No compression when engine is cold

[Krtek]

Thanks for elaborating. I know how the marks should line up and how to put the engine in TDC (and that that's not when the spark plug sparks, if all is well), but it's nice of you to take the time.

I'm still not sure how a stretched chain would make a bike harder to start after two weeks than it is after two hours, though. In both cases, it's cooled down.

Any thoughts?

[StumpyFingers]

Thanks for elaborating. I know how the marks should line up and how to put the engine in TDC (and that that's not when the spark plug sparks, if all is well), but it's nice of you to take the time.

I'm still not sure how a stretched chain would make a bike harder to start after two weeks than it is after two hours, though. In both cases, it's cooled down.

Any thoughts?

normally when an engine is cold the timing is slightly different.

Try this , when the bike has been running and you switch it off, put the choke on.
when the bike is cold see if it is easier to start.

I used to do this on an armstrong and it worked most of the time.

[Krtek]

normally when an engine is cold the timing is slightly different.

Fair enough, but then it's cold in both cases. Still, the difference in how hard to start it is is very noticeable (and makes you want to drop the bike into a river).

Try this , when the bike has been running and you switch it off, put the choke on.
when the bike is cold see if it is easier to start.

I used to do this on an armstrong and it worked most of the time.

Good one, definitely worth a try. Thanks!

[Stoneshop]

normally when an engine is cold the timing is slightly different.

Maybe on an engine that's got points on the camshaft, but I fail to see how that can occur with a CDI triggered off the crankshaft.

And that would influence the lack of compression, exactly how anyway?

Try this , when the bike has been running and you switch it off, put the choke on.
when the bike is cold see if it is easier to start.

Put the choke on before you stop the engine, letting it run rich for a moment? Or after (how would that help)?

[Stoneshop]

Exactly the same problem with an XR250 I have, twas the cam chain, stretched it was!

Check the timing marks ALL line up.

So, you're working the kickstart when you last used the engine a couple hours ago. It's as cold as the engine would be after those hours. Compression fine.

Then you perform same after having the engine sitting for a week or two, and find you have bugger-all compression. That would be caused by the timing chain suddenly stretching to allow the camshaft to do a quarter turn, from cooling that extra tiny bit during the week? Remember, BOTH valves ought to be closed during the compression stroke and the power stroke, so IF it's the camchain that's causing this, either the intake or the exhaust valve has to be commanded open by the camshaft. That would mean it's suddenly turned some 90 degrees.

Somehow, I don't think so.

[Stoneshop]

It's gone. So, what was the cause? I don't yet know exactly, but it was in the head cover, so either the rockers, or the rocker shafts. Not the decompression doodad, I checked that that didn't touch anything anytime when not activated.

I had the engine block out, to try and locate the source of this pesky problem, and also to fit a head cover with a tacho drive. With the cover off I tried the kickstart and noticed I now had heaps of compression (and the bike had been standing for at least a month, more than long enough for the problem to manifest itself under normal circumstances). So clearly it wasn't anything to do with the rings, or the valves/valve seats, or the timing.

Fitted the new cover, tightened it down and the compression was still there. Put the block back in, adjusted valve clearance, kicked it, and it started first try. It did so too a few days later, at -5C.

So it must be something to do with the rockers, binding after the oil has drained.