Sage advice from our friend John Twist of University Motors concerning timing settings with various types of distributors used with MGB.

On 3/31/2015, John Twist wrote:
"The distributor has three distinct functions. It makes the spark (with the coil) by opening the points; it times the spark by means of springs and weights and a vacuum advance, and it distributes the spark. All these functions are contained in a unit about the size of your fist. Nearly all distributors need mechanical attention from the years of wear, abuse, and neglect.

Prior to 1968 the distributor was the 40897 with ported vacuum. This offered the best performance for the engine. After 1967 the manufacturer fiddled with the timing, nearly every year, and in more than one market, to meet the pollution standards set up for each year. After 1967 there are another 20 distributors (at least) which were fitted. Again, this was to achieve compliance with the pollution regulations for that year and that market.

The 1973/1974 distributors are wildly different from the earlier one. The 1967 distributor has an advance of 20 degrees on the crankshaft. Working backwards from 32 degrees maximum advance, the static timing should be 12 degrees. (Static – engine OFF, NOT running). In practice, the idle timing is about ten degrees above static – around 20 degrees. The vacuum advance works at higher speeds.

In great contrast, the 1973 distributor 41491 has an advance of 40 degrees on the crankshaft. working backwards from 32 degrees maximum advance, the static timing should be NEGATIVE eight degrees. At idle, the distributor is advanced about ten degrees, which is STILL after top dead center. The distributor advance at 1500 rpms is about 16 degrees – moving the timing to eight degrees the way I have it figured. The factory spec is 12. The vacuum advance on this later distributor (operated by manifold vacuum – highest at idle and deceleration) gives enough advance to pull the timing up another eight degrees which gets the idle timing close to 20 BTDC.

There are two rpm ranges where the engine must perform the best – idle and 3500 rpm. It's not usual to run the car at an intermediate speed for much time. In the city you spend a lot of time at idle. On the road you're running at 3500-4500 rpm to stay with traffic. There are the two places the timing MUST be correct to offer driveability and power.

When the distributors were new the rate of advance was within a small variation. But – the intervening years of use, neglect, and abuse have changed the rate of advance – so it is no longer possible to assume that an advance of 12 degrees at 1500 means that you will achieve 32 degrees maximum. If the springs are stretched, you might get only 25 degrees (as they've expanded more by 1500 than they should), or, conversely, the springs have been changed to stiffer ones or the distributor is sluggish from lack of lubrication and you might get 40 degrees total.

So what's the big deal about 32 degrees maximum? Timing is CRITICAL! Let me state that again: Timing is CRITICAL!! "Back in the day"� people fiddled with their timing until the car seemed to run the best. This is NOT a proper tuning method. Let me explain. We had an enthusiast attend our "Tuning for Speed"� class about ten years ago. He drove to Grand Rapids, Michigan, from Hershey, Pennsylvania in his 1967 MGB. That's a run of about 600 miles. Our first day in the class was spent properly tuning his MGB to achieve as much power and driveability without replacing major components. We checked his timing to find 27 degrees BTDC maximum (around 4000 rpm). We changed it to 32 degrees. The next day we put his car on the dynamometer to find 62 horsepower at his rear wheels. That's a good figure for an earlier MGB. We retarded the timing to 27 and did another run. This time we got 57 horsepower. Five degrees = five horsepower. Timing is CRITICAL!

So – to answer your question. Disconnect the vacuum advance. Use a dial back timing light set to 32 degrees. Run the engine up until the timing stop advancing (full mechanical advance). The timing should be 32.

If you do not have a dial back timing light then you must make a new mark on your front pulley with a paint pencil. Roll the engine until 12 degrees before top dead center. Put a paint mark on the pulley in front of the last (zero) timing mark – the last one in a clockwise direction. Now, when you time the car, align that mark with the 20 degree mark on the timing cover (the first timing mark in a clockwise direction), as 12 + 20 = 32. All of our MGs use 32 degrees except the MGA Twin Cam.