Piston top analysis

[drjack]

I purchased a 3 foot camera scope for my Android tablet, to save time and trouble of dismantling the engine top. I found the piston tops of our 951 was mostly blackened with carbon, except for a dozen 1/8" shiny pitting spots about the center area of the piston top, and shiny strips along the two outside edges where intake and exhaust ports would be.
By looking at the attached photo, our pistons match closely to the image that says "too lean" ( on the right) . But I'm questioning the validity of this chart. You would think too lean would be the piston that's completely shiny ( left image) ......or, am I wrong?

I rebuild the carbs, last spring, with genuine Mikuni carb kits, and set the low speed to 1.25 turns. Based on my description, and if this verifies a lean condition, should I open the low speed jets a little more to richen the mixture?

 

[ORION '95 Speedster]

I think that the spark plug condition will be easier to read and more accurate than the amount of carbon stuck to the piston VS how much carbon has flaked off.

 

[Sportster-2001-951C-Stock]

How well does it run? IMO, increasing the idle speed mixture by backing the screw out some amount won't gain an appreciable part throttle fuel increase but this will (and should) richen the idle mixture.

ie: The LS adjuster is for tuning your idle mixture.

 

[drjack]

The engine runs great, after rebuilding the carburetors. Previously, I experienced some hesitation, and actual engine stoppage around the 3500 rpm region. But now, we have quick and crisp throttle responses from any rpm position.
The one noteable change I did during the carb rebuild was close off the HS jet setting. It was originally opened one full turn.

 

[Sportster-2001-951C-Stock]

I haven't seen my pistons in two years so can't say what the wash looks like. I'm using 87 octane pump gas with 10% ethanol and the same #80 jets Seadoo used in the 1998 951 calibration. It blows 122psi as of last year.

Stale fuel is not allowed, low quality fuel leads to detonation. I mix nothing with my fuel, I just make sure it's never stale (87 octane with 10% - a few months old is approaching stale, pump it or use it up).

My thought is aluminum pits and carbon out to the edge and perhaps beyond down the skirts into the ring land area is indicative of being too lean (hot pistons can be wear issue too) and this is pretty common due to the pilot holes tend to close up with a slight bit of native corrosion that isn't removed by using conventional solvents and compressed air.

Lean hesitation part throttle is the big clue of being dangerously lean.

Best I can tell I don't need more fuel at this point, judging by the 4-stroke burble I hear from about 2200~2800RPM, it's not pronounced but it's there and hopefully all the fuel I need.

I have to say, the 1st thing to do is restore the pilot holes, this is the root of 99% of hesitation, lean stalling and part throttle detonation issues, IMO.

Best I could tell from my experimentation with my boat, the large venturis don't become dominant till about 70% throttle due to that's where my hesitation would disappear, which was quite pronounced hesitation and lean stalling. If you add fuel using the HS adjusters, I expect you will (and should) see a drop in WOT and not much benefit.

Pilot holes (low speed venturi) are the key.

We use our boat every weekend, I estimate about an hour of engine time is accumulated and almost 5 gallons burned with an average cruise of 50~55% throttle at ~5200RPM and of course a short WOT blast once or twice.

 

[drjack]

Sportster, I agree the fuel quality used is a very important. I was informed that any fuel with ethanol will absorb moisture. Most brand name high test fuels contain zero ethanol. Our marina supplies ethanol free fuel, and that's what I have been using all this year. There is a difference.....the engine just performs and feels right.
During the carburetor rebuild, I did pay attention to those 3 small pilot holes. Two of them for the mag carb were mostly plugged. I believe I succeeded in cleaning them but they are so small it's difficult to ensure they are 100% whistle clean.
The end result of a vibrant running engine was indicating to me that, I believe, the rebuild was worth the effort.
The pitting I see in the center part of the piston is not indicating any surface damage, but more like a clearing of carbon.
I guess working with a fresh set of plugs to analyze what is happening will be the best way of indication.

 

[Sportster-2001-951C-Stock]

Yes I'm quite sure my low speed venturi pilot holes are slightly restricted, this is the only explanation I have for why installing larger low speed jets resolved my hesitation issue.

 

[Sportster-2001-951C-Stock]

I think that the spark plug condition will be easier to read and more accurate than the amount of carbon stuck to the piston VS how much carbon has flaked off.

IMO, reading the piston crown tells you more about the piston temperature than reading the plug does.

 

[Chester]

IMO, reading the piston crown tells you more about the piston temperature than reading the plug does.

:thumbsup: Absolutely!

Chester

 

[ORION '95 Speedster]

IMO, reading the piston crown tells you more about the piston temperature than reading the plug does.

If you know what your doing, have a micro bendable camera, the extra time, and inclination - I would imagine that the piston would be able to provide valuable information that could be used in conjunction with all of the other indicators. I certainly would not turn away any valuable hints that the motor can offer, however I dont think that most peoples skill level or available tools would allow for regular inspection of the piston. But I agree, it would be useful to look at the piston regularly. Regardless of your skill level, if you own a 2 stroke engine - you change spark plugs. Teaching people how to read plugs would save a lot of motors - then if they have the inclination they should definitely research piston indicators.