disaster strikes -- stuck cam bucket

Don't think the valve had siezed.  The wear pattern says inadequate valve clearance/insufficient lubrication over a lengthy period to me... the hardened surface wears off both the bucket and cam and the cam lobe starts hammering away.  The top of the bucket becomes swaged and starts to jam in its' bore, until the jamming force is greater than the force exerted by the valve spring and sticks somewhere down low with the valve wide open.  The valve then gets clouted and bent by the piston and no longer returns to its' closed position.

Whatever caused this to happen, it took longer than a happy little Sunday afternoon ride.  ::)

piet
 
sfcpiet said:
Don't think the valve had siezed.  The wear pattern says inadequate valve clearance/insufficient lubrication over a lengthy period to me... the hardened surface wears off both the bucket and cam and the cam lobe starts hammering away.  The top of the bucket becomes swaged and starts to jam in its' bore, until the jamming force is greater than the force exerted by the valve spring and sticks somewhere down low with the valve wide open.  The valve then gets clouted and bent by the piston and no longer returns to its' closed position.

Whatever caused this to happen, it took longer than a happy little Sunday afternoon ride.  ::)

piet
Click to expand...

1.  The head work included new guides & springs.  The original valve seats were resurfaced slightly, but not replaced.

2.  I checked the valve clearances several times.  The initial clearances were set as follows:
      1          2          3
EX  .24      .24        .28
IN  .23      .25        .25

3.  I rode it 200 kms, over 5 or 6 short rides.

4.  The following clearances were observed at the first service:
      1          2          3
EX  .21      .26      ZERO
IN  .25      .21        .22

5.  Unless the #3 exhaust seat suffered a massive recession in short order, is lubrication the ONLY potential cause?  I checked the oil pump action after the head was on, and oil came out of each block hole.
 
beauchemin said:
5.  Unless the #3 exhaust seat suffered a massive recession in short order, is lubrication the ONLY potential cause?  I checked the oil pump action after the head was on, and oil came out of each block hole.
Click to expand...

I had a lot of valve/seat recession on my 74 3C recently but over a longer distance, say 1500km. 200km seems a bit too radical unless the head's made of chewing gum.. :o

Is it possible the valve has stretched? It does happen (am finding this on old Triumphs). I take it your #3 Ex valve is no longer it's original length: are you able to measure the stem thickness? compare this to the other valves and see what you find..

Cheers: AndyW
 
If it ain't lack of lube, then it has to be insufficient clearance.

Have found several stretched exhaust valves over the years, even a couple of inlet valves as well.  Always down to incorrect float levels (either maladjusted or faulty float valve) and a heavy right wrist... while not giving a fuck about the slight falter at around 5500rpm.

Perhaps a "pre-stretched" valve was re-fitted, they don't last very much longer once the poppet head has softened.  A side-on pic of the valve head with a straight edge resting on it would be nice, shows up valve stretch.

Shim sizes may not be the key, valves are often tipped to regain a bit of shim thickness, especially after seats are skimmed.  Would need to measure the length of the valve stem above the collet groove first.

What's the float level on #3 carb?

piet
 
Hi Piet

I am going to demonstrate my ignorance, again........

if the float in the carb for number 3 cylinder is either maladjusted or the carb float valve is faulty, how would that result in an exhaust valve/bucket seizing on cylinder number 3?

Paul LeClair

 
Paul,

Incorrect float height will have only an indirect influence on the cam bucket. 

With a too low fuel level, burn temp in the combustion chamber will rise and the exhaust valve can begin to glow to the extent that it becomes soft.  The hammering it recieves from the valve seat will cause it to stretch, taking up the valve clearance in the process.  Without clearance, the cam will quickly ruin itself and the bucket.  If action isn't taken by now, the worn surface will work itself away(as can be seen in the photo), the now soft bucket can be distorted by the forces exerted by the cam lobe and can eventually jam in the bore.  This will inevitably  happen close to full valve lift, keeping the valve open while the piston rises and gives it one hell of a whallop, bending it instantly.  The valve now cannot return to its' closed position, even if it is pushed upwards by the piston, causing power loss and even more noise emanating from the cam box...

Valves generally don't sieze without reason, usually overheating in one form or another.  If valve/guide clearance is insufficient, this may be the cause.  But normally all valves would be fitted to the same clearance, why is only one afflicted?  Haven't seen a siezed cam bucket in a Laverda engine either, at least not as sole damage, usually a follow-up of a previous issue.

Definite conclusions can only be drawn once the valve is removed and the stem closely inspected.  Only speculation up till then!

piet
 
All three float valve heights = 18mm.

The valves aren't out yet, but the plugs and header pipes might be shedding a clue:

1) The #3 spark plug had a much cleaner (more white) insulator and ground electrode, while the others were brown/black.  However, there were no deposits on the insulator or electrodes.

2) All three exhaust pipes had been re-chromed -- #3 was the only one showing a golden discolouration at the first bend (closest to the head).

I have been under the impression that a gold discolouration is normal, but I guess if only one exhaust pipe is showing it, then there may be a problem.

Plugs:

normal_3cl_plugs.jpg

 
Possibly a tight clearance issue between the bucket and bucket guide not allowing for expansion when hot? Perhaps the recent headwork allowed for a juggling of the buckets during assembly. Now there's ?2 worth!!
 
I just re-read this phoenix thread. Was hoping for a photo of the #3 combustion chamber at the very least.  ???
 
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