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Discussion Starter · #181 ·
I don't have access to a compressor sadly. But either way, I can see the piston going up and down but no air is moving in/out the plug hole. The fact pretty much no air is moving suggests stuck valve?

Whatever the problem the solution is going to involve removing the head?
 

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That and the failure of a wet compression test too raise the compression. However if the compression really is zero you will not have the bang part of a four stroke cycle and the bike should be running, if only barely on the other cylinder that apparently has 50 psi. Does seem to defy logic a little.
 

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Discussion Starter · #183 · (Edited)
Last time I rode it it wasn't happy at all...that's why I decided to wait it out until my throttle cables arrived and 'start again'. It ran, but not all that happily. There were no obvious failures/bangs/clouds of smoke though. The spark plugs are clean (i.e. not covered in metal) so I'm hoping I don't find any holes!

I suppose I just need to have a look.

Do these bikes have interfering valves? If one had always been stuck would I have smashed a piston into it?
 

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They can do, only time it's happened to me was when an inner valve spring broke with the speedo in 3 digits. Had to make it too the pub on one cylinder and then home again!
 

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Hi Mr D,
If you have a valve that is stuck open, even a little bit, the tappets to valve clearance on that valve will be obviously large.

A blown head gasket wil have a distinctive steam engine ‘chuffing’ sound, soapy water blowing bubbles when the engine is kicked over, will confirm.

A holed piston will leave debris in the crankcases, removal of the drain plug will reveal this.

Valve clearance, drain plug removal and spraying with soapy water around the head gasket joint should only take 10 minutes to check.

Regards
Peg.
 

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The solution to all those problems is going to be the same though? Remove the head?
Probably yes but worth exploring Peg's first point before you do that. Remove the rocker inspection covers and spark plugs then turn the engine over gently with the kickstart. If a valve is stuck open you'll see it.
 

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Discussion Starter · #188 ·
Yeah fair one.

You need to remove the covers and slacken the clearances right off anyway to make head removal easier so you might as well look?
 

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Discussion Starter · #189 · (Edited)
Had a really quick look at the Bonnie today while I was in the garage doing something else.

The right exhaust valve/rocker isn't moving very much at all. The other three visibly move (I'm just looking through the rocker box inspection covers while turning the rear wheel in 4th, no tools involved) but the right exhaust doesn't.

It gives a feeble little wobble at the appropriate point in the cycle, but that's it.

If the valve was stuck open the rocker would still move?

If it's stuck shut I shouldn't be able to turn the engine? Or do the springs/pushrods/rockers have enough 'give' to get past a stuck valve? It doesnt feel any harder to turn when you get to the offending point in the cycle.

Edit: Brainwave moment....the right is stuck fully open. I looked at the left and turned the engine over until the left was open. The right and left look identical in that situation. That still doesn't explain in my mind why the rocker is also stuck? If the valve is stuck should I not be able to 'flap' the rocker at the point where the cam is closed but the valve is still stuck open? Do the rockers/rods ever get stuck?
 

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Just to be clear, the right exhaust valve is stuck in the open position - so you can see the head of the valve inside the cylinder, if you take the rh spark plug out. Yes?

If that's so, the rocker should still move as the engine rotates as it's driven by the exhaust camshaft. It may actually not move much though as you rotate the engine as normally it's forced to return as the exhaust valve is closed by the valve spring. See if you can move the rocker with a finger. So, if the exhaust valve is stuck open, the rocker will lose contact with the top of the valve as the camshaft rotates to a point where the valve would normally be closed. You should be able to move the rocker up and down by hand by maybe up to 10mm.

However, by this time, you may well have detached the pushrod for that rocker either from the cam follower, or it's dropped out of the other end of the rocker arm. So, the camshaft may not now move the rocker because the pushrod is now detached. Neither is a disaster but you'll need to work the pushrod back into position once the exhaust valve has been freed up (not easy!). You'll need to slacken off the tappet too to give you some room. You don't need to take the head off just yet, but you do need to get that valve moving. It may just be coked up. Try some penetrating fluid down the valve stem from the rocker box end. Make sure the piston is well down the bore and give it a gentle tap with a hammer - and I mean gentle! Just to see if the valve will move. You may be able to get some small grips on the top end of the valve stem and see if you can rotate it a bit. If it really won't move then the stem may be bent - but that can only have happened if the valve has hit the top of the piston for some reason. Are there any witness marks on the top of the piston that can be seen through the spark plug hole if you rotate the piston up to TDC? But go gently!

Whatever you do, go gently. If you try to rotate the engine now and the pushrod is not seated correctly top and bottom, you risk bending that pushrod if you try to force the engine to turn over.

No need to panic, this is solvable! Even if it's a head off job, that's not so serious.
 

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Discussion Starter · #191 · (Edited)
Not quite...I can see that the rocker doesn't move through the rocker inspection window. The end where you do the clearance adjustments.

If the valve was stuck but the rocker were free I'd expect to be able to rock the rocker back and forth through it's full stroke when the cam says "valve is closed".

I.e. when the left valve is open the right should be closed. At that crank position I should be to able to move the right rocker? Edit: When I say "should" I mean should be able to if the right valve is stuck open when it's not supposed to be.

I can't . That suggests the rocker/cam follower/push rod is stuck?

The engine is pretty easy to turn over. It did however run (briefly) like this, so if anything is bent it's properly bent!
 

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If the lh piston is coming up and the lh exhaust valve is opening then that's the exhaust stroke, and the rh cylinder is then on its compression stroke, so the rh exhaust valve should be closed at that time. Equally, the rh inlet valve should also be closed. You should not then be able to move the rh exhaust rocker arm, other than by a few thou (the tappet clearance).

As you turn the engine over, just be sure you know where you are on the cycle.

It may be useful to rotate the engine so that both the lh valves are closed and both rockers will just move a couple of thou, as that's the top of the compression stroke, and the exhaust valve on the rh side should be about to close and the inlet open.

If the rh exhaust valve is stuck down, you should be able to move the rocker with a finger - as it's probably got detached either top or bottom (or both). Just check that the rocker isn't binding somehow on the pushrod if it's got detached and jammed in alongside the rocker arm - not sure if that's possible, but have a look.

If these investigations don't bear fruit, the next step is to take the rocker box off. That'll free up the rockers and pushrods etc. If the exhaust valve can then be persuaded to move, all you'll have to do is replace the rocker box and re set the tappets - provided nothing's damaged. Getting the pushrods back into the balls on the end of the rockers is a bt of a faff, but if I can do it anyone can. It's not a major job though. I think you have head bolts that go through the rocker box on the 650 though so you'll need a torque wrench to put them back.
 

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Hi,

If the valve was stuck open the rocker would still move?
Nope. The valve spring is what keeps the cam follower "doing what it says on the tin" when the cam lobe starts to allow the valve to close. So the valve spring also keeps the rocker on one end of the pushrod and the other end of the pushrod on the end of the cam follower/tappet. So, as soon as the valve spring didn't uncompress, the clearances in the valve train went massive and - at least - the pushrod popped out from between the rocker and the cam follower. However, the pushrod can't go very far, likely hence the "feeble little wobble at the appropriate point in the cycle".

'Fraid minimum exhaust rocker box off to see what you see. :(

Hth.

Regards,
 

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Discussion Starter · #194 ·
At right that last bit was what I was missing...

So the rocker WOULD have been free floating like I was thinking, but only for a moment before those free floating bits caused a problem.
 

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As Stuart says, once the valve sticks open (and the tappet clearance goes from 4 thou to 400 thou or more) the bottom end of the pushrod can be lifted out of its socket on the top of the cam follower. That lift is probably more than 1/4" and that can prevent the rocker from rocking - which is why I was encouraging you to go gently as forcing the engine over when the rod is displaced can result in damage.

Getting the pushrod back into the cam follower socket is not difficult when the rocker box is off - you need to turn the engine over few times with a finger on the top of the pushrod to be sure it's dropped down into its socket and is staying there whilst you re-fit the rocker box, and then finagle the top of the pushrod onto the ball end on the rocker arm. It's just a bit fiddly. The bottom end of the pushrod can sit on the upper edge of the cam folllower; you think it's gone in, but it's not. When you come to adjust the tappets you'll find out............

If you are contemplating taking one or both rocker boxes off, order a couple of rocker box base gaskets now. Some of them have a pair of holes for the pushrods and those make it easier (IMHO) to get the pushrods back into place as the gasket holds the pushrods upright while you replace the rocker box. However, I have heard it said that these gaskets can cause wear around the pushrods if they are too close a fit. You pays your money.....
 

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Discussion Starter · #196 ·
Who's a good supplier of gaskets? There are a lot out there for these engines...

I suppose I should probably buy a whole head set. I need to figure out why the valve is stuck which will probably involve removing the head?
 

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I'm no expert on suppliers, but there are a few that I understand to be reliable, LP Williams, TMS in Nottingham and Grin are good, and I've used Monty's and found them to be excellent too..

I believe that there are various rocker box gaskets of differing specs. Stuart and others will be able to advise better than me. I've used the std thick paper ones with WellSeal and that seems to have been fine.

If you take off the rocker box you may be able to free the valve up, if it's simply coked up, and you might get away without a head off. Tapping the top of the valve repeatedly with some penetrating fluid on the stem, and some patience, may work.

Can you see any witness marks on the top of the piston where the valve has hit it? Not sure if the 650 is an interference engine, and if not there shouldn't be any witness marks anyway. If there has been a coming together of piston and valve then the valve may be bent and it's a head off anyway. Whilst it's handy to have a gasket set in stock, if you have a copper head gasket then that can be re-used if you anneal it first, so you'd only need the rocker box gaskets right now.

If you can't get that valve to free up, it has to be a head off, but it's a shame to go that far without an hour or two of trying to free that valve up first.

Taking the head off is not a huge job, but you have to disturb the exhausts and re-sealing them can be a bit of a faff (I use a high temperature silicone), but taking the head off would give you the chance to have a good look at its condition and lap the valves if necessary. You'd also get the chance to have a look at the bores with an eye on the need for future maintenance.
 

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Discussion Starter · #198 ·
I also want to know if it's got high compression pistons or not! My wife's Uncle says "no", the last time I spoke to my FiL about it he said "yes".

I've heard about annealing head gaskets before, but for the cost of new ones (especially when you consider I don't have a decent blow torch!) it ain't worth it. Do you still use copper even if you buy new ones, or is it better to use a modern substitute now?
 

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If the pistons have a flat top, then I think they are low, or low-ish compression. I've just put a set of LF Harris 7.4:1 pistons in my 750 and they were fairly flat on top. Interestingly, the supplier said that that's all LFH make now as their market is 'older gentlemen' who want an easy to start engine, not one for scratching around corners with the knee out. Sounds about right.

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The gasket set that I bought for the 750 rebuild had the modern composite gasket, but I annealed and re-used the old copper one, reasoning that I've re-used them before with no problem, and if it failed I'd still have the new composite one to use, which is a use once part anyway. I used Permatex copper sealing compound too, and it's fine so far.

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AIUI, there are pros and cons to both the copper and composite gaskets, but I have copper on both the 750 and my 500.

A gas torch is very useful if you have an older bike. You may only think you need it once, but once you have it, you'll use it over and over again. Probably one of the most useful tools you can have. If someone's used Loctite on a fastener, you'll need heat to get it off. I'm afraid I learned, the hard way, years back, that if you need a tool that cannot be fabricated with what you have, buy it. You'll use it over and over again and save the cost anyway by not ruining a part that you could have re-used if you had the proper tool. I recently bought the tool to drift out the tappet blocks from the barrels because I knew I'd need to change the tappet block o ring. I can't see me ever using it again, but the cost of new tappet blocks, or even a barrel, was far more than the £25 the tool cost, and I had the peace of mind to know that I was unlikely to damage the valuable parts I already had. And if I'd taken a chance and not changed that O ring, it would only have leaked, and then I'd have to have the barrels off, again, and new gaskets etc, maybe a broken piston ring etc, and bought the tool anyway, so it was good value in the end.

Having said all that, if you have a gas hob, you can anneal the head gasket on that. As long as SWMBO isn't watching that is.....
 

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I forgot. Apologies. Ahem. If you take the head off, try to hold the pushrod tubes down so that they don't pull off the tappet blocks at the bottom, thereby disturbing the seals. You'll need new O rings at the top - and you need to buy the Viton rings for this.

If you do have to change the seals both top and bottom, that gets a bit more complicated as you need to check the height of the pushrod tube (PRT) when assembled and before the head is torqued down. This all gets a bit more involved as to get it wrong may result either in a leaky PRT or worse, a damaged cylinder head. I've been lucky with mine and the std seals have been fine, but it does need to be checked. If you get to that stage, there's plenty of guidance on here as to how to do it - you only need a set of feeler gauges.

EDIT: Have a look at these:


 
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