Piston Burner Part DEUX

[SkiDaddy]

On 3/9/06 I lost the front cylinder, and attributed it to a clogged cooling line (first pic). Well, it happened again today, and both pissers were flowing normally both before & after.:banghead: :banghead: :banghead: :banghead: (all other pics). Rear cylinder is fine. In both cases part of the upper ring was missing when I pulled it apart

I'm at a loss as to why this happened, or exactly what did happen. Did I lose a ring in the exhaust port, leading to the melted piston? Front exhaust port width is 56mm; rear exhaust port width 55.4 mm. The bore & chanfer job was done by Paul Lehr, so I'm sure it was fine.

Both carbs were jetted the same, and the ski was hitting fine off the bottom before this happened (in the surf BTW). Piston wash shows jetting was OK.

Any ideas what happened? Can this cylinder be made reliable?

 

[Speedfreak]

Did you do a leakdown test before you pulled it apart? I had a kawasaki do the same thing. Finally found a crack in the case causing an air leak. I do leakdown test a couple of times before I run a motor now.

 

[SkiDaddy]

I did do a leakdown last time I rebuilt it, but didn't do one today. Guess I'll put the jugs back on & test it tomorrow.

 

[SkiDaddy]

Been reading up - If it went lean it should have had ~some~ signs of detonation, & there are none. Also, it was running fine, hitting off the bottom fine - if it were lean it should have hesitated off the bottom.

I'm thinking it wasn't from being lean at all.....

 

[keefer]

Looks like the EGT was real high and caused the piston to fail from being super heated. I had a 760 do that to me once and it was caused from too much compression running 93 octane fuel. My cheapo comp guage was reading low and I thougt I was ok for pump gas. My main jets were also too lean and it caused the failure. Are you running any static advance on the timing? What ignition are you running? Too much advance up top can be bad when running a freestyle motor at WOT for too long. All being said these are examples of several things that could contribute to this. A leak down test for air leaks is a good idea, but you need to call Paul and he can probably help you since he did the port work and should know your entire setup.

 

[Matt_E]

It doesn't look like a ring snag. The piston crown by the exhaust port was melted/disintegrated.

 

[Water Worx]

It was caused by deto,it melted the piston then with no piston to support the ring, the ring failed.

 

[butti]

what was the compression?:reporter: :biggrin:

 

[Matt_E]

what was the compression?:reporter: :biggrin:

Nice emoticons! :biggthumpup:

 

[SkiDaddy]

Comp. 190 on a cheapo gauge. It's been running this comp. for 9 months though, including hammering it many times at extended (for a standup) WOT.

BTW, the cylinder was originally a RIVA pump gas ported cylinder - Paul touched up the porting (he couldn't put metal back into the exhaust port), bored, honed, & chamfered it. It's not really his work but RIVAs.

If it was deto of any sort why is the top of the piston & dome completely free of any deto pits?

If the EGT was super high why was the other piston pristine? You'd think since the 2 cylinders "share" the exhaust manifold high EGT would affect the other piston too...

 

[keefer]

My 760 failed in the exact same way on the front cyl only, although not as extreme as yours.The rear cyl looked fine. You must be on the edge with that setup. 190 is pretty risky on 93 octane, especially at high RPM. Either change to smaller domes or start running race fuel. Find someone who has an accurate guage and be sure of the comp. Cranking comp is a baseline but it does not tell you what the cyl pressures are at high RPM. Porting, pipe, etc. start coming into play. Do yourself a favor and talk to Paul, he is much more knowledgeable about what you should change to fix your problem.

 

[Water Worx]

Comp. 190 on a cheapo gauge. It's been running this comp. for 9 months though, including hammering it many times at extended (for a standup) WOT.

BTW, the cylinder was originally a RIVA pump gas ported cylinder - Paul touched up the porting (he couldn't put metal back into the exhaust port), bored, honed, & chamfered it. It's not really his work but RIVAs.

If it was deto of any sort why is the top of the piston & dome completely free of any deto pits?

If the EGT was super high why was the other piston pristine? You'd think since the 2 cylinders "share" the exhaust manifold high EGT would affect the other piston too...

Deto does not show on the dome,it usually shows as a sandblasted on the outer edges of the piston,this is not something that happened over night it has been going on for a long time.

EGT can be different on each piston with same jetting in both carbs,same compression,etc,etc. Each cly is different,my 720 doodoo front cly always is higher than the rear by 100 degrees.

Severel things could cause this,my first guess is your "cheapo" comp guage is reading low & you are probally higher than you think & you are probally running right on the edge & running pump gas so its a slow death.:frown:

 

[Matt_E]

This time the gas from the pump may have been a lil bit below the limit.

 

[Water Worx]

This time the gas from the pump may have been a lil bit below the limit.

I would bet that this didnt "just" happen but has been picking away at the piston for some time.

 

[Matt_E]

Very true.

 

[kid4now]

What type of coil are you running? Are you running any advanced timimg at you stator?

Thats deto. I`ve gone through the same thing. For me it was a piece of junk msd improved coil. One plug wire was messed up and would miss fire intermitatntly but you could not tell when riding. So i changed back to a stock coil and backed off on the advance I was running and never had the problem again. I did go through 2 or 3 pistons though.

 

[SkiDaddy]

So others have had a hole melted in the SIDE of the piston? I haven't been able to find any info on that, the pictures I've found always show the top being melted.

I'm running 33cc domes, but the exhaust port is raised a tad over 1mm so comp. is less than w/stock porting. I always run fresh gas & never set my cans in the sun.

I run an MSD enhancer & JSS rewound stator w/3 degrees initial (static) advance.

2 weekends ago I was tuning the high speed adjusters w/a Tiny Tach. After tuning I pulled the head off to check piston wash & there was no sign of any problems. Before this weekend I richened up the pilots from 75 to 77.5, and lesaned the mains from 137.5 to 135. But this should have equalled the same mixture at 3/4 throttle or so right?:dunno:

 

[Water Worx]

Not necessarily,I would def go back on the mains & maybe back the timing up a little & also check the squish clearance.

 

[eel]

I recently had an engine come in with similar damage. ended up being caused by a type 9 pipe internally cracked at the stinger end.

 

[Waterdawg Kustomz]

Also, 190 lbs. of compression is border line pump gas. With the quality of some pump fuels you have to be careful. Gas is from the pump is garbage and your motor will let you know that!!!!! Anyways, that sucks and I feel your pain. Keep positive, it will all be good.

Jeff