When your engine develops a new noise the wise thing to do is to think back over ...what have I just done ?
It's part of Airhead wisdom that any axial play in the rocker arm bearing will contribute to an increase in valve noise. As I understand it the main reason for this is the rocker arm tip is slightly off center on the valve stem for the purpose of slowly rotating the valve so the rocker get pushed axially against the bearing support block on every operation of the valve which can create noise.
Anyway the left hand cylinder seemed slightly noisier than the right so when I checked the valve clearances recently I thought I would have a go at closing up the rocker arm clearance on that cylinder.
I wear an open face helmet without ear plugs on my short commutes and I suggest the real reason the left cylinder seems nosier is because its staggered further forward than the right and therefore not so shielded by my knee but in the true spirit of engineering adventure I didn’t let that stop me.
The specified clearance for early rockers is zero but still free to move over the operating arc of the arm. On pushing the rocker arm there should no axial movement visible other than the seeing some displacement of the oil film.
The method of adjustment is to slacken the two head nuts holding the bearing support blocks while applying gentle pressure to the blocks with a G clamp. Something will be needed between the G clamp and the block to ensure pressure is applied to the blocks and not the rocker shaft. Suitable sized sockets are often suggested but I found that brass nuts off 15mm plumbing compression fittings are just the right size and being shallower much easier to use. After carefully torquing up the head nuts the rocker arm should be checked for binding. This is how Snowbum describes it :-
It is also possible to overdo this end play adjustment, that means having the clearance too tight. The result is that, at approximately normal valve tip adjustment clearance, there is enough rocker END binding at full lift to hold the valve open, almost defeating the tired old valve spring. The resultant snap when the valve does close is a loud clatter. It is necessary to TEST move the rocker over a WIDE arc to test for any binding. To do THAT, you must back off the valve adjuster relatively fully in order to be able to move the rocker arm in that wide an arc....this is done each time you ADJUST the end play (which is seldom).
Well I’d just checked the valve clearance and they were spot on with no change over the last 2000 miles so I thought I’d leave well alone and skip this step. You can guess the rest
And so we get to the real purpose of this post which is to describe to you what a binding rocker arm can sound like. In my case not a loud clatter as such more a light knocking sound and engine speed related at that. No noise perceived at idle or above 3000 rpm but under load at around at 2000 – 2500 rpm a light knocking. Couldn’t even hear it with earplugs. I know what big end knock sounds like and this wasn’t it. Still any load related noises are a bit scary.
It was a while before I made the mental link with my recent attempt at rocker arm adjustment and after backing off the valve adjusters and checking properly sure enough there was some binding. Not so much I would have thought that it could defeat the valve spring and I can’t explain why the noise should have been load related but when the binding was removed the noise went away and another bit of Snowbum wisdom was learned.
LEARN FROM THE MISTAKES OF OTHERS. YOU CAN'T LIVE LONG ENOUGH TO MAKE THEM ALL YOURSELF!
Topic: Rocker Arm clearance and how not to do it (Read 1845 times)