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The
most common cause of combustion problems such as pinking in MG
engines is not fuel as usually assumed, but distributors not
performing to spec, especially wear in the centrifugal advance
mechanism of the distributor.
This slackness results in the distributor giving too much advance
too soon with obvious results.
Often this is "tuned out" by simply retarding the
whole distributor, a move which certainly stops the pinking but also
leaves insufficient advance at mid to high engine speeds leaving the
engine well down on power and sluggish.
Even setting the advance to the correct spec using a timing
light at idle doesn't solve it, as the distributor has already used
up most of its advance at idle when worn like this and so will end
up well short of where it should be at mid-high speeds.
The following example on pre-73 MGB's illustrates what to do:
Confirm the problem by checking the advance at 2500 RPM with a
timing light (vacuum disconnected) - it should be 30 degrees total.
It should drop steadily through
23-24 deg @ 1750 RPM to 14 deg at 700 RPM.
If it doesn't meet this within a degree or so at each speed (many
can be 10 deg or more out) the
only cure is to strip the distributor and rectify the advance:
Check the take-up point of the heavier spring - the cam should have
moved about 1/3 of the way to its stop when the heavy spring takes
up.
Typically, in most worn MGB distributors the cam will advance nearly
all the way before the larger spring cuts in! This can be 90% cured simply and cheaply by closing up
the hook on the heavier spring a bit so that it takes up at the
correct point.
Once this is done, reset the ignition timing & check the
newly adjusted advance with the light again, followed by any final
adjustments necessary from road test.
You won't believe its the same car after that!
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