OzVMX Forum

Clubroom => Tech Talk => Topic started by: Nathan S on March 08, 2009, 10:10:20 pm

Title: Is this bad? :P
Post by: Nathan S on March 08, 2009, 10:10:20 pm: This may actually be a repost - my apologies if it is - I've been gunna post it up for over 12 months now, can't recall if I have done so already, but I rediscovered the pics today.

Anyhow, going back to late 1997, I had my little DT125 racer running with a DT175 top end. The air cleaner fell off in practice, so cleaned out the carby, reattached the filter and went out for more practice.
Got a few laps in before it just died in the arse.

Pushed it back to the pits, and pulled the spark plug out to discover that the plug gap had closed up, and there was a tiny rectangular lump of something jammed down next to the porcelin.

Slapped a new plug in and it fired straight back up.

Raced the bike that weekend, and two more HEAVEN meets. Toward the end, it was clearly down on power, and a bit low on compression, but I never thought much of it - it had been flogged hard, and it wasn't even a fresh top end to begin with, so you gotta expect these things.

Anyhow, early last year, I got the ported 125 top end back from Uncle Lozza, so the 175 top end came off. This is what I found:

(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/Juha098.jpg)
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/Juha099.jpg)
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/Juha101.jpg)

I think the only correct word is "oops".
:D
Title: Re: Is this bad? :P
Post by: Lozza on March 08, 2009, 10:32:53 pm: Well it doesn't look good ;D At first I thought there was a bit of ring in there but that looks all there. A close up of the crown/head would tell more,could be bad det marks.That won't explain the bit stuck in the plug though, unless the triangular lump caused it all ;D
Title: Re: Is this bad? :P
Post by: LWC82PE on March 08, 2009, 11:00:04 pm: ive seen that sort of thing on a YZ490
Title: Re: Is this bad? :P
Post by: Nathan S on March 08, 2009, 11:00:12 pm: The lump in the plug was a stone. Before it got stuck, it was pounded into a really neat shape (if anyone knows what a generic surface mount resisitor looks like, then that would be close).
But yeah, the whole top ring and upper land have just disappeared - and the thing still ran great and never even stalled!

The loss of compression was largely due to the remaining ring being stuck for about 25% of its circumfrence.

Those Yammies are built tough! :)
Title: Re: Is this bad? :P
Post by: pancho on March 11, 2009, 10:37:53 am: that pic of the top of the piston reminds me of a section in Phil Iving's book 'tuning for speed' the discussion on detonation "the top of the piston looks like the mice have been at it" cheers.
Title: Re: Is this bad? :P
Post by: oldfart on March 11, 2009, 04:27:42 pm: possible water in fuel ???
Title: Re: Is this bad? :P
Post by: Big John on March 11, 2009, 06:41:34 pm: Yes melting a pistons is bad usually caused by running the bike to lean. Lift the needle or get a larger main jet. The fuel also cool the motor. I learnt this the hard & expensive way.
Title: Re: Is this bad? :P
Post by: oldyzman on March 11, 2009, 08:13:32 pm: Good show Nathan, Behind your bike must have stunk like an aluminium smelter...
Brett
Title: Re: Is this bad? :P
Post by: Doc on March 11, 2009, 10:52:51 pm: wow, looks like ingested some serious crud before you got the filter back on Nathan ;) I kinda lean toward the big end having a little more freeplay now too eh :D
Title: Re: Is this bad? :P
Post by: asasin on March 13, 2009, 05:35:19 pm: Thats exactly what happened to my RM S piston after som p..ck put beer in my feul tank. >:(They wont run on that sort spirit.Causes severe detination I think.I also put a seal on my bike tanks when parked at nite in motels now ;) ,lesson lernt! :'(