I have my hypothesis pretty well worked out but figured I'd lay it out there and see what you all thought. The engine has a few hundred miles on it and this happened in 3rd gear, about 4,500 rpm so nowhere near where I figured things would go *boom*. I'm wondering if this could be contributed to a loooong, gradual acceleration in 3rd gear on the dyno as opposed to running it up through the gears and keeping it in the power band?
No idea as it soft seized and the dyno rotated the engine over a few times. It kicked over after the fact but with obvious diminished cylinder pressure. Not sure where in the stroke it seized.
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
Your plug looks quite dark (despite the piston debris stuck to it) so that doesn’t initially indicate lean running....though there maybe a ‘hidden’ lean spot in the mid-range, amongst an otherwise over-rich carb setting. Your dyno operator would have been able to see any lean A/F readings from the word go, no matter how short the run, and would certainly have abandoned the run had they seen such problems?
I’d think if it was a timing issue, then you’d have seen a much neater hole popped in the crown of the piston....rather than the evident smearing of the piston up the cylinder and destruction at the exhaust side.
Other things you need to consider are compression, lubrication, and exhaust....that piston has run hot for a reason...was your fuel-oil mix spot on? Is the exhaust stinger size matched to the cylinder capacity sufficiently to allow the heat to bleed off, and what comp ratio/ign timing combo were you running?
All these numbers need putting into a chart, so you can then look at the engine puzzle as a complete picture...to see how one thing effects another.
Out of interest...how long did your dyno session go on and how many runs did you complete before this happened? I’m surprised this happened on a dyno, is your operator of choice a scooter specific company....or someone used to dealing with 4-stroke motorcycles?
garry inglis wrote: ↑Wed Dec 13, 2017 9:03 am
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
Hi Garry,
I get your meaning now. The bottom of the four transfer ports and the exhaust are even at 66mm below the top of the barrel. With a 64mm stroke and 2mm+/- squish, the piston crown should be even with the bottom of all five ports at BDC. Does that make sense?
As you've mentioned, it doesn't appear to have been running lean and timing doesn't seem to be off, at least at that moment. I can attest to the AFR being in the 12:1 range almost across the board (apart from closed throttle/coasting) as I tested it with an AF Meter. Timing was 24 degrees at 4,500, which was where it soft seized. Compression is stock (11.5:1 uncorrected) with 2mm squish (also stock). Lubrication was 40:1. Header pipe diameter is 40mm on the cylinder, stinger is 26mm, which puts it at 65%, which seems ok.
Was the water cooling up to snuff? Looks like a simple it got hot and seized. If it runs a thermostat in the cooling system, the thermostat might have given up. Timing, 24*, seems high.
.
id check on the stinger size and have that as the failure ,at 300cc id have 26mm as absolute minimum size up to approx 30mm as about right check the vent size on the baffle cone and the stinger tube , mount plate , silencer baffle tube and that it hasnt collapsed ,, 1 more possible is have you shortened or modded cut/welded the header pipe
did u get a power run ?
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