The stock answer from most untrustworthy mechanics is, of course, "They
*all* do that." With MGBs, that's not entirely true: only the GOOD ones
do that. British Phlegmsucking Leyland finally fixed the B Series'
tendency to run-on, and did it by leeching most of the fun out of the
motor in the process.
There are several things you can do to mitigate things. First off,
realize that the B is a Sports Car, and use 92-octane gasoline. Yes,
the '74 is supposed to have 8.0:1 compression, but if you're runnign
on to this degree, I suspect it's raised either to the pre-72 ratio
of 8.8:1 or, as you hint, perhaps even a little higher. (Remember
that overboring an engine will raise the compression ratio, as will
truing up the head and block, so even if the rebuild didn't include
high-compression pistons or a good deal of head-skimming, it's still
likely to be part of a point higher than stock.)
After this, you'll want to make sure that you use plugs that are
colder, not hotter, than stock. Many people run hotter plugs in
old British cars on the assumption that the hotter plug will keep
from loading up with oil or unburned gasoline, or will foul less
easily. That's only a cover-up for other problems, and at 5000
miles on this motor the issue shouldn't come up. Check in the
mfgr's chart for the plugs you like and try running one range cooler
than stock. At the worst, you might have to run an "Italian tune-up"
from time to time (that's where you blast onto the freeway in first or
second gear, taking it to the redline and holding it there for as long
as you can stand it.)
Other things to watch out for are timing -- make sure you set the\
timing with the vacuum advance *disconnected and plugged* (though
that should move things the other way from run-on, by retarding
rather than advancing the net timing) -- and temperature. The B
runs on mainly because of the shape of the combustion chambers, which
have a beak-like splitter between the valves in each chamber. This
can retain heat, particularly if there is carbon build-up in the
motor. Other problem areas are the spark plugs and sometimes even
the threads on the spark plug hole.
So in order of complexity:
1. Use 92-octane gas, and use something like Chevron or Mobil, both
of which have good additive/detergent packages that will help clean
your MG's combustion chambers.
2. Replace the plugs with the next range colder, and be sure every
few days (or as needed) to run the car well into its powerband so that
you keep them clear.
3. Flush the radiator, check the thermostat for proper opening and
flow, and use no more than 50% coolant in the mix. (Due to some magic
properties of chemical solutions, the more glycol you have in your
coolant, the hotter the car will run. I know that the actual percentages
are 54 and 46 to hit the optimum solution, but I can't remember which is
the water and which is the Prestone so I usually just eyeball it about
halfway up the jug.)
4. In an extreme -- and usually unnecessary unless you've got a full-
race motor with a long-period cam and 12:1 or more compression -- you can
reshape the beak in the combustion chamber (as part of a full polish
and CC job) to remove the hot spot that it causes. This is great fun
and rewarding to do, but takes a lot of work to get to and isn't
necessary or worthwhile unless you're also doing a valve job, as the
required grinding means you have to disassemble and clean the head.
My money's on #1 as the most probable solution, but keep #2 and #3 in
the back of your mind.
--Scott
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