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PostPosted: Mon Aug 08, 2005 7:22 am 
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Location: Mid Wales,UK
Hello
Can anyome enlighten me on this one? Our Ram Stinger 100 is bored out to 53mm (45mm stroke), ported cylinder, worked head, now on 3rd piston change! It runs on a Dellorto 21mm phbg orginally jetted at 115 main jet, now on third piston gone down to a 107 and popped another 1!
Plug test always o.k until it pops then it looks lean but if you jet up it runs too rich God Knows on this one! :???:
Did a compression test before last piston popped and pressures on 2 readings were 160 p.s.i and 165 p.s.i looking for a quick answer, is compression too high (we run with NO head gasket) or should I look towards the carburretor on primary jet?
Forgot to say exhaust is an FMF Fatty off a KTM 65 cut to fit and works great.
This has been built for hour and a half Enduros and completed everyone but pops a piston when my lad goes on a flat out track!
Any suggestions welcome but I can tell you how quick to change a piston!!
Plenty of power on tap but running out of pistons.Look forward to some interesting replies on this one Regards Rod & Jak


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PostPosted: Mon Aug 08, 2005 7:27 am 
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Sorry all forgot to say we run 40:1 premix and use octane booster mixing all at 19 ltrs fuel, 1 ltr pro boost and 500ml of Putoline MX7 Fully Synthetic.
Regards again Rod & Jak


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PostPosted: Mon Aug 08, 2005 8:58 am 
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First three things that come to mind are :
What is the stinger Diameter, Not enough bleed off?
Squish band is incorrect, so there is not enough cooling of the piston dome.
To hot of a plug.

Fourth thing: Put the timing back to stock.


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PostPosted: Mon Aug 08, 2005 10:08 am 
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Hi Billy
Thanks for the prompt reply.
The Bore is 53mm, the squish is at .7 measured on micrometer off a length of solder! possibly a bit tight ?
The plug at the time the last piston popped was a NGK9HVX. But it did do the last Race on a NGK10HVX and that day was the hardest I have seen my lad race, he did manage a second in class, beating for once a Trinity engined Eton Viper !
The piston popped the next day when he was 'playing' on a flat field but I had changed to a 9 HVX that morning.
Hope this helps
Regards Rod


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PostPosted: Mon Aug 08, 2005 10:11 am 
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Sorry forgot to mention, the hole in the plug is about a quarter of an inch in circumference and is right under the plug !
There is no hesitation on the throttle he actually got the holeshot last time out.
Rod


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PostPosted: Mon Aug 08, 2005 10:12 am 
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Oops missed last thing.
Timing standard, not altered at all
Thanks
Rod


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PostPosted: Mon Aug 08, 2005 6:01 pm 
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My boy's quad did the same thing recently after switching fuel types. We were also riding on flat ground for about 3-5 minutes and thats all it took to burn a hole right in the top of the piston as you describe.I had played with the jetting right before it happened cause it wasn't running right and thought I had it worked out but I was wrong. I think to run this other fuel I'll have to jet up more than I had thought and run it longer when doing a plug chop.


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PostPosted: Mon Aug 08, 2005 9:50 pm 
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Detonation or abnormal combustion:
The effect can be alleviated by various methods such as raising the octane rating of the gasoline used, promotion of charge turbulance,squish effects,improvement of scavenging efficiency, stratifying the combustion process and, naturally lowering the trapped compression ratio.

Quoted From Blair's Two Stroke Engines Book


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PostPosted: Mon Aug 08, 2005 9:51 pm 
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My guess would be the nonexistent head gasket. Once the head starts leaking you could possibly burn up piston. The junky mallossi heads with the oring will blow and do the same thing. Did you notice any oil/dirt around the head gasket area? It may be too dirty to tell. Another possibility would be detonation from low octane and high compression. If the pipe is working as it should the pipes pulse going back in cylinder will make "corrected" compression way higher than an at rest (uncorrected)compr. reading from gauge.


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PostPosted: Mon Aug 08, 2005 9:57 pm 
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To bad most of that went way over my head, I might have a better understanding of whats happening.


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