Big problem this time - bike sounds like a sewing machine and no power

[lb99]

pics of dropped valve carnage

these were ones that actually came adrift during opreation, so the valve head was bouncing around on top of the piston (not much space)

[xwhatsit]

Hahahahahah. OMG I hope it doesn't look like that ^_^.

No, you're right, it is experience. If money was out of the question, and the inconvenience of being stranded on the North Shore when I live in South Auckland discounted, I would actually prefer it this way, having an old broken bike. If I had some CBR250RR I probably not only would've hurt myself on it pretty badly by now, but I wouldn't've had the chance to learn all this stuff about what really makes an engine work.

The only problem of course is tow trucks, buses, and the cost of parts for a 1980s bike on a student's income.

[xwhatsit]

OK found out the problem. Didn't take long. Check out the pics.

Some theories on why this has happened:

1. Didn't tighten cam sprocket bolts tight enough
2. Camchain was so loose it came off to the left of the cam sprocket, pushing the sprocket out until the bolts hit the head and got pulled out
3. I don't know how likely this one is, but the camshaft got stuck somehow, but the sprocket kept wanting to go around and pulled out the bolts

[gijoe1313]

Dizzam! That looks seriously wicked cool and sick at the same time! You'll end up being a master bike mechanic in no time at the rate you are going! Glad to hear that you managed to get your bike back and yourself safely home - jinkies, this is like the odyssey of Ulysses, new twists and challenges around the corner!

Big ups to having the bloody minded perserverance to mastering your ride!

[xwhatsit]

Hmm. After thinking about it, I'm wondering whether I should just get an entire new engine. If the motor had seized because valves had hit the cylinder, they just would've gone straight through the piston, right? And looking at the position of the lobes on the camshaft, I don't think that's happened.

What I think really has happened is that the camchain has become jammed, and the crankshaft hasn't been able to turn because of that. So, I've probably stripped the crankshaft timing sprocket as well. Also, there'll be metal shavings and all kinds of shit in the bottom of the sump. So that will have to come apart too; when that happens, I'm definitely expecting to find all sorts of other problems as well. All the evidence points to this being a stuffed engine before I even bought it.

Maybe I should just get a new engine?

[Ixion]

Mechanical jam, I suspect cos you didn't torque the camshaft sprocket retaining bolts correctly. Too loose, or too tight and the threads stripped. On some bikes these are a replace each time deal.

Unlikely to have damaged the engine sprocket, probably just came off, takes a lot to damage those morse sprockets.

Can't see if the camshaft or sprocket is actually significantly damaged, or whether it's just a reassemble properly, with Loctite (always Loctite those bolts, though some disagree)

Shouldn't be much metal in the lower half, probably just drain oil, refill (one of the few times flushing oil is a good idea), discard and refill.

Of course the cam shaft will have stopped when the chain jumped (how worn is the cam chain, anyway?), so it is possible that a valve and piston made intimate connection (ie fucked each other) , but sounds like you might have been lucky.

'Course, if you can source a GOOD repalcement engine, go for it.

EDIT Hm, looks in one picture as if the camshaft sprocket mounting lug is actually smashed. Which emans a repalcement camshaft.
Anyway, loosen up the jam and see if the bottom half turns OK.

Sorry, I suspect this one is down to mechanic error (ie you f'd up.)

[xwhatsit]

Yeah... I think maybe I might see if I can get a hand putting it back together next time lol. Any intelligent KB mechanics willing to work for beer? I'm sorry I can't really pay for your time.

Well I managed to blunder through this far just by ignorant stubborness, but I think (like when I crashed the first time) I've worked out exactly what I'm capable of. Learning, though, yes. Would like to learn more about mechanical stuff.

Yep the camshaft is pretty owned. I don't know if you can tell in the picture, but it looks like one of the camshaft bolts has actually pulled the entire thread out of the camshaft, so it would be toast anyway. I think I would like to replace the camchain anyway, according to Mr Haynes it was getting slightly long in the tooth anyway, and it's probably been a bit hammered by all this.

I'm just a bit worried that if I pull the bottom end apart (to check lower camchain sprocket, if deemed necessary) I will find piston rings, bearings, clutch etc etc need doing as well. I would imagine I could spend close to the price of a new engine in replacement parts anyway, the way some of these parts seem to be priced.

Thanks for your input, Ixion, always appreciated.

[Ixion]

Pull the top end, figure out what's salvagable , what needs replacing and post the list. See if anyone here can help, must be quite a few bits for those bikes around.

You don't need to pull the barrel or crankcases to check the lower sprocket just pull the outer cover off. But with the head off you may be able to shine a very bright light down the cam tunnel and check the lower sprocket with a little mirror on a stick.


Experience is a stern teacher sometimes, but you'll remember the lessons.

Anyone got a box of CB250rs bits in the shed?

[scumdog]

I would REALLY check that none of the shafts involved are bent or out of round, there's been a shitload (technical jargon) of force put through them during this mayhem.

[xwhatsit]

What shafts do you mean? Crankshaft et al? I'm replacing the camshaft anyway so I don't have to worry about that one.

Hmm might have to take off the crankcases anyway.

[Ixion]

You'll know if the crankshaft is bent or out of true, because the primary gear pinion runs off the end of it. Any misalignment will be pretty obvious with binding of the drive as you turn the engine gently over.

[El Dopa]

Yeah... I think maybe I might see if I can get a hand putting it back together next time lol. Any intelligent KB mechanics willing to work for beer? I'm sorry I can't really pay for your time.

Well I managed to blunder through this far just by ignorant stubborness, but I think (like when I crashed the first time) I've worked out exactly what I'm capable of. Learning, though, yes. Would like to learn more about mechanical stuff.

Mate, you're doing fucking great. I've been in a similar position to you, and it sucks, but you do learn a shitload.

I semi gave up long before I was into the sort of stuff you're getting into (I bought a new bike, so I didn't have the motivation to work on the old one any more, especially after the ballache it had caused me).

Keep smiling, even if it is through gritted teeth.

[lb99]

OK found out the problem. Didn't take long. Check out the pics.

Some theories on why this has happened:

1. Didn't tighten cam sprocket bolts tight enough
2. Camchain was so loose it came off to the left of the cam sprocket, pushing the sprocket out until the bolts hit the head and got pulled out
3. I don't know how likely this one is, but the camshaft got stuck somehow, but the sprocket kept wanting to go around and pulled out the bolts

ahh, well you live and learn, we all have to do it. I do know a dude who has a heap of bits, its just pinning him down thats the problem, I can probably get an XR cam which I think has more lift and duration than the CB, I'll track him down on the weekend.
hopefully you can get away with just the cam and some valves, would you put an XR motor in if I could put you in touch with one?, its the same just silver with higher compression, possibly a different cam, and different gear ratios

[xwhatsit]

Thank-you lb99! I would put in an XR motor, the problem being is I'm not exactly the richest guy in the world right now; spent a good chunk of my money on that head, and now it looks like it may be trashed (will post up photos soon, the bearing surfaces are really badly scored, and the old camshaft was jammed in, I had to hammer it out, so I think it's stuffed). So... I would, definitely, I would love to start afresh without all the problems of the old motor, but there's a couple of things: a) how much would the motor cost, and b) it would have a new engine number; does this mean the $400 COF routine?

I don't need valves, hopefully, I haven't actually taken the head off yet as I didn't have time today, but I have four valves from the old head which are OK I think. Camshaft, definitely, and the cam sprocket looks stuffed to me (again, I'll post photos, but one of the bolt holes in it is actually stretched, it's not round any more). Cam chain would be prudent, as I think it was approaching the service limits anyway, and will definitely be stretched now after the carnage I put it through.

One particular question is that when I pulled out the camshaft, I heard a `tick, tock', and no, it wasn't a clock, but I believe it was one of the cam sprocket bolts falling down into the sump. Arse. Will this require me doing some major disassembly, to get it out? If I'm changing the cam chain I will probably need to pull a lot of stuff off anyway, so I suppose it's not the end of the world.

Thanks for everybody's help once again. If I was on my own I'd be stuffed! I suppose this can be one of my `motorcycle stories' I can tell the young bikers when I'm old and grizzled...

[Ixion]

Well, some slight positives (or at least amelioration of the negatives). Different engine doesn't need any paperwork - since it's more or less the same engine model. If the tick tock was a bolt you'll prolly need to pull off the drive side cover - it will have fallen down through the cam chain tunnel and now is somewhere down by the lower cam chain sprocket. Unless you're rather unlucky (and it dives down a passageway into the crankcase itself) you should be able to retrieve it . Try not to rock the motor or turn it on its side. And you'd need to pull the cover to change the cam chain. It's not a big job , usually just remove kick start, few bits and pieces and take out the screws. Try to get the gasket off intact so you can reuse it.

Pity about the bearing surfaces. But sometimes they can be cleaned up, especially if the damage is a short duration event like this rather than prolonged grinding wear.

Post some piccies of the head once you have it off.