Pacific Coast engine puzzle

[solo1]

My oldest son has a 97 Honda Pacific Coast . 800cc Vtwin liquid cooled.  At 135,000 miles one cylinder went dead.  The problem was the spark plugs.  These plugs have about 30k miles on them.  The tech pulled the plugs and found this.  One plug is missing part of the center electrode and completely missing the outside electrode.  Neither plug show high temps, no melted electrode, no preignition, no signs of hammering  The other plug has the center electrode bent or broken and broken porcelain but it was still firing.

The tech was thorough, he checked the cylinder with a borescope, no pieces of the plug in the cylinder, no damage to valves or the cylinder wall.

It looks like a failure of the plugs, one maybe but two?  Keep in mind this is a simple two valve vtwin, one plug for each cylinder.

Any other ideas?

[Michvalk]

Wrong plugs? Different brand? Never have seen plugs do that before. Well, I have, in an alcohol burning funny car, and the plugs were the wrong heat range. I would say running real lean, but I would have noticed how bad it was running before it got that bad, I would think. Got me stumped!

[Valker]

Plugs too long? If that is caused by pre-ignition, there will probably be piston damage.

[R J]

Wayne, 1st place I would start is the parts book.    Checking to see if I got the right plug.      I'd don't recall my old PC having a deep throat plug.    From the looks of it, I'd say the piston has slapped that plug a few times.

[Bone]

Has he got any friends ?  We used to poor a half cup of oil under the non HD's. Finding nothing in the cylinder sounds like a prank.

[solo1]

There wasn't any noise from the engine. no knock, no pinging, no nuttin'  no sign on the outside electrode that it was hammered. In fact the gap was over .050. I've seen wrong temp plugs and there is no sign that they saw high temps like globules  of carbon or burned porcelain.
I'll check with Wayne but I think that these are the right plugs,  seems to me that they would've failed long before 30k miles on them if they were the wrong heat range to say nothing of holes in the piston.  

As far as a prank goes, they'd have to go to a lot of trouble to change plugs.  The engine is completely surrounded by Tupperwear.

[Bugslayer]

It looks like a failure of the plugs, one maybe but two?  Keep in mind this is a simple two valve vtwin, one plug for each cylinder.

 

  I sure though  my old '90 PC800 had two plugs per cylinder.

[solo1]

The '97 had only one spark plug per cylinder.  However, I was wrong; it is a four valve engine, not two valves.

[shooter64]

With that many miles could hardened carbon be building up on top of the pistons and starting to touch the plugs?

[RiderNJ]

I had a misfire in my GMC 5.3 L Envoy.  Everything was fine...  15 miles into my 30 mile ride home,  suddenly developed a miss.  I pulled over,  checked the plug boots and all were secure.  Took it easy getting home.  Pulled wires one at a time...  bingo... pulled the plug and center porcelean had disintegrated... piece of center electrode gone.   Never had that happen before.  Turned the engine over to blow out any remaining debris...   threw an old plug I kept from last change in there...  fired right up.  Never an issue before or after. 

Could have been the running to get home that wrecked the plug...   ??   At 3000 rpm that's 50 per second..  guess it  wouldn't take long to do the damage.
RiderNJ

[YoungPUP]

HIgh RPM  HEavy load.  Maybe got lucky and pushed the debris out of the cylinders when the plugs came apart?  (Where's the smiley with crossed fingers?)

[olddog1946]

If those pistons and plugs were banging each other you'd have heard and possibly felt it happening. I wouldn't overthink it. It is possible that those plugs had a minor/unnoticed crack when installed and over time (heating and cooling) came apart. Put in new plugs, fire it up and listen..if it runs fine, ride. IF not, get ready for some work.

[gregc]

  Don't think the piston or carbon could have done that.  It would have to hit the very lowest point on the plug first, the ground for the electrode.  My guess is, very hot cylinder temps from lean condition. Remember the 15% methanol in todays fuel, it is causing many strange problems.

[tonyfan70]

  Don't think the piston or carbon could have done that.  It would have to hit the very lowest point on the plug first, the ground for the electrode.  My guess is, very hot cylinder temps from lean condition. Remember the 15% methanol in todays fuel, it is causing many strange problems.

See my post in the thread about cleaning spark plugs with a sand cleaner.

CLIFFS NOTES: Loves Truckstops is paying for damages to smaller engines caused by ethanol content being too high in gasoline they sold.