Distributor advance rate adjustments: compensating for high mileage wear
The most common cause of combustion problems such as pinking in MG engines is not fuel as usually assumed (provided the engine is running on minimum 95 octane), 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.
In the following example to illustrate what to do, I have used pre-73 MGB’s as an example as they are one of the worst culprits. However, all cars with old distributors will experience wear changes the advance rate and adversely affects both power and economy. Although what follows is for the early B, the principle is the same for most distributors – only the actual advance specifcations will vary, the procedure remains the same.
The specifications you will need are the static, idle, maximum and intermediate advance figures. These can be found in the workshop manual. Key points in interpreting this data are:
- Remember to identify whether the specs are referring to crankshaft or distributor degrees – timing figures are usually crankshaft degrees, but advance figures can be given in either.
- The advance figures given in the manual are the amount of advance caused only by the distributor. To get total advance you need to add to this figure the static advance. In our example, the MGB manual lists:
- static advance as 10 degrees
- maximum advance as 20 crankshaft degrees
-so the total maximum advance is 10 (from the static setting)+20 (from the distributor) = 30 degrees. The total intermediate advance figures are worked out in a similar way. In our example, the book lists the advance at 1500 RPM as being 15 degrees, so the total advance at 1500 RPM will be 25 degrees.
Start off confirming the problem by first checking the advance rate of the distributor as it is. Using our example, we set the advance at idle to the factory speified 14 deg, then 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 – in our MGB example: 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!
For other engines, you can work out the correct take-up point of the second spring by plotting the intermediate advance figures on a graph. Where the graph line changes gradient is where the second spring should take up. Work out what proportion of maximum advance is taken up at this point, and that’s the proportion the distributor cam should have moved towards its stop when the second spring takes up.
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 relatively insignificant compared with the additional advance generated by the wear in the spring hook and post.
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!