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What destroyed my pistons?

  • Thread starter Thread starter Paul.S
  • Start date Start date
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Paul.S

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Several months ago I opened my gs1100 up wide open for about a mile (about 150-160mph) and then it suddenly lost power. It had done that before and after I would let off, the bowls would fill back up with fuel and I was good to go. I then started running petcock in the prime position and that fixed it I thought. The motor was allready about 210 degrees from driving 85mph the last 40 miles at about 7000 rpm (factory gears). Anyways, this time the power never came back so I towed it home. I found two destroyed pistons. #2 and #3. Those will run hotter because they are the inside cylinders. I thought what surely happened was the factory petcock didn't keep up with my 33 smoothbores and leaned it out this time doing it in for good. But now I am thinking it might have been piston clearance and ring gap related too. I had about .0008" piston clearance on an MTC 1198 kit. MTC recommends .0005"-.0015" side clearance so I was in that range. My ring gap was about .014" top and .020" bottom ring. My A/F ratio was set at about 12.5 to 13.5 at WOT of course if the bowls were full. This bike had made over 100 passes at the local strip in the 10s with not one problem before this happened. Here is one pic and I will upload another one: Thanks!

2017-09-12 22.15.31.jpg
 
Don't know enough looking at the damage to say what went first, but I would for sure get a Pingel. I had starvation problems with a stock bore, 0.340 cams, 4:1 and BS36 carbs.

With the Pingle I never starved the engine with sustained 135 on 1166cc (closed circuit track).
 
posplayr said:
Don't know enough looking at the damage to say what went first, but I would for sure get a Pingel. I had starvation problems with a stock bore, 0.340 cams, 4:1 and BS36 carbs.

With the Pingle I never starved the engine with sustained 135 on 1166cc (closed circuit track).
Click to expand...

Yep, I went ahead and bought a single outlet pingel for it. Also, I forgot to mention cylinders 3 & 4 looked perfectly fine.
 
Paul.S said:
posplayr said:
Yes, it has 750 gears and oil pressure was more than 15psi at about 8000rpm.
Click to expand...

I noticed that on mine I could suck the oil pan dry on a high rev(10K). At a sustained 6000-6500 (guessing) RPM in 5th gear I probably never sucked it dry.

Things happen fast, but that was my inclination. I had a 15 psi pressure gauge on the handle bar.
Click to expand...
 
Do you have a big oil cooler that is mounted ports down?

I think you probably subtract the cooler capacity from your oil pan level so you need to over fill by the cooler amount.

That is why some people mount the coolers upside down, the cooler does not drain when the engine is off increasing the oil capacity with the same crank case level.

I only used a small 550 oil cooler, but with the Series R/R and an oil sprayer on teh stator the engine stayed below 220-230 F in almost all conditions
 
One more thing. If you use the universal ring gap charts, my top ring gap should be .0135" and bottom ring .016". The bore is 76mm. I called MTC and they recommended .016" top and .022" bottom. I guess I will go with MTCs recommendation to be safe when going back together. The rings new out of box measure .013" top and .019 bottom. I had new sleeves put in by MTC and my side clearance is on average .0012" with the tightest spot being .00075" and loosest spot .0015". Of course this is measured with generic brand micrometers and dial bore gauge so accuracy may be off a little.
 
Paul.S said:
One more thing. If you use the universal ring gap charts, my top ring gap should be .0135" and bottom ring .016". The bore is 76mm. I called MTC and they recommended .016" top and .022" bottom. I guess I will go with MTCs recommendation to be safe when going back together. The rings new out of box measure .013" top and .019 bottom. I had new sleeves put in by MTC and my side clearance is on average .0012" with the tightest spot being .00075" and loosest spot .0015". Of course this is measured with generic brand micrometers and dial bore gauge so accuracy may be off a little.
Click to expand...

DUNNO, my machinest gapped all my rings to match the bore.
 
posplayr said:
Do you have a big oil cooler that is mounted ports down?

I think you probably subtract the cooler capacity from your oil pan level so you need to over fill by the cooler amount.

That is why some people mount the coolers upside down, the cooler does not drain when the engine is off increasing the oil capacity with the same crank case level.

I only used a small 550 oil cooler, but with the Series R/R and an oil sprayer on teh stator the engine stayed below 220-230 F in almost all conditions
Click to expand...

Mine has a lockhart oil cooler. Not sure what size. My oil temp gauge must be out of calibration because everyone complains about high oil temps but mine would allways cruise at 190 degrees and about 200-210 if you drove it hard on a hot day. The hottest it ever got was that 220 or so when the pistons let go. I live in Texas too. I did notice all the oil drain holes in the cases were ported.
 
Paul.S said:
Mine has a lockhart oil cooler. Not sure what size. My oil temp gauge must be out of calibration because everyone complains about high oil temps but mine would allways cruise at 190 degrees and about 200-210 if you drove it hard on a hot day. The hottest it ever got was that 220 or so when the pistons let go. I live in Texas too. I did notice all the oil drain holes in the cases were ported.
Click to expand...

You should check the sensor, 190 is extremely cool for a big bore like that. It is probably not that far off so it suggests a big fat cooler robbing oil from the crank case when running.
 
posplayr said:
You should check the sensor, 190 is extremely cool for a big bore like that. It is probably not that far off so it suggests a big fat cooler robbing oil from the crank case when running.
Click to expand...

To answer your prevoius question, my cooler is mounted with ports facing down. I allways would over fill the oil so it would show full right after I turned motor off.
 
Paul.S said:
To answer your prevoius question, my cooler is mounted with ports facing down. I allways would over fill the oil so it would show full right after I turned motor off.
Click to expand...

Send the pictures to MTC and see what they think
 
I would maybe try shooting that pic to Rapid Ray. Melted pistons tell a story. To me, that looks like detonation issue, but I am far from an expert. Seen a lot of snowmobile engines with pistons like that but 2-strokes are vastly different with burn downs.
 
I would think detonation. The huge chunk out of the top of the piston in the first photo didn't happen from lack of oil. Too much timing or too lean. Dar
 
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