Grenade finally went off!

Seems like pre-ignition. Mixing in extra oil is often to your benefit, especially when rolling WOT all day. What was the trouble with the CDI? could it have caused pre-ignition? Rolling down the hill with a closed throttle while the rear wheel pushes the crankshaft may cause the cylinder to dry up and seize, but not detonate. Unless it seized and the CDI fired off one last time, blowing the hole out and un-seizing the piston... :P hehehe
 
Adding more oil will not help, and in fact can lower octane with will encourage detonation, and a lean fuel to air ratio.
 
bowljoman said:
Seems like pre-ignition. Mixing in extra oil is often to your benefit, especially when rolling WOT all day. What was the trouble with the CDI? could it have caused pre-ignition? Rolling down the hill with a closed throttle while the rear wheel pushes the crankshaft may cause the cylinder to dry up and seize, but not detonate. Unless it seized and the CDI fired off one last time, blowing the hole out and un-seizing the piston... :P hehehe
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The screw coming off the cdi that has the plug cable on it snapped off, probably from enough high frequency vibes and some lateral pressure from the cable being too long. I'm pretty sure that caused a huge change in timing and that's what did it, but I couldn't say for sure. But it didn't seize, as there are no markings or anything on the cylinder walls and the piston still moved freely. Plus I heard the spark snapping noise of the broken cable screw JUST before it happened.
 
k.mah said:
The screw coming off the cdi that has the plug cable on it snapped off, probably from enough high frequency vibes and some lateral pressure from the cable being too long. I'm pretty sure that caused a huge change in timing and that's what did it, but I couldn't say for sure. But it didn't seize, as there are no markings or anything on the cylinder walls and the piston still moved freely. Plus I heard the spark snapping noise of the broken cable screw JUST before it happened.
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im with you there. Having spent enough time working at planet hotrod and building out monster engines, your image of the piston is a classic shot of pre-ignition catastrophe.
 
bowljoman said:
I just know from experience with high compression running high-octane in 11000 RPM engines. 25 to 1 mix btw :P
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Except these aren't high octane/high compression 11,000 rpm engines.

Just "adding oil" will not help the situation when he already has a detonation problem...but you know that from building out monster engines, right? ;)
 
sounds like you could use a piston that could withstand higher temps. i got some in the swap and shop.

i got to say tho, that is the most mangled china doll piston ive seen yet
 
Bikeguy Joe said:
Except these aren't high octane/high compression 11,000 rpm engines.

Just "adding oil" will not help the situation when he already has a detonation problem...but you know that from building out monster engines, right? ;)
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He had a puch 70cc hi hi compression head. He had ridden 14 miles already at WOT which is not trivial. He is doing 9000 RPM estimated and keeping up with highway traffic. No trouble at all, until the moment it blows up.

Yes, I have built monster engines from 22CC that do 16000 RPM's, to supercharged 454s running 7 to 1 compression @14 pounds of boost, to Hilborn injected alcohol under 24 inch velocity stacks. So yeah, after reading his saga, in my opinion, had his CDI stayed where it should, he'd still be running strong. I think its reasonable to believe running 30-to-1 saber was not the issue...

We are all ****ing experts around here. Go ride a bike or take a running leap through a rolling rim...
.flg.
 
moonerdizzle said:
sounds like you could use a piston that could withstand higher temps. i got some in the swap and shop.

i got to say tho, that is the most mangled china doll piston ive seen yet
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Ahhhh I just bought a new one too! Are those the ones with the Teflon coating or whatever it may be?
 
Bikeguy Joe said:
Somebody has his panties in a bunch.
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I just get sick of sarcasm from know-it-all builders who fail to read.
Leaning out an engine with 2-stroke oil does not make a detonation. It actually cools the cylinder. And he would not be making 9000 RPM under load either.

But of course you know that, great god-father of motorized bicycles.
 
I just hate it when know-it-all's have to "qualify" themselves with all their great experience.
Leaning out a mixture with too much oil lowers octane. Lower octane promotes detonation.

BTW, I didn't name myself the "Godfather of motorized bicycles".

But you knew that, right? ;)
 
BTW! This is what my cylinder looks like. Ill be cleaning it up before re-assembly and re working the ports to better benefit flow to certain areas now that my knowledge on what to do is much higher than it was when i initially did this.

 
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the only thing really holding these motors back from high rpms is the stock cdi. if you make your own cdi that doesnt advance the spark like the stock ones you can get higher rpms out of your engine. 9000 isnt out of the question
 
moonerdizzle said:
the only thing really holding these motors back from high rpms is the stock cdi. if you make your own cdi that doesnt advance the spark like the stock ones you can get higher rpms out of your engine. 9000 isnt out of the question
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:o Seriously?!
I've been trying to get the parts to build one from a single vendor so i'm not spending crazy amounts on shipping, and been slowly deciphering the plans for making your own that are here on the site.

I think i'm stuck making the jump to build one now! especially if it gives me that much more potential (not to mention i just port matched cylinder to jug, and increased size while promoting better flow dynamics)
 
Not the Radio Shacks I went to...had to go to Mouser. Bought enough parts for 3 CDIs for about $30 delivered.
 
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