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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: (NOTE although what follows is for early MGB, the principle is the same for most distributors, though the engine speed at which maximum advance occurs will differ for different cars - see page Setting Ignition Timing - strobe timing at full advance )
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.
Most of the time the problem is only a bit of wear in the secondary spring hook and/or its supporting pins, allowing the distributor to advance too far before the second spring takes up. The primary spring is seldom a problem in the MGB as it is under
constant tension when the car is running (and so doesn't chatter around the take-up point) and is also much lighter.
Check the take-up point of the heavier spring - when the heavier spring takes up, 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!
New distributor springs are not available, but the problem is not insurmountable. Although the springs do lose a bit of strength with age/use, this is seldom significant.
It's easy to fix: It 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. To compensate for the normal amount of weakness in the secondary spring of a high mileage MGB,
over-tighten the spring a little further. I usually close the hook up so that it takes up after one third of the cam's movement from fully backed off to where it hits the stop. This is a degree earlier than original and gives a good approximation to the overall shape of the std advance curve with the average high mileage distributor.
Closing the hook up is a bit fiddly: I grip the base of the hook (where it joins the main coil of the spring) in the corner of a vice, then tap the hook end-on with a small hammer.
Modified engines may require stronger springs to slow down the rate of advance (refer to the page on Setting up ignition timing for modified engines.) If the distributor springs are not strong enough, a stronger spring can be obtained from another Lucas 25D distributor. All BMC cars of the period had the same basic distributor, but varied in the spring strength and the point at which the secondary spring takes up. Suitable candidates are Mini or BMC1100/1300.
Once you have attended to the springs, install the distributor, 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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