I had not thought of that, an oil delivery problem could be showing up at
high rpm as lifter noise. If that's the explanation then that could be
pretty bad if it continues as there would be an oil problem to the crank
too.

"Calif Bill" wrote in message
ink.net...

"JamesgangNC" wrote in message
ink.net...
It is odd for an engine to start making a noise above 3/4 throttle and
not
make it below. It's pretty hard to diagnose a noise over the internet
too.
One man's clack is another's thump if you know what I mean.

Is it rpm dependent in that if you vary the throttle a little it gets
faster
or slower?

Lower end noises such as rod or main bearings tend to be deeper
sounding,
more of a knock or thud. Rod bearing and wrist pin issues will change
sound
if you pull the plug wire on the problem cylinder but that is a lot more
difficult to do to a 4 cylinder than it is to a 6 or 8 cylinder because
pulling a plug wire has a more dramatic effect on a 4 cylinders
performance.
True clacking usually is a valve train noise that is caused by too much
clearance. Hydralic lifters can cause it to come and go based on rpm
because they use the oil pressure to stay pumped up. But usually the
problem is the other way around, a clacking at low rpm that goes away at
higher rpm.

I'm a bit stumped by a clacking that appears at high rpm. If it is
valve
train it will sound like it's coming from the valve cover.

"B. Alexander" wrote in message
...
Hi,
I have a mercruiser 3.0 with an Alpha 1 outdrive. the engine runs
great
up
to about 3/4 throttle, but when I go to w.o.t. the engine still runs
fine
but it starts to make a loud clacking sound. I sat beside the engine
with
the cover off while someone else was driving to try to determine if it
was
caused by something vibrating, but it appears to be coming from inside
the
engine. Any ideas what this might be?
Thanks,





Could be low on oil or a bad filter or a bad oil pressure bypass valve and
causing lifter collapse or a bad lifter.