*Clunk*, *knock*, then no compression. And then back to normal!

[xwhatsit]

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.

[Ixion]

I have (very rarely) experienced similar on big singles. Happened last a year or so ago on the Yamaha (interestingly, also just after I raised the gearing).

I stalled it in second gear, it made nasty clogga clogga noises and died. Thumbed the electric starter, motor turned over excessively fast with more noises. Oh oh , shit. Then fired , and all was well

Without any evidence or logic whatsoever, I suspect the automatic decompressor system.

I wouldn't worry about it. Just one of those big single things, all part of lifes rich tapestry. 's called character.

[xwhatsit]

I thought you might say something like that :laugh: With the design of the automatic decompression system this motor has, I can't see it sticking open like that; that is, unless the knocking and stalling was unrelated, and it just stuck open when I went to start it.

Still, like you say, put it down to character. My bike's got character -- the character it plays is a high-strung, expensive, hypochrondriac, needy girlfriend. At least I have fun riding her I suppose.

[Max Preload]

I'll put my money on it just running on a little because of the way it was stalled.

[mouldy]

First things first , a 4.10 is a low profile 4.00 , I used to run a 110/90 18 BT45r on mine when I was richer . As to the noise no idea mines never done it but decomp might have stuck down but if its running fine ignore it .

[Ixion]

I thought you might say something like that :laugh: With the design of the automatic decompression system this motor has, I can't see it sticking open like that; that is, unless the knocking and stalling was unrelated, and it just stuck open when I went to start it.

Still, like you say, put it down to character. My bike's got character -- the character it plays is a high-strung, expensive, hypochrondriac, needy girlfriend. At least I have fun riding her I suppose.

Anything automatic or electronic is always automatically suspect.