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

[imdying]

Somewhere in the depths of my memory I seem to remember someone saying CB250s did this if you put the oil filter in backwards at some stage. No idea if that's possible of not, someone here may be able to confirm or refute it. Not saying yours is, just that it may have been in the past.

[xwhatsit]

Hmm well the CB250RS doesn't have an oil filter as such; maybe you are thinking of the CB250, rather than CB250RS?

[Posh Tourer :P]

Ahhh.... errr.... yes it does....

[lb99]

it has at least one strainer, I think you take off the clutch cover to get at it from memory, I think the CBX250rs has a strainer and a filter though

[xwhatsit]

Yes lol I know it's got an oil strainer, but I think it would really only filter out pebbles or small rodents. Sorry, mixing up semantics.

[Posh Tourer :P]

Probably true enough....

[Pixie]

Your starting and idling problems are most likely related to the cracked head.The motor will have lost compression and that will affect both.No point in trying to diagnose these faults until the head is sorted.
If you are using the old valves,it would be wise to check they aren't bent.
They should be lapped in when they are fitted to a diferent head.
If you choose to use epoxy to try and repair any threads etc.Use a hi tech product filled with metal or ceramic.Araldite is a waste of time.
A & E Bearings stock a range. ph 09 838 8268

ps most bikes these day run their camshafts in plain integral bearings.eg in the head material

[xwhatsit]

Well, I've installed the new head. With all the stripped threads before, I can see now that they weren't actually stripped threads; they were helicoild that had come out! The new head too is full of them. Do aluminium threads just not have enough strength to retain helicoils? They seem weaker than the original threads. The thread in question that I stripped with the tachometer drive, was too a helicoil, so is it possible to put another helicoil in there now or is it stuffed for good?

Anyway the new head works a treat. The valves all seem in much better nick than the old ones. After putting it together, however, and putting in the sparkplug and going to kick it over, I just about crapped myself I was so scared! As I kicked down, the kickstart lever just stopped dead! It felt like the piston had gone up the cylinder and hit something solid, like the spark plug, so I quickly whipped out the plug with shaking hands and checked to see the end of the sparkplug was not squashed in, but it was fine. I then thought about it and only half screwed in the plug (with lead off of course) and gently kicked it over, and I could hear gases hissing by the plug. So it was just compression! I checked the decompressor cable and made some adjustments, which made it slightly easier to kick over but it's not anywhere near as easy as before. I now really have to stamp on the kickstarter hard if I'm to get the engine to turn over. I'm slightly worried about the bastard kicking back hahaha. At any rate, mostly the engine starts second kick now, although after riding to the house of family to pick up some mail I left the bike outside and it started raining, it was super super hard to start then.

At any rate having full compression is a whole new experience. Man can this thing haul arse now! Now when I'm in fifth gear and doing 60kph and snap open the throttle the bike actually does something! I'd heard Brits on the net say the bike is capable of 90mph (~150kph), and didn't believe them in the slightest, but now I can believe it. The exhaust note is still not what it should be due to a very corroded set of split collets on one exhaust (but not the other... from being on the side stand? It's the lower side), so I'll have to pick some of those up sooner or later. However the engine note is quite different, has quite a different timbre; I can defnitely hear more induction noise when I fiddle with the throttle. I wonder if that's because it's not sucking air through the cracked head now.

Contrary to Pixie's advice (I didn't see it yesterday lol) I used araldite to seal the gap for the tachometer drive. It seems relatively effective but it's only a short-term fix. I can imagine with sustained high temperature it'll probably not last too well. The bit where the head and the valve cover mate leaks a small amount of oil. I can see on the new head when it arrived, there was a small amount of orange sticky stuff left on the top surface. Is this some kind of sealant? Should I look at getting some?

Cheers everybody for all your help. I'm touching wood as I type this that I won't need much more!

[xwhatsit]

Ahem. So, after that rather enthusiastic previous post, I'm feeling rather shit now. If I had beer I would cry in it.

Was over North Shore to surprise girlfriend at work, bike hauled arse over Harbour Bridge (my first time over). After that I headed down Lake Rd in the wrong direction, worked out what I was doing and turned around. Went 200 metres down the road and the engine made a godawful grinding sound, rear wheel braked heavily (may have locked, I'm not sure), so I grabbed the clutch. Engine seems pretty seized. After waiting 2hrs for tow truck, finally got home (missed work, boss will be pissed).

Had plenty of oil. Maybe a blocked oil route? Could I have done this? Cam chain come loose and piston met valves?

This was 30km after changing the head. I would love to -- get this -- ride a bike.

Argh.

[xwhatsit]

OK, about 20 seconds after I made that post I've gone and fiddled around with the engine at around half past 10 at night. Took spark plug off, engine remained impossible to turn over even with it out. Took off the inspection plug on the (clutch? alternator?) so I could turn it over with a socket wrench. Would not budge; but turning it backwards it went around very easily (very very easily!) until it stopped hard again. Poked a screwdriver in through the spark plug hole and the piston is definitely free and going up and down. Seems to be rather `rattly' but could just be the gearbox in neutral (it seems to make a little ratchety noise when I'm coasting in neutral normally). I've decided on the camchain-is-screwed-piston-meets-valve theory. Which is... not too horrific, as I have four more valves from the old head, and I'll need a new camchain (how much for one, do you think?). Maybe a new camchain tensioner? Well those are my theories for now.

Almost a shame the piston wasn't stuck in the bore, then I could bore it out and have a CB275RS (and I've seen a guy selling a wide range of oversized CB250RS/XL250S pistons on ebay). Offtopic, but how much does a bore-out increase capacity? Enough to make my bike illegal for a Learner to ride? Not making plans, just curious.

Well I'm certainly learning quickly. Despite the rapidly alternating periods of happiness and depression, I like to fool around with this engine. It's in my blood, Dad was a rather good mechanic. If I can't finish my uni degree in compsci, I'd love to learn to be a mechanic. Don't know how good I would be at it though, my first attempts have been pretty hopeless hahah.

Oh I'd love a couple of shots of vodka right now...

[AllanB]

Crack up thread!

Good luck with that engine - sounds like it has seized now. Did you set the valves after putting the new head on?

If you ever need work head to India - you'll be a master mechanic there!

Keep us posted.

[xwhatsit]

Hahaha cheers AllanB, that made me laugh... some Indian guys at work, when I first pulled up on the bike, leaking oil and making oil smoke come off it when it idled, they thought it was the best bike ever! Looked at it, marvelled at its technological advances; didn't have the heart to tell them it's from 1980 or so haha.

No I didn't set the valves. Do you mean the valve clearances? Can they really get that far out of whack, to cause a seizure? I don't have a feeler gauge set. I checked the cam sprocket and chain timing very very carefully. I was worried the first time I went to kick it over and it had massive compression, I thought maybe I had the cam in such a way it was on the wrong stroke (the timing marks on the alternator would still show TDC whether on compression stroke or not, right?). But it ran fine until it blew up, so I must've got that right.

It's all coming apart tomorrow. I'm kind of afraid of the carnage that awaits me haha.

[lb99]

This sounds exactly like my CB250RS, head off, head on,head off, head on head off....you get the picture, your cam chain might have slipped, were you hammering it at the time? was it properly adjusted? , it sounds like the problem is definatley to do with the top end, hopefully your new head is ok, as well as your piston. dont worry about the capacity on a rebore, I've never seen a cop strip a motor to measure the capacity, in fact that frame would be a good candidate for an XR500 motor, the bottom end is the same but the barrel and head are bigger I think

just strip it slow and careful like, look at each component for signs of torture, esp. the cam bearings ( to see if it siezed the cam), I don't know how you'd go with a guarantee on the head if it dropped a valve, but as you say you have some spares, fingers crossed.

[xwhatsit]

Hahaha yeah I remembered you saying about your bike when I was waiting for the tow truck. I haven't got a top end rebuild `down to a fine art' yet, but I'm getting faster lol.

No I wasn't worried about cops hitting me up for a bored out motor (that would suck!), I was just wondering how much a bore out increases capacity. Are we talking 275cc here? Not actually wanting to do it, just curious. Yeah heheh heared about the XR500 swap, but that would change the engine number and make my life difficult with WOF, wouldn't it?

I wasn't exactly hammering it at the time, but I was accelerating away from 20kph, so the engine was loaded. If we're talking of the cam chain slipping and valves hitting the piston, is the piston, piston rings, bottom end likely to be OK? The one thing I wondered about was the piston rings, I've heard they're quite brittle and I envisioned the piston smacking into the side of the barrel when it hit the valves and breaking the rings.

Cam bearings, can this seize a camshaft outright? Or are you talking about now it's damaged, to check if the bearings are stuffed as well?

Indeed, fingers crossed.

[lb99]

well, if the cam chain slipped then the piston would smack into whatever valve was open at the time, bending the valve, sometimes ramming it (bent) back into the head, and/or smashing the piston, and continuing the shock of a sudden stop down though the piston and (maybe) bending the conrod too, this is a worst case senario though, the cam chain on my Z440 snapped while I was puttering along, and it just stopped dead, so I pulled the clutch in and coasted to a stop.
after taking the head off all it had done was to bend a valve and mark the top of the piston, so I replaced the valve and I was away again. If the piston is ok, then the rings probably will be too
So its not all doom and gloom, its called exprience, and it happens to most of us, just crack her open and have a look, it probably is able to be fixed.