“ABW” is fitted with a Luminition optical ignition system, ruling out points issues. But last time the problem was the distributor cap – burnt out by the fitment of the wrong coil by the last owner. Next to check is the plugs and HT leads, but no obvious issues are apparent; spark is good on each cylinder.
So what about the distributor rotor arm? I’ve read plenty of bulletin boards bemoaning the quality of replacement rotor arms. To my surprise the rotor arm doesn’t look like a regular MG Midget item. What is going on? Examining the existing rotor results in more questions about MG Midget distributors.
Firstly which distributor should be fitted? From 1961 to 1974 MG fitted the Lucas 25D unit to the Midget; from then to the end of production in 1981 the Lucas 45D type was fitted. Both were fitted to MGBs and other Austin Morris vehicles. So, being a 1972 model, “ABW” would have originally had a Lucas 25D type.
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| Lucas 25D distibutor |
The 25D unit is considered to be a reliable unit with an abundance of spare parts. The mechanical advance mechanism is generally reliable and dependable, with original components of high quality hardened steel. The use in multiple engines, and multiple applications of the A Series engine in particular, means that many advance and retard curves are available.
The main alternative is the 45D unit fitted to the later MG models. Not only are second-hand items widely available, but many brand new reproduction units are available.
Reports suggest that the quality of some replacement units is variable, with lower quality bearings (though not always an issue for low mileage classic cars).
The existing rotor arm looks similar to that fitted to the 45D type distributor.
Of course, the engine doesn’t know whether it is being serviced by a 25D or a 45D distributor; no more than it knows whether the distributor is electronic or has points. All the engine is interested in is that a spark of the right strength is delivered as the right time.
What exactly does the distributor do – beyond delivering the spark? The most important function is to place peak cylinder pressure at 17º-20º above top dead centre (ATDC) by advancing the ignition spark as engine speed increases. The difference in crank speed between idle and maximum engine speed is about an order of magnitude. However the rate at which the fuel burns is relatively fixed in comparison. That means that the mixture needs to be ignited earlier and earlier as the revs are increased. Igniting the mixture too soon drives the piston down before it has reached the top of the stroke – “knocking” which will damage the engine. Leaving it too late reduces power. Not surprisingly, car manufacturers are incredibly conservative, preferring low power rather than warranty claims!
The stock ignition curves that MG used when “ABW” was manufactured never envisaged the engines would be operating 40 years later, let alone using lower octane unleaded fuels or coping with engine, carb, or exhaust modifications.
The distributor contains a cam and two springs. Under the cam are a pair of counterweights that fly out as speed increase and cause the cam to advance. Different cams can be used to adjust the maximum advance figure. The cam opens and closes the points… and is increasingly becoming obsolete with the introduction of after-market electronic systems! However, the principle remains the same. The other item that can be adjusted is the springs. The length and rate of the spring can also be manipulated to change the advance curve.
“ABW” is fitted with a performance distributor from Aldon Automotive (http://www.aldonauto.co.uk), a long-established supplier of performance parts in the West Midlands. Aldon have been successfully modifying distributors for many years, supplying its own make of performance distributors amongst other items. Their units that are based on Lucas and Bosch distributors, with a modified advance curve to suit the engine specification. It appears that “ABW” has an 100AR series Aldon unit. A quick call to Aldon has a replacement rotor arm sent, arriving next day – excellent service.
The rotor arm is definitely the same as that used on the later 1500cc Midget unit. Reassuringly, Aldon sent me Bosch rotor – hopefully of good, durable quality.
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| Lucas 45D distibutor |
Reports suggest that the quality of some replacement units is variable, with lower quality bearings (though not always an issue for low mileage classic cars).
 |
| MG Midget ignition rotor arm Lucas 25D4 type 1961-1974 |
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| MG Midget ignition rotor arm Lucas 45D4 type 1974-1981 |
The existing rotor arm looks similar to that fitted to the 45D type distributor.
Of course, the engine doesn’t know whether it is being serviced by a 25D or a 45D distributor; no more than it knows whether the distributor is electronic or has points. All the engine is interested in is that a spark of the right strength is delivered as the right time.
What exactly does the distributor do – beyond delivering the spark? The most important function is to place peak cylinder pressure at 17º-20º above top dead centre (ATDC) by advancing the ignition spark as engine speed increases. The difference in crank speed between idle and maximum engine speed is about an order of magnitude. However the rate at which the fuel burns is relatively fixed in comparison. That means that the mixture needs to be ignited earlier and earlier as the revs are increased. Igniting the mixture too soon drives the piston down before it has reached the top of the stroke – “knocking” which will damage the engine. Leaving it too late reduces power. Not surprisingly, car manufacturers are incredibly conservative, preferring low power rather than warranty claims!
The stock ignition curves that MG used when “ABW” was manufactured never envisaged the engines would be operating 40 years later, let alone using lower octane unleaded fuels or coping with engine, carb, or exhaust modifications.
The distributor contains a cam and two springs. Under the cam are a pair of counterweights that fly out as speed increase and cause the cam to advance. Different cams can be used to adjust the maximum advance figure. The cam opens and closes the points… and is increasingly becoming obsolete with the introduction of after-market electronic systems! However, the principle remains the same. The other item that can be adjusted is the springs. The length and rate of the spring can also be manipulated to change the advance curve.
“ABW” is fitted with a performance distributor from Aldon Automotive (http://www.aldonauto.co.uk), a long-established supplier of performance parts in the West Midlands. Aldon have been successfully modifying distributors for many years, supplying its own make of performance distributors amongst other items. Their units that are based on Lucas and Bosch distributors, with a modified advance curve to suit the engine specification. It appears that “ABW” has an 100AR series Aldon unit. A quick call to Aldon has a replacement rotor arm sent, arriving next day – excellent service.
The rotor arm is definitely the same as that used on the later 1500cc Midget unit. Reassuringly, Aldon sent me Bosch rotor – hopefully of good, durable quality.
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| Aldon AR100 distributor |
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