Engine Died, No Compression. Thought Cam Chain but then...

Ok, so long story short my '80 XS400 has been off the road for over a year as I (slowly learnt to paint, then) repainted it. I took it for it's first ride in that long and it was glorious. It and I was loving it. So then I thought I'd ride it to work, and it got there fine but then when I tried to leave it wouldn't start until I'd tried kicking it for about 20 minutes. I had an audience and everything by then end. A guy even clapped when:

Eventually it started, and I was so scared of it stalling (and taking another 20 minutes to start again) I was blipping it and keeping it revving at about 2000rpm at the first set of lights. I took off fairly quickly but after about 100m it died at the top of a hill. It literally just sounded like a "Pfft" and then the revs were at zero and I was coasting down the hill. I first thought the battery had disconnected itself but all the electronics were on. I was still coasting, even dropping the clutch to hope it would hill start but it just kept coasting, not slowing down at all, telling me I had no compression at all. Eventually it stopped halfway up another hill and I tried kicking it again after shifting through the gears a few times and it was the same. I could even kick it in gear and I'd just hear this "whir-whir-whir" of the piston just going up and down with little resistance, and definitely no sparky sounds. I could also even push it while in gear. But, putting it in neutral, I had a long walk pushing it home to mull over what I thought might have happened...

My first thought was the drive chain, but it was still attached. Then I thought gear box, but that didn't seem right. It seemed to me like the top end wasn't talking to the bottom, because there was no spark when I kicked it. Cam Chain was my theory by the time I was home, but by then it was dark and I was swimming in my jeans and work shirt with sweat, so it would have to wait till I could find out.

First thing I did when I got back to it was take off the point cover and kick the engine over again (I could do it by reaching over and doing it with my hand, there was so little resistance). Cam shaft didn't move. Cam chain was sounding about right. I then took off the valve covers and saw the attached. Can you spot the difference? Yep, the left inlet valve doesn't have a lock nut. Makes me sick to think where it might be and what it might have done in there.

So, anyway, my questions for you guys are:

1. About how screwed do you think I am?
2. Can I take off the top end without removing the engine from the frame?
3. Will I be able to replace the cam chain (if that's still the main issue) without removing the engine from the frame?

As you might be able to guess, I REALLY don't want to take the engine out, because I don't have a bench to put it on anyway, or, I'm assuming, the strength to mount it back on again by myself once I'm done.

The factory manual gives you instructions I'm sure I can follow (and I've seen other forum topics that say this), so that's not the issue, I'm more after some rough guidance, rather than a how-to.

Any help/sympathy is appreciated.

You will at least need to take off the rocker box and try to find the nut. You can do it with the motor in the frame. If you find the nut still in the rocker area and everything else looks good you should be ok. While you have it off it is a good idea to replace the tach seal and cam seal. Remember to use a light coat of three-bond or yamabond on the mating surfaces when you put it back together. There are lots of posts in the forum on this. Just use the search function to find them.

I had a tappet locknut vibrate loose last summer and it definitely made the engine run badly, but I found it in one of the valvetrain oil galleries and reinstalled it and all was well. No real cause for alarm.

You can find it by mounting a magnetic retriever on the end of a piece of flexible wire. (Make sure it's very attached, I bought a telescoping one from Autozone and then bent it and didn't care if it broke. Tank off you can get in and out of the top end easily. Also, you could just leave it there. It's not gonna move much. But we all know you're gonna fish it out. )

it's sitting directly below on either side of the casting inside the top end. I've had that happen to me 20 miles from home with a girl on the back.

That was the beginning of the end!

Ps, buy new tappet screws, make sure, they're not over long. Once the screw starts to back out on its own it means its worn from vibration. I made do putting Grey permatex on mine for a while, but you'll need good metal to metal to keep it living.

Ok, thanks very much for your advice everyone, I FINALLY got around to taking the rocker cover off and it was as I had come to expect... The valve locknut that vibrated itself off must have jammed itself in the cam chain and has completely wrecked:

1. Itself
2. The cam chain
3. The cam sprocket

The roof of the rocker cover was also pretty scratched up, with a few deep burrs in it and lots of metallic shards around.

Is the cam chain something easy to replace? I'm going to have to take the engine off the frame and split the crankcase aren't I? Can you buy chains and sprockets somewhere or is it going to be I have to hunt around for a wrecker?

Is it going to be worth it? It's not really in that great condition, one of the sidecovers fell off and smashed the ride before it died... A part of me is starting to think it might be...

What all parts do you need? I may have them but shipping down to there might be pricey from Canada.

You should be able to replace the chain without splitting the cases but you'd need to break the chain to do it. Definitely easier than taking the case apart though.

Thanks for the offer, I really appreciate it, but yeah, I imagine shipping to Australia is a little over the top.

Is it really possible to break the chain then put it back together while in position on the sprocket?

I think I only really need a new sprocket and chain. And that jerk of a locknut. The chain is just in the worst position to replace it easily.

You can unbolt the camshaft sprocket and slide it off. Break the chain, then wrap the chain around the crank in whatever way you can, then reattach the link. Then reinstall the chain on the cam sprocket and voila.