What year is your BRG, Alan? If it's post-1972, the 18V engines
don't generally tend to run on as much as the earlier ones. The
comment about "only the good ones do this" was, I should have
pointed out, mainly meant as a term of endearment when one's
early MGB sputters a little after shutting off... It's entirely
possible that you have an MGB that is both good AND doesn't run on. :-)
As for the bore/CR issue, consider what compression ratio is,
a ratio between two volumes. The first volume is that of the
cylinder and combustion chamber when the piston is at BDC; the
second is the volume of the cylinder and combustion chamber
at TDC. When you overbore an engine, you are making a vastly
more significant change to the first volume than to the second.
It's probably not enough to cause dieseling/pinking under normal
conditions, but in a borderline situation it can -- and since
the running-on problem with Bs is pretty well documented,
Bs pretty much define "borderline condition" in their normal
state. With plugs that are too hot, a less-than-optimal
cooling system, or even messy castings inside the head, it's
easy to push a B into the area where it diesels.
Fortunately, this only seems to affect the cars after shutting
them off; they're fairly resistant to detonation up to about 10.5:1
compression ratios, assuming you use 92-octane gas.
For people who are consistently experiencing the running-on problem,
the solution is almost never to lower the CR. Always address the
borderline-defining conditions first, as I outlined earlier. If that
doesn't work, and it's not something you can live with, then you
might consider polishing the combustion chamber surfaces in the head.
But I don't think that wil be necessary unless there is another problem
that requires pulling the head.
--Scott
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