replacing a burnt exhaust valve-any handy hints??

david vaughan-birch

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Hi-I need to replace an exhaust valve on an 1150. I'm following Steptoe's handy guide to replacing a gasket to get the head off and I've also got a Haynes, but is there anything else I should know before I start in and change the valve? (its the second exhaust valve I've had go in 25k miles, both on the same side strangely-wonder why?). All hints appreciated!
 
You'd possibly get more replies if you hadn't closed the thread - now reopened!

Greg
 
Is it the same valve each time?

If so there must be a good reason, they don't do that commonly. Damaged seat? Incorrect clearance? Damaged/worn valve guide? Less likely if it's the same valve but:- Incorrect timing; valve or ignition?

Does the engine suffer from pinking?
 
Is the valve sealing properly, how do you lap them in ?.
 
I didnt do the last one but I'll find out which one it was-Jim Hill put the last one in so I'm sure it was done right. Jaythro reckons the mixture's too lean and the valve clearances are out? For what its worth the surfaces of the top of the cylinder and the exhaust valves feel "gritty". Fortunately the cylinder itself doesnt seem to have been damaged by the broken bit of the valve!
 
i did an exhaust valve on mine about 2000 mls ago and if you take your time & lap the valves its not a big job.
as to the reason yours keeps packing up well thats a question for a clever bloke.
BTW mine pinged a lot before it broke up on me and it was probably leaking a bit and that caused a hotspot & caused it to fail.
A good indication on if yours is running lean is to look at the plugs & check the colour.
 
Valves breaking up is not a 'burnt out' exhaust valve.

Burning an exhaust valve is quite literal - the valve material melts (usually in one place on the valve rim).

A burnt out exhaust valve is characterised by:

  • Low compression
  • Difficulty in starting
  • Lumpy low speed running (similar to a misfire) that gets better as the revs rise
  • High unburnt hydrocarbon exhaust emissions

Cause of valve burning is usually wrong valve clearances.

When rebuilding your cylinder head, carefully check the valve seat width. Race engine use narrow valve seats (but they are regularly rebuilt and actually don't do many miles). On a road engine, if the seat is too narrow, it restricts the valve seat's ability to wick away heat from the valve heat and can accelerate the onset of a further failure.

Greg
 
thats allmost a photo copy of what minle looked like.
the rest of the valve & 'seat' looked a bit dodgy so I guessed it wasnt sealing and burnt away part of the valve.
Like i said i got a new valve laped it in (and did the other exhaust valve to clean it up) and now all is well
 
Hard to tell from that photo but looks as though the exhaust valves have receded into the head, that would close the clearances and cause the burnt valve.
 
That's encouraging news John! thanks-I'll try it with a new valve and relap the other one, and I'll CAREFULLY check the clearances this time! Could anyone tell me whether a standard valve compressor will work or do I need a special one?
 
I used a standard valve compressor on mine, I seem to remember having to use a socket on the tool to 'shim it up to get the tool round the head but I'm sure you can get a standard tool to work. just remember to lap it in right and it should be good. I read on the other bmw gs forum that exhaust valves going are not unheard of on our bikes.
Good luck and get a few new of the half moon valve things to make life easier when they ping off and go into another dimention when taking the valves out :)
 
You need some engineers blue to check that there is not a gap at that point letting hot gasses seep by and erode the valve
 
I used to fill the combustion chamber with paraffin to check valve sealing.
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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