Nuff said :-)
I've seen a few of these now and deliberately ran mine down so I could monitor the symptoms as they occured last year.
All four stroke engines have valves and all fourstroke engines need their valves adjusted - when you hammer what is in effect a steel nail into an aluminium cylinder head up to 10,000 times a minute then something is going to give.
The valves are held tight into the head by return springs and have the cams mounted above them - between the two are the infamous bucket shims. the shims are set on manufacture to have a cold gap of 0.15mm about the thickness of a piece of writing paper. this allows for heat expansion in the valve.
On the inlet valves the cooling action is done mainly by the fuel/air mix. On the exhaust valves they desperately need a heatsink which in this case is the cylinder head every time the valve closes the heat dissipates into the engine. Without this heat sink the valve will get super heated and soften.
As mentioned above if the head starts to give or the valve wears away the gap will close and eventually be none existant, this is where the trouble starts as when hot the valve expansion will make it stand proud losing its heatsink and allowing the hot gases to burn past causing damage to either the valve or the head. Fortunately most of the time it is the valve that suffers as the head is considerably more expensive.
Damage to the head, or valve will cause a loss of compression
The first symptoms are always stalling. If you leave the bike running it turns itself off or it will stop at lights or junctions. Generally it will start again straight away as it takes a fraction of a second for the valve to cool enough to restore compression.
After a while the exhaust pitch will change due to the fuel air mix igniting in the exhaust. Fuel economy and top speed will also start to suffer as the piston works at less an less compression.
It will start to smell petrolly as fuel mix escapes into the exhaust and fails to burn.
eventually it will fail to start or run without a lot of throttle once it gets to the 40mph on max throttle then its needs to be kept at home or garage as it will fail almost straight away in the six mile trip to and from my work it died completely.
I've done this to a removed head for ease of explanation I'll cover removal in another post.
A typical burnt valve - I dont know why but its generally the offside valve which goes, notice the huge chunk missing.
To fix it you need to remove the valve and put a new one in - a multi valve tool is around 20 quid from machine mart. A new valve is about 17 quid from bmw. In theory they are a universal item but I havent tried to source any. Its possible to get titanium or hardened valves which wont burnn but then the damage will be to the head and that cost many bucks more to sort out.
To get to the bust valve out you need to strip the head down to the bare minimum - in most cases you will have allready removed the cams inorder to get the head off. the bmw manual however has the mechanic take the cogs off - these are threadlocked on and a pain if you do it wrong so its better to leave them on.
Pull off the shim to expose the top of the valve, the valve is held under constant tension by the spring via a couple of collets, we need to release this tension by first compressing the spring and removing the collets -
Place the tool either side of the valve and wind it in -
Once compressed the collets can be removed, whatever you do dont lose them -
When the valve is out check the seat for damage if its scored then you'll have to regrind it -
Here we have the old burnt valve next to its new replacement.
Put the new valve in and recompress it to put the collets back in. A dab of grease on the valve stem will hold them in place, another dab of grease will make them stick to a screwdriver so you can place them properly. The valve has a groove cut into it for the collets -
Nice, new and in position -
Now we need to adjust the clearances, in the factory they are set using 2.9mm for the inlet and 2.8's for the outlet. In generall I swap the 2.8's to the inlets and scrounge some 2.65's for the outlets. The bmw garage will have a tray full of them from K series bikes and other C1's take the old ones in and they'll swap them over for you. The shim size is written on the inside of the cavity.
With the head still off the engine reassemble the cams, the holders and do up tight. the clearance is measured between the top of the shim and the base of the cam
You should then be able to pass a 0.15 blade between the cam and the follower(shim) if so then you're sorted, if not then another trip to the dealer. You can get away with less of a gap but it might mean another blow out later on. With a bigger gap the engine will naturally take up the slack. The inlet valves need a smaller gap because they dont expand so much and needn't be worried about, I adust them out of habit.
Once you have the clearances sorted out then you can put the head back on the engine and reassemble as you took it apart. This should give you a few thousand more miles before it needs doing again.
A yearly clearance check will however pre-empt such a catastrophic failure and can be done by any competant mechanic, person or dealer without the need to remove the engine or other such nonsense valve check takes an hour tops, shim readusting 2 hrs, valve replacement 3-4 hours - :-)
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