Did you "time" the engine when reinstalling the fuel delivery pump? You
could be delivering fuel a little early into the cylinder - ok with the
engine cold but once warmed up you are getting a little pre-ignition.


"Alisdair Gurney" wrote in message
...

"Phil Bailey" wrote in message
om...
I have just completed a partial rebuild of a Yanmar 1GM (6.5 HP), or
at least I though I had! Some background: A bent con rod was causing
very difficult cold starting, although once started it ran very well.
The new con rod (plus big end bearing shells) has reduced the piston
top clearance to 0.9mm (from more than 1.2 mm originally, the
thickness of my solder wire) and the piston height above the cyclinder
top surface has increased from 0.15mm to 0.4mm. This has considerably
improved cold starting. The rings were replaced at the same time. Oil
pressure is fine

The injector has been serviced, the engineer said that it was seized
(the engine had been stored for 4 years) but he had managed to free it
up and it should now be OK.

With the new con rod fitted, on the bench the engine starts and runs
fine most of the time. However,it occassionally and suddenly develops
a very severe knocking which sounds mechanical as if the engine is
being hit with a hammer. So far this has only happened at low revs and
I have immediately stopped the engine. On restarting everything is
fine again until it happens again. It does not seem to be associated
with anything else.

One thought which occurrs to me is that the reason the con rod was
bent originally could be due to water entering the cylinder, possibly
via a cracked head. The knocking could then be the sound of the
resulting hydrolicing with the piston hammering into the water at TDC.
Having taken the head off again I find nothing to suggest this.
Everything is evenly covered with a thin layer of carbon. If it realy
was something mechanical, worn crankshaft bearing for example, I do
not understand why I do not hear the knocking all the time. It must be
something to do with the reduced piston top clearance and consequent
increased compression as before replacing the con rod no knocking was
evident.

Any suggestions for the cause of this problem would be gratefully
received.

Thanks,
Phil

Two suggestions:

1. Take off the water pump and examine the bronze bit that fits inside the
engine block for wear. There are three bolts attaching it to the engine
block and it's easy to remove (you don't need to take off the front of
the pump with the impeller). I had a similar sounding knocking problem
a couple of years ago when the large nut holding the governor in place
loosened and bits were moving more than they should, occasionally
catching the water pump where there's now a small flat spot.

2. It's normal - at low speed, say less than 1000RPM, they ARE clanky.
It seems to be just above the stalling point where there's little or
no energy left in the flywheel when the fuel combusts.

Alisdair