Detonation, lean seizure, or....?

[garry inglis]

When you originaly set the piston within the barrel at bdc was the top off piston level with the transfer ports and usually 1/2mm over the exhaust port if below you would open up the exhaust port but if it was 4/5 mm over the exhaust port at bdc it get to hot on the edge that's showing in the photo

[holty]

when i built mine, i have a packer plate between the case and barrel, i had to machine the packer to the correct thickness so the crown of the piston was level with the bottom of the exhaust port at bdc, when the piston was at tdc it was about 1.5mm from the top of the barrel, which meant i had a massive squish when the head was fitted, as they have a recess , so i had my head skimmed flat to take the squish to the 1.5mm i was after, i think the .7mm that you had would sure be a factor in detonation, your barrel is not the same as mine as mine is non rave, i am going to increase my stinger size anyway , better safe than sorry, a work in progress, you will get it sorted im sure, as with all one off builds you will have teething troubles, especially when your the first to build one, good luck ryan.

[RManson]

Ordered a set of 1.2mm base gaskets, apparently the thickest base gaskets available, and dry assembled everything to see where we sit. This is with a new cylinder/piston/head and the 1.2mm gasket:

Squish is 1.016mm (0.040”) with 0.381mm (0.015”) positive deck height.

Stock Skidoo spec is 1.5-1.7mm (0.060-0.070”)

Cylinder Head Volume is 32cc measured which results in a corrected combustion ratio of 10.08:1 (5.96:1 corrected).

Stock Skidoo spec is 28cc (based on 11.37:1 uncorrected compression ratio, 6.11:1 corrected) Ski Doo specs 11.5:1 stock compression.

I'm not sure I put a lot of faith into the volume measurement as much as I do the squish as the two don't really jive when compared to the stock specs. With the lower squish, you would think it would have a lower head volume, given stock piston and head arrangement. Based on the stock Skidoo specs, the squish is a tad tight, but still seems reasonable.

IMG_6320 by PotvinV8, on Flickr

Looking at the exhaust port, with the 1.2mm base gasket, this raised the exhaust port above the piston ring at BDC, but leaves the crown of the piston still slightly proud of the exhaust port.

Here's the piston as it travels towards BDC for reference of ring location and crown...

IMG_6317 by PotvinV8, on Flickr

And at BDC...

IMG_6318 by PotvinV8, on Flickr

I suppose i could raise the cylinder further with a packer plate to get the top of the piston even with the bottom of the exhaust port at BDC but that would require machining the top of the cylinder and head to get the squish/combustion chamber volume back in line. My goal was to avoid machining either so as to keep things within stock spec once assembled (ie cylinder and head relationship) but that may not be possible. Thoughts?

[RManson]

P.S.

Garry, I think you nailed it! The high combustion ratio and the piston crown hanging over the exhaust port was too much for the piston to handle is my guess.

P.P.S

Holty, I ordered a pipe for a KTM 300 just for kicks and guess what? The stinger is the same diameter as the one I used on the dyno (26mm). Talked to a tech guy at FMF and he said the 250 and 300 pipes are the same, which leads me to believe the stinger diameter is probably okay. Will be having a look at the length, etc and will be watching the EGT gauge when things go back together but I would think that of all the pipes FMF sells to all the groms beating the crap out of their dirt bikes and they don't have an issue, we should be okay.