CB750 DOHC: Ticking sound

Oh, I'm crap with electrics. Really. It's just something I've never
'got'.

So why does a four keep running on three if you pull off a plug cap,
then?

 

Yet you somehow feel that you MUST comment on something that you admittedly
"never got".

Maybe someone else will waste their time trying to explain but since you
probably still won't "get it", I certainly am not.

Go back and re-read the previous 6 posts or so.

 

Oh dear, oh dear, you really are being objectionable.

Right through this bit of the thread, I have made it plain that I'm not
particularly good on electrics

I've used 'AFAIK'.

I've admitted I'm not good on this, and I admit it again. I'm interested
in learning new stuff. On other stuff, a lot of stuff, I know a great
deal. Electrics has always been my blind spot.

I've read Olson's posting, and I still don't really understand it
because... well, read the above again.

 

You must've forgotten - that was your old Vauxhall, the one with the
distributor.

 

Ah. Thanks. Understood now.

 

Fair enough, learn something new every day.

Now I'd best go and google Kirchoff's law. One of these days I'll
actually understand electrical stuff but I'm not going to hold my
breath.

 

The coil is effectively strung between 2 plugs, so the voltage has to jump
the spark gap at a plug at each end to complete the circuit.

If you only lose one cylinder with a plug cap off then the spark is
completing the circuit by some other means, like dodgy insulation on the
floating lead or where it joins into the moulded coil body.

A point raised earlier in the thread, if the leakage spark is allowed to
continue any length of time will burn a carbon track in the insulation, the
leakage track will eventually become permanent - so no spark at the plug gap
when the plug cap is put back on.

 

Ok - I pulled the pipes on the side where I'm hearing the noise, looked
closely at everything, and I don't see any cracks or uneven surfaces. I
replaced the gaskets on that side, put it back together, and it still sounds
the same.
It also doesn't change if I push on the pipes.
I tried running with one plug at a time unhooked (I put an old plug in the
disconnected cap and wedged it against the block just in case there's
something to that). It still ticks no matter what.

So... I pulled the valve cover again. The only thing I can see is maybe the
upper cam chain slapping against the top chain-guide a little at one point
in the rotation. But that would make a sound in the center, and the one
thing I can tell is that the sound is closer to #1 or maybe #2.

I checked the valve clearances again for the umteenth time. What would you
consider a clearance excessive enough to cause noise?

I guess I'll order a cam chain, but just put it back together and ride it in
the meantime.

 

That depends on what the recommended clearances are.

Does this sound go away ......or change much when the engine is thoroughly
warmed up?

In addition to a valve clearance problem, it could be a weak (or maybe even
cracked) valve spring, a sticky valve guide or a burnt and leaking exhaust
valve.........but the cam chain would still be a good place to start.

 

ian field wrote:
....

Thanks, when I queried that I had compression tests in mind. I
believe a number of bikes won't crank with the kill switch on,
certainly I've had a couple of single-cylinder ones like that. I've
no idea what 'any length of time' could be but I imagine it could vary
depending on other conditions. When I've watched people taking
(lengthy) compression tests, I've wondered once or twice why they
didn't ground the plugs, for safety's sake.

 

The book-recommended clearance is .08 mm +.05 -.02
I was using .09 +.05 -.02 just to avoid excessive tightness when hot.
Now I've brought that max back down to the book max of .13, and tightened up
a couple of valves that were on the high side of the range, to bring them
more to the center.
Still ticking. (Maybe it's actually a Timex.)

The sound doesn't go away after a 5 mile ride.
But it does go away sometimes under load, then come back when I let off the
throttle.
Which leads me back to the worn cam chain theory.

I don't think it's a sticky valve guide or a burned valve, because I just
re-worked the head. All the valves travelled freely, and I lapped them and
made sure they didn't leak.

Oh well, when my new chain arrives, I'll see what that does.
'anybody managed to swap an upper cam chain with the engine still in the
frame? Seems like it would be doable. I'm not looking forward to pulling
the engine again.

 

....

There are many people here who know more about engines than I do, but
if I'm not mistaken, nobody asked if the four headers heat up equally,
say thirty seconds after starting from cold. One guess I have would
be that if they do, that might eliminate a valve as a possible
culprit. Also, a couple of times, I was able to figure out which
cylinder by putting a heavy screwdriver on each port and laying my ear
on the handle. Just a couple of easy suggestions from an admitted
aamateur...;

 

My reactions precisely

 

Um, in the case of my old air-cooled Kwak four, about two or three times
the recommended max clearance.

It's an odd'un and no mistake.

 

Hot/cold is figured into the published specs. Don't try to out-smart the
engineers; you will end up out-smarting yourself !! ;-)

 

Ok - I've swapped the upper cam chain. (The lower one is still using hardly
any of the range of the tensioner so it's fine.)
Still ticks.
I'm hoping it's some shim bucket or other such part that I swapped into this
head, with a clearance on the tight side, that sticks when it heats up, but
might wear in.
OTOH it occurs to me it could be a clogged oil passage, and something's not
getting oil.

At this point I'm inclined to ride it till it seizes up, then push it off a
cliff.