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1981 Honda DOHC 750
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Hello-
I've got an 81 Honda DOHC 750cc, just under 20K miles. Its a strong
runner, responsive in all ranges, but once it gets past about 4000 RPM
there is a distinct top-of-engine "clicking" noise, quite rapid. Its
not that loud, but noticeable to me. It gets more insistent the higher
the RPMs go.
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A distinct possibility is that one or both of your TWO cam chains is
There are TWO camchain adjusters on the cylinder and you can adjust
them yourself with the assistance of a repair manual or have them
adjusted by a shop mechanic.
Maybe it's not the chains though.
Does the clicking get worse as the engine warms up, or does the
clicking sound subside a bit?
The former symptom indicates that your problem might have nothing to
do with valves that need adjustment. The problem might actually be
pinging, which is pre-ignition taking place before the spark plug
actually fires. This happens when the spark plug gets so hot the
ground electrode starts glowing red.
Preignition is caused by lean fuel/air mixture, advanced ignition
timing, plugged up exhaust baffles, or a spark plug that has the wrong
heat range.
Preignition can be detected by removing the spark plug and examining
the ceramic insulator inside the spark plug. If an engine is pinging
when it gets hot, you'll see little black specks like pepper burned
onto the spark plug nose.
If the top end clicking is louder when the engine is cold, you might
have one or more valves with clearances that have loosened up. The
valve stem will expand a bit, getting very slightly longer as the
valve heats up, and the noisy valves will quiet down a bit when the
engine is hot.
But valves don't always loosen up as you put more and more miles on
the engine. Valves tend to get hot because of the lower octane fuel
used in modern motorcycles and the lean fuel/air mixture mandated by
the environmental agencies.
Valves get so hot they cannot be cooled by contact with the valve
seats. Instead, the valve will weld itself to the valve seat, just
tiny welds.
The next time the camshaft opens the valve, a tiny bit of metal will
be ripped off the valve and seat and will be blown out the exhaust
pipe as a hot spark.
The valve and valve seat will erode and the valve will "sink" into the
head. This takes up some of the valve clearance, and valves that
aren't making any noise at all are more in danger of burning than
valves that go clickety-click.
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There seems to be no performance problems whatsoever but I've been
keeping it below 4000 as much as possible in case something is coming
apart. This means basically no freeway travel.
Below 4000 rpm there is no clicking noise at all. Gear selected makes
no difference; if I bring it up past 4000 in 2nd gear the clicking is
also present.
Do I need my valves adjusted, or is there a different common cause for
this? Tell me I don't need the whole head rebuilt.
Any thoughts welcome.
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Checking your valves would be a good first step. if there's a manual
camchain tensioner on that thing, or even an automatic one, check that
too.
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