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Chicagoland MG Club: Driveline
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  Chicagoland MG Club:Driveline
Go Brit Baby! - Tech Sessions

Steve Merical Dizzy for Distributors

In August we had two tech sessions. First, we installed an overdrive transmission into a MGB - a nice upgrade for anyone looking for better highway cruising. The second tech session was on distributors.

Distributors can be mysterious - Tech Coordinator included here. Sure most of us realize that the points open at a specific time releasing a spark that goes on to fire a plug which ignites the gas in the engine. An electronic unit duplicates this effort without the need of points.

We may also understand that to set the timing we rotate the distributor housing and use a timing light on the pulley marks. However, when the topic turns to specific rpms and the timing advance associated at these rpms; our eyes begin to glaze over and we seek shelter in a safer environment…Say, what tires should I use?? Never mind, let’s make time for timing.

Here’s a quick breakdown on the process. The distributor (dizzy) receives power from the coil and then triggers, plug wires, a spark at the spark plugs at precise moment. The dizzy rotor rotation is put into motion by the engine’s camshaft. The dizzy points open as they come to a lobe on the distributor cam.

When the points are closed the coil is charging. When the points open, the coil sends power through the dizzy, which times the spark event. The condenser acts a kind of power stabilizer and storage tank for the voltage going to the coil. If the condenser wasn’t present, the points would quickly become pitted and the engine would run poorly.

The dizzy’s job is to correctly time this spark event which ignites the fuel mixture in the cylinders. Ignition timing is in reference to the start of this burn cycle before the piston reaches top dead center (TDC). So, if your workshop manual calls for a setting of say 10 degrees BTDC (before top dead center) it means the gas starts to ignite at a piston position of 10 degrees BTDC. Consult your workshop manual for your MG’s setting.

Distributors may have a vacuum advance or a mechanical advance. As the rpm changes, the vacuum/mechanical advance rotates the dizzy contact breaker plate to advance or retard the ignition as needed.

Okay, so what about advance curves? Well, when the engine is running at a low rpm there is more time for the gas mixture to burn. As the rpms in the engine increase the pistons move faster and faster. So, this burn needs to initiate earlier for the best effect, perhaps 30 degrees BTDC at a high rpm. This is the advance curve.

Why is this important? Performance. The ignition advance curves it’s way up from say 10 degrees to 30 degrees to initiate the optimum burn as the rpms increase. If the burn starts too early or too late your engine won’t be performing at it best. In fact, it could lead to problems like detonation, poor throttle, and bad gas mileage. Timing is everything!?!

Too much ignition advance may cause detonation. This can create a pinking sound in the engine, as the end gas burns unrestrained by high pressure or heat in the piston chamber. Think you can live with it? In extreme cases it can eat away at the piston and then the piston rings. Before I freak anyone out too much though, please note that some of our classics probably have a number of distributor faults and these hardy engines just keep going. Nonetheless you are warned.

If you take a distributor apart, you will find a couple of weights and springs in the bottom of the housing next to the advance plate. The dizzy advance plate will have degrees advance stamped into it (example: “10”). You can repair or change your dizzy advance curve by changing these components. Also, be sure to check or change your points, condenser, cap, and rotor. Make sure that shaft wobble is minimal especially if you are using points. Electronic ignition (EI) is less fussy about wobble because there is no side pressure on the shaft with EI. Then set the timing as part of your tune up for the car. You want the correct set up for your MG’s engine and performance requirements.

On this tech session we took CMGC Pres. Dave Bralich’s MGB distributor completely apart and checked all the parts and components for wear. We found a ground wire needing replacing and a badly stretched weight spring. Other members brought along their distributors for analysis too.

We also took a few minutes to help set the timing and adjust the carburetor on a 1980 MGB. This car had failed its emissions test recently. So, they wanted us to have a look. The happy owners came back a little later to report that the car had just passed the emissions test.

For more information on distributors and advance curves please visit Paul Hunt’s website at This site has great information on the different Lucas distributors models and their relative advance curves. Remember, when someone asks you about tires switch the topic to advance curves. Time flies when you’re having MG dizzy fun.

On Sunday, September 16th we will fit a pair of Speedi Sleeves to fix a leaky rear axle. Have you got oil on your rear break shoes, drums, and wheels? This is the tech session for you! Come to the tech session at 10 a.m. in St. Charles, IL. Feel free to contact me for more information.

Steve Merical   630-443-4985

©2001 Chicagoland MG Club, All rights reserved.