CB750 DOHC: Ticking sound

This is about a 1980 CB750 (DOHC).
http://mysite.verizon.net/barrettmanor/EngineTick.mp3
is a link to an mp3 I just recorded of the worrisome ticking sound it's
making.

It doesn't do it until it warms up a bit.

I've done the cam-chain auto-adjustments, although the upper chain is near
its limit.
To me this doesn't sound like what I'd expect from a loose chain though,
which I would think would be more of a continuous grating as each link
rubbed, rather than a tick per revolution.
It coming more from the #1 side, near the top, but it's hard to pin down
more than that. If I had to guess I'd say around the #1 or #2 intake
valves. (I even tried the old trick of listening through a tube placed at
various spots but that was no use at all.)

I've checked/set the valve clearances multiple times, and they're all
between .004 and .006" (.102 - .152mm).
I don't see any broken links in the chains, or any obvious wear marks.

Is there a minimum recommended shim thickness? On the exhaust side I'm down
to a 260 on one valve.

With the engine out, I've turned it over by hand countless times, and
haven't spotted anything. There's no vertical play in the piston rods.

Any suggestions on what to check next would be appreciated.

 

"Ticking" noises can often be traced to exhaust leakage.

However, it's usually the warm up that results in a dec-
rease, rather than the onset. But, I'd cross that off the
suspect list by having a good look there.

 

<Nods>

 

I put in new exhaust gaskets when I put it back together a week ago, and
re-torqued the exhaust manifolds just before I made that recording. I'll
check again though.

 

Did this start suddenly, gradually, or you don't
know ?

If the chain's near its limits anyway, I'd consider
changing it. Made an amazing amount of difference
when I swapped mine out.

Wouldn't hurt to swap it if it's already near its
replacement interval and might get rid of some
extraneous noise even if it's not the noise you're
trying to locate.

 

Check the cam chain link by link, looking for a link that is more worn
out than the rest of the links.

 

I've never had a camchain that suffered like that, and such a fault
would not cause the aforementioned ticking noise anyway.

My own experience of ticking noises is that they have always been down
to one of two things: a slightly blowing manifold or an over-wide valve
adjustment gap on a single valve.

 

Agreed, agreed, agreed.

Coo. There's smart.

But, but, but, all fours idle at 1200-1400rpm, don't they?

<Runs and hides>

 

<Impressed>

That's a damn good idea.

 

Unless......it's a double fire arrangement with 2 plugs in series. If that
is the case, you must ground the plug so the "other" one will still fire.

 

I don't think I've ever removed a plug cap on a Jap four and had *two*
cylinders die.

 

I'll bite - How could any computer-less bike with pickups or ignition
pulse generator(s) on the flywheel rather than camshaft (like the
older inline cb750's) not have a waste spark?

 

I've been told that's desireable to avoid the chance of burning a
coil. Is that what you mean by 'carbon tracks'?

 

That's what I generally do anyway. Because when I pull a plug, it's
almost always because I want to make sure it's sparking, IYSWIM.

 

Yes, they do run on the wasted spark principle. But the plugs aren't
wired in series, AFAIK.

Next?

 

Hmm, are you sure about that? So if one plug dies you lose both
cylinders that coil fires?

It doesn't sound right to me - surely then a 400/4 wouldn't drop onto 3
in the rain but straight onto 2?

Other thing that springs to mind is that some of the Laverda 180 triples
use ND coils, one single output and the other twin output. I'm told
suitable single output ND coils are rare secondhand and the common dodge
is to buy a pair of coils from a 4 and just tuck the surplus lead out of
the way. I could be wrong here as I've never run a 180 triple myself.

 

I made up some plug extenders which run between the
plug top and cable and allow shorting plugs to ground
with a screwdriver. Useful for setting idle adjustments
and cable slack.

Mine were made of 1/8" diameter aluminum standoff
spacers tapped to 4mm with a 4mm bolt extending
out one end to connect to the cable and the 4mm
hole at the other end threading onto the spark plug.

Using an insulated screwdriver, you can now short
between the exposed part of the extender and the
cylinder head to cut out one cylinder at a time.

 

Well, we finally found something you DON'T know. ;-)

I had a Kawasaki I4 from the mid to late 80's that I quite assure you WAS
equipped with 2 plugs in series......with the block as an intermediate
conductor. Since that was a fairly common engine in the Ninja and other
models at the time, I assumed it was a common arrangement in other engines
too. Maybe not.

That fact alone doesn't also make it a double fire but I'm pretty sure the
plugs were paired so that the 2nd one was indeed firing at the end of the
exhaust stroke (wasted).

 

Yes.....IF the plug fails in such a manner that it goes wide OPEN and the
spark also can't jump to the block. A relatively rare occurance, I would
think.

 

It happens

Which is why I added AFAIK....

But I've still not had (AFAIK) a four drop onto two pots on the rare
occasions when I've removed a plug. I'll have to test this again when I
get home.