MSX 140 centre cylinder, apparent lean failure?

[GGWA]

Hi all , new to this site and looks like some good info. Had many cars and boats but just got my 1st ski. Am auto mechanic / engineer by trade.

I bought a 2003 polaris msx140 ho here in western Australia not running with pto cylinder water induced.

Just done full top to bottom rebuild and fired up great.

Whilst checking the oil output to each cylinder at cranking speeds I thought it was a bit low after bleeding it so decided to add some premix for 1st start up around 15L .
All was well on about a 1hr ride with low then mid throttle with occasional short wot . Run great and brought it home , used about 7L.
Took it back out 3 days later added 30L 98 octane unleaded and all was good cruising around. On wot attempt for around 10 sec motor just cut to a lock. Fired back up 5 min later and ran ok then did something on wot again.

Took home and tested . Comp 130 psi on pto and mag but zero centre. Stripped and had had burn on top of piston towards exhaust side with a hole and complete cylinder score grab.
Looks definitely like a lean out not lubrication problem.

Would run fine for ages on max half throttle but will lock on wot any longer than 10 sec only on middle cylinder.
Checked fuel pressure on water while cruising and a couple stabs and maintained a steady 20 psi.

Maybe crook injector and or ecu? Not sure if fuel reg mod has been done.
Any advice would be appreciated.

[K447]

Welcome.

Adding oil to the fuel tank on a Ficht fuel injected engine does not provide any additional lubrication to the engine. The fuel is injected directly into the combustion chamber, where it is instantly ignited and burned.

More info here
http://polarispwcknowledge.shorturl....t-things-to-do

Has this engine been rebuilt recently, or perhaps the center cylinder/piston was replaced? Wondering if the piston was incorrectly sized.

Lean burn on these engines tends to create a crater hole in the center of the piston, not the exhaust port area.

[GGWA]

Hi K447 ,
Yes I just recently rebuilt it . New bearing rods pistons , re nikasil bores etc. Used spi pistons and clearances were perfect. Bores were diamond honed then plateu finished.
As said pto and mag are still perfect just centre cylinder problem. Yes I agree being Di not fuel and air case reed induction your comment makes perfect sense regarding premix.
Burn started from centre and got gradually worse towards exhaust side of piston.

[HiPeRcO]

Welcome!

Can you post pictures of the damaged cylinder?

[GGWA]

Hi mate, not the cylinder but I will try send one of the piston.

[GGWA]

[GGWA]

Seeing these things are common to run on the lean side , after scouring the internet and once I get to the bottom of this and fixed was wondering if anybody has used the evo tech fuel controller to make things better and possibly safer?

[K447]

Seeing these things are common to run on the lean side ...

When running properly the Ficht system has tight control over the fuel:air ratio. The system works very differently to a tradtional carb engine, including how the fuel is mixed into the air. In stratified combustion mode there is a lot more air in the chamber than a carb engine would have. The fuel injector plume is aimed directly at the spark plug tip and ignited near simultaneously. This creates a fuel efficient burn and less fuel consumption per traveled mile.

Tradtional carb guys might consider this 'lean' but it is simply efficient. Adding fuel would just make a little more power, which is not the same as more rich at the original power/RPM/throttle level.

At higher RPM (above 5500 ish) the Ficht system seemlessly changes to homogeneous mode. This mode is similar to carb engine combustion conditions but also tightly controlled.

When a Ficht engine is 'running lean' there is something wrong that needs to be corrected.

[HiPeRcO]

Its possible that failure was caused by a broken ring. This would let fuel escape, causing a lean meltdown.

[randys]

The most common causes for a burned hole in a piston are:

• air-leak in intake manifold or in crankcase seals (creating a lean mixture)
• lean carb jetting (or fuel injector clogged)
• too advanced ignition timing
• too hot a spark plug range
• too much oil ratio (displacing fuel=lean)
• 87 octane fuel (too much oil also reduces octane further)

from your description you should probably look at air leaks in the recently rebuilt block, a clogged fuel injector and replacing fuel without oil.