Strange thing happened today going to work. I have a new rear tyre on, which is much taller in profile (it's a BT45 4.00-18, closer to the recommended figure which is 4.10-18) than the old one, which was much lower in profile (100/90-18 -- I see these all the time on CB250RSs, can't work it out, as 4.10-18 is a lot bigger, isn't it? The 100/90 worked very well though). Coupled with me fiddling with sprockets recently, the result is 1st gear is a bit taller than I've got used to lately -- so I was very silly and stalled the bike hopping up onto the footpath where I park my bike.
It didn't just stop like it used to; with the fatter spark, it kept wanting to turn over long past when it would normally cut out. So it made a horrible bottom-end knocking noise, *klonk klonk klonk* and then it stopped. Fine. I'll push the bloody thing to my parking spot.
On a whim, I then tried to kick start it, in case I had somehow fooked the engine (it was a really horrible noise, I promise). Nothing! Turned over fine...
So, bump start. Go to pull it back to TDC against compression, but it just kept going. Next to no compression. Getting scared now :P I jump on it down the hill anyway, eventually fires, runs like a dog for a few seconds, then magically it goes back to normal. Ride back up the hill, park it again, starts first kick. Running sweet since then.
Why?!?! I can only assume losing compression and then getting it back like that is a result of valves sticking open or something? It did something like this once before -- the engine got super hot, then I left it out overnight and it got super (frost) cold. Valves seemed to be sticking open, but cleared eventually by themselves. Why would a horrible knock (detonation? I've switched to 91 and it pinks sometimes when hot at too low RPM when I open throttle too far) cause valves to stick?
All very strange. Did an oil change tonight for interest's sake, didn't see anything interesting floating in the old oil.
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