Article 6 – The Distributor Advance Mechanism

Tech Article 6

Troubleshooting your XK Engine – The Distributor Advance Mechanism

Last month we looked at the operation the distributor in the XK spark ignition system. In this month’s article, we are going to discuss an area that can be confusing, that being timing advance mechanisms.

As we established in our last article, the ignition coil is able to generate short pulses of high voltage electricity. This electricity is sent down the ignition wires to the spark plug, where it generates a spark that ignites the air/fuel mixture. This needs to occur at the precisely correct instant in order for your XK engine to operate at peak performance. But due to the changing driving conditions under which the engine will operate, the right time for the spark to occur is a moving target.

As we discussed in the last article, when the engine is running at its idle speed, the distrutor is set to fire the spark about 10 degrees before the piston comes to its Top Dead Center position. This gives the fuel/air mixture a small but sufficient amount of time to ignite and begin burning fiercely as the pistion travels downward in its power stroke. As the engine speeds up, the spark needs to fire earlier in time, as the fuel/air takes the same amount of time to fully ignite but the piston is moving much faster. On the XK engine, the distributor is designed to “advance” or to move forward in time the point at which the spark fires. As the engine speed reaches 3000 rpm and above, the distributor fires the spark approximately 30 degrees before the piston reaches Top Dead Center. It does this with a “centrifugal advance mechanism”. The centrifugal advance mechanism is buried down inside the distributor. It basically consists of a set of weighted, spring loaded arms that move outward as the distributor shaft spins faster. The motion rotates the plate that the points are mounted on and causes the points to open sooner, advancing the time as which the spark fires.

The centrifugal advance mechanism is built into the distributor and, while it is adjustable, it is beyond the scope of this article to explain how to rework a distributor advance. But what is possible and highly recommended is to confirm that the advance mechanism is working and advancing to the proper limit. This can be done quite easily if you have purchased a dial back timing light as I recommended in one of my previous articles. Checking total advance is easy. Hook up the light per manufacturer’s instructions, start your engine, and confirm the idle setting, which should be around 10 degrees. Then take the throttle linkage and advance the engine speed. You will see the mark on the crankshaft appear to move around the circumference of the pulley. This confirms that the advance mechanism is not stuck or jammed. When it reaches the limits of its travel, dial it back to the fixed pointer with your timing light. Again, it should reach its maximum limit of movement around 3000 rpm, give or take a few hundred. Now check the number of your dial back timing light. If it is around 30 degrees, that is good news. I found one of my to be around 40 degrees, which was a little alarming! One problem is that there were many distributors produced for various Jaguar cars over the years and some have found their way into the wrong engine from which they started. And it is possible that the advance limit has been changed intentionally by someone thinking they were finding hidden horsepower. More advance can indeed help horsepower but it can also burn holes in pistons! For me, I’m sticking with the factory stock numbers. While you are checking the advance, see if the mark on the pulley is jumping around. If it is really jumpy, the shaft bearings on your distributor may be worn out. Symptoms such as no advance, incorrect total advance, or worn distributor bearings will require that a professional work on your distributor to get things straightened out. But it must be dealt with to make your XK engine run to its full potential.

I recognize that some of you are thinking I must be crazy to be lying under that engine pointing a timing light up as it races over 3000 rpm! I’m not that crazy, although I’m not sure what Jaguar was smoking when they point the timing pointer on the bottom of the engine. But here’s a tip. The timing light will work on any spark lead. Hooking it to the number 2 lead will cause the light to fire as the mark on the pulley is up around the 10:30 position. It is a simple job to make a supplemental stationary pointer off one of the timing cover bolts up at this position which will allow you to check timing and advance while in a somewhat safer position leaning in over one of the front wheels. The pointer can be a bit of stiff wire. Get it close to position and then move it into a final position so you get the same idle reading as from the mark on the bottom of the engine. It makes life much easier.

Vacuum Advance Module as Seen on the Side of the Distributor

I will conclude with a discussion of vacuum advance. My 1967 XK engine has a vacuum advance module mounted on the side of the distributor. As I mentioned above, more spark advance can be beneficial to engine performance, under the right conditions. Under light throttle or cruise conditions gas mileage will be dramatically improved if the spark is advanced another 10 degrees or so. The vacuum advance serves this function. The vaccum adance connects to the front carburator via a small rubber hose. Pull this hose and plug it when checking the regular or mechanical advance. As a quick check your vacuum advance, I have found that sucking on this hose will cause the vacuum advance to engage to the point that the engine will speed up. It is hard to check the precise operation of the vacuum advance much more than this without some special equipment. What you can check is the vacuum source on the carburator. Note that the connection to the carburator is “ported vacuum” which means the tap is upstream of the carb butterfly plates. A small automotive vacuum gauge is an inexpensive purchase. Use it to confirm that there is no vacuum at the vacumm advance port on the carb when the car is at idle. As you crack the throttles, you should see the vacuum jump up on the guage. As you open the throttles to their fully open position, the vacuum should subside back to almost zero. This makes sense, as you only want the vacuum advance working at small or light throttle openings associated with steady cruising. You want the vacuum advance to be gone when you are at full open throttle conditions. Finally, the vacuum advance on an XK engine should not be confused with vacuum retard found on other cars, which serves a different function and uses manifold vacuum as its source.

Assuming that you have sucessfully performed the checks I have discussed in the last three articles and have replaced any suspect components in your ignition system, you should now be confident that your ignition system is in fine working order. The last step is checking carburation, which will be the subject of the next few articles.

Disclaimer – Automotive work can be dangerous if proper safety procedures are not followed. In homage to our litigious society, I must state that I cannot be held responsible for any real or perceived mis-information that may be contained in this article. A good shop manual is mandatory before you attempt any work. Read the safety section of your manual. If you have any questions, contact me at hdferris@bellsouth.net so we can hopefully get questions worked out before a problem is created. Previous Technical Articles will be posted at my website http://www.newhillgarage.com

Required Tool:

Dial Back Timing Light

Vacuum Guage