When broken, it is usually done by your ring compressor clamping a ring over an alignment pin. How is your ring end gap?

Does you trigger base move as you advance the throttle? Is you throttle cam in place with its bushings? Any broken reeds? Any intermittent ignition parts (fail when hot)? Are you using NGK original spark plugs? How are the injectors?

A pic of the other 3 pistons would help judging fuel burn and spark.

 

[DomsX20]

I am familiar with your "mark on top toward the exhaust port". Snowmobile,
motorcycle, etc.

The reason for a mark is to align the pins in the piston grooves that align the rings such that the end of a ring will not catch in a port and destroy a cylinder bore.

Yes, your pistons are upside down. Up means toward open sky. However, I don't think that is all of your problems. Your compression numbers are a little low. In a perfect world, compression would be 140psi. One common reason for low numbers in a fresh rebuild is a broken ring. Our rings are thin and somewhat delicate. When broken, it is usually done by your ring compressor clamping a ring over an alignment pin. How is your ring end gap?

Does you trigger base move as you advance the throttle? Is you throttle cam in place with its bushings? Any broken reeds? Any intermittent ignition parts (fail when hot)? Are you using NGK original spark plugs? How are the injectors?

A pic of the other 3 pistons would help judging fuel burn and spark.

Tim, I appreciate your thoughts and help. I do agree that compression is a bit low. But the low RPM and low top-end speed make it "feel" like 2 dead cylinders rather than a slightly under compression issue. I am still in the process of disassembly so I will let you know the ring end gap when I get to that point. To answer your other questions

- Yes the trigger advances with the throttle. I actually replaced the trigger early in my troubleshooting. I had some brittle trigger wires due to corrosion. Replaced trigger and double, triple checked ignition timing at idle and WOT. All appears within spec.

- Throttle Cam and roller bushings are in good shape. The butterflies also operate as expected.

- No broken reeds (picture attached, still installed). I actually replaced these during troubleshooting with some Boysen power reeds due to some gaps that were out of spec.

- Intermittent ignition parts - Recently yes, but this was not an issue at first.. Just last year, it began to have issues starting when hot... Several years ago, I replaced the port and starboard temperature sensors, all 6 CDMs, the rev limiter and ignition controller without any change performance. The replacements were all CDI brand. Also have changed out the Air Temperature Sensor.

- Spark Plugs - replace these every year with NGK BPZ8HS10

- Injectors were professionally sent out for test and service - no change to performance

- Initial Piston/ring observations. As requested, I took a picture of the starboard side. There is no scoring on any of the cylinder walls. Perfectly smooth on all 6 cylinders. I have not pulled the pistons out but it appears that all of the piston rings are in tact. I will know more in the next couple days.

I have spent a LOT of time and money trying to diagnose this problem and gave up about 3 years ago since the boat ran OK. I finally decided to dig back in... I was really hoping I found the issue with the pistons being upside down.