Washington, D.C. Centre
the Spark

(or Timing is Everything Part II)

by Jim Lunson


Where to set the timing? Following up last months discussion, let's set the timing. I won't go into the basics, but look at the specifications for your car and it will say something varying between 10-15° BTDC at 600-1,000 rpm, with vacuum hose removed and plugged. This is a good place to start, but after many years of car changes and improvements, to gain maximum performance from the engine, some fine tuning is usually necessary. The final setting also depends on modifications (carbs, distributor, points, advance curve, etc) on the car and the primary use of the car. Timing is set by rotating the distributor in relation to the position of the pistons (clockwise to advance and counterclockwise to retard).

For most of us MG drivers, street and highway driving is where we want optimum conditions. One good method to obtain the best setting for this use is to take the specified setting noted above and then advance the idle timing 3-4° more by again loosing the pinch bolt on the distributor housing and rotating the distributor slightly clockwise. Relock the bolt and run the car at 40 mph up a long fairly steep steady grade. If there is no pinging from the engine during this run, advance the distributor another 3-4° and try again. Keep repeating this step until pinging occurs, then back off to the previous setting and you're done. This system is not too precise but works pretty well as it finely adjusts the timing to match the carburetor and cam as well as the gasoline grade being used and the engine compression to provide a good setting for average driving conditions. The idea is to get the timing optimal where it is at the driving conditions most often encountered.

Another idea put forth by John Twist, renowned MG expert, is to simply adjust the distributor to 32° advance at 4,000 rpm. Again, this setting is fairly easy to obtain and will give good performance at average driving conditions. This is a setting that he has found to be very good for average driving conditions and again totally does away with trying to set it at idle.

One trick in using this method is in getting this reading using a standard timing light. With the engine off, rotate the crankshaft pulley by hand until the timing mark aligns at the 20° mark. Then place a second scratch on the pulley back at the zero point. Once this mark is made, run the engine and set the timing as you would usually, only rev the engine to 4000 rpm and use the second mark to set the timing at 12°. This then yields a setting of 32° (20° + 12°) which is what we are looking for.

Both these methods work well and take into account all the factors involved. Plus they give a good setting at the main usage the car normally gets. It may not be the best at idle, but that is not the goal. Of course, if racing or lots of high speed interstate driving is anticipated, changes need to be made, following the same principles noted above.

Manifold vs. ported vacuum Advance. As noted last month, most MGs require both vacuum and mechanical advance. This vacuum advance difference occurs where the little vacuum hose (or tube) to the distributor connects to the engine. Manifold advance obviously connects to the manifold, sometimes through the gulp valve, sometimes directly off the center pipe that connects the two carburetors feeding the engine. Ported advance on the other hand connects to a small port on one of the carburetors (usually the rear one). The difference is which side of the carburetor throttle plates the hose connects. There is much debate as to which system is best.

Up to about 1967, all MGs used a ported vacuum advance. This provides no vacuum advance to the distributor when the engine is at idle, slightly reducing the engines efficiency. Then, when the accelerator is pushed (such as when starting from a stop light), the throttle plates open, giving a surge of vacuum inside the carburetor. This surge then goes directly to the distributor, instantly boosting the advance. This increases engine performance just as you need it to accelerate. Idle is slightly weaker but initial acceleration is better.

Starting around 1967 (in the middle of MGB production) emission controls started to come into play. One of the early methods to improve exhaust emissions was to relocate the vacuum hose to feed directly off the manifold. This advanced the timing at idle which improved idle operation and reduced emissions, but sacrificed that first burst of acceleration. Some say this hurt the MGB performance while others say the improved idle compensated for the loss. Two further changes to manifold vacuum advance were made later as emission requirements tightened. First was the gulp valve addition which adjusted the manifold vacuum with the air pump. This valve reduced the vacuum pressure depending on acceleration needs of the engine, further reducing the vacuum advance available to the distributor. And lastly, with the introduction of the single carburetor on the MGB, an electric switch/solenoid (TCSA) was inserted in the vacuum line, cutting off all vacuum advance to the distributor except in forth gear. These changes hurt engine acceleration as the use of vacuum advance was greatly restricted and the distributor had to rely much more on the mechanical advance system to maintain performance (more reactive and slower - remember last month).

Some MG owners removed the TCSA switch and run the hose line directly to the distributor giving vacuum advance in all gears. And some have tapped the hose directly off the manifold to bypass the gulp valve, also increasing vacuum advance. These changes get into modified cars as opposed to keeping everything exactly stock, but they improve performance. It is difficult to switch to ported vacuum advance unless you change carburetors to get one with the port built in.

One problem I encountered in fine tuning the timing on my car concerned the pinch bolt and clamp. On the MGB, this bolt is way down beside the engine block. My idea when fine tuning was to get a wrench with an extra long handle. Then I could loosen and tighten this bolt without reaching down next to a hot block. This worked great until one day on a Sunday drive, suddenly the engine started running very rough and would hardly idle. Upon checking under the hood for the problem, I reached for the distributor. It came completely off in my hand. Seems that with the longer wrench, I was really tightening this pinch bolt to where it squeezed the clamp so hard it forced the distributor up and out of the clamp. So, when setting the timing, tighten the bolt to hold the distributor where you want it, but don't over tighten it or you may lose all your timing work.

There are many other ideas and tips on setting the ignition timing on our cars, but this gives a few of the latest ideas on what goes on and how to get the most from our MGs. So keep the cars running, and perfect your timing as you roll along.


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