Faulty Rings: high oil consumption and/or fouled plugs/low power;

Faulty Valves: Incomplete filling and scavenging of cylinders/burned valves/low power;

Faulty Head Gasket: Low power, rough running, poor fuel economy, or worse.

Adjusting the Valves

Rule #4. It’s my belief that the more an MG owner knows about the inner workings and hidden mechanisms of his car’s engine, the easier time he or she will have keeping it tuned and running properly.

A text insertion here explains the intricacies of the valve train, importance of proper valve clearance and why you should check valve train clearances before doing a compression check.

The valve train is something akin to the anklebone connected to the knee bone, the knee bone connected to hipbone affair. The camshaft down in the bowels of the engine has eight lobes plus three gears. One gear is connected to the timing chain. Another powers the oil pump and yet another turns the distributor camshaft. Riding on the lobes are eight tappets (or lifters) and atop these are eight pushrods. The pushrods have cupped tops which accept domed heads located on the ends of the rocker arms. At the opposite end (the valve end) of the rocker arm, the end is flat and rides just above the top of the valve when it’s closed (sticking up). The valve is forced up and closed by springs wound in opposite directions to prevent their interfering with each other. When the push rod on the opposite end of the rocker arm rises, the rocker arm rotates and pushes down on the valve stem, compressing the spring and opening the valve (pushing it down and away from the seat in the head).

A certain amount of free play, or valve lash as it’s sometimes called, is needed to allow for differences in the expansion rate of the various parts, plus wear and tear. After initial adjustment, it’s usually wear and tear that necessitates re-adjustment of the valve lash, for it must be set precisely to account for erosion of the various parts. A final objective is to make sure the valves seat properly when closed. The valve end of the rocker arm must not touch the end of the valve stem when the valve is closed. The space between them is a mere .012 or .019 inches, depending on the series car you own. It’s this clearance that must be checked and adjusted as needed.

When hot gases are exhausted through an open valve, it becomes extremely hot. The exhaust and all the thrashings about going on under the hood are generating that heat. At 50 mph, the valves are opening and closing 25 times/second, resulting in considerable heat buildup. In the course of the complete cycle, the valves are closed about 66 percent of the time and open about 33 percent of the time. This is important, because when the valve seats, the seat itself acts as a heat sink, transferring some of the heat away from the valve. Should the valve remain open even a tiny bit, the extreme temperature (1400 degrees Fahrenheit) of the exhaust gas can melt the valve and possibly ruin the seat, necessitating a valve job. With a leaky valve, the engine will suffer a loss of power. Therefore, it’s absolutely critical that the free play in the valve train not only compensate for expansion, wear and tear, but also allows the valve to seat properly. Because you want these clearances to be spot on when driving the car (hot), they must be adjusted when the engine is hot. They cannot be set cold. You can test this by doing a proper hot adjustment of the valves and letting the engine cool. Check the clearances when the engine is cool and you’ll find not only are they different from one another, they’re far from the .019-inch settings called for when the engine is hot. This accounts for the clattering noise when starting a cold engine. With properly adjusted valves, much of this noise disappears as the engine warms up. MG engines are famous for this sound.

• Remove the air cleaner and ducting (do this after driving the car and the engine will cool down before you finish getting the stuff off).
• Drive the car for thirty minutes or until the engine reaches operating temperature.
• Return to your garage and remove the rocker arm cover.

• TB/TC/TD MK II to engine number 24115 = .019 inches
• TD/TD MK II from engine number 24116 = .012 inches
• TF/TF 1500 = .012 inches
• Measured on the valve side of the rocker arm.
• If you’re not sure which type of head you have, use the .019-inch clearance.

• Have three feeler gauges ready: .018-, .019- and .020-inch.
• Turn the crank until the #1 valve is fully closed (See below).
• Slack off adjusting nut from screw on push-rod side of engine using a 1/4-inch Whitworth or 5/16-inch British Standard (BS) wrench.
• Insert the .019-inch feeler gauge on the valve side between the valve end and rocker arm foot.
• Turn adjusting screw until blade is just trapped and tighten the nut.
• Check clearance using the Go-No-Go method.
• Insert the .018 feeler gauge. It should feel loose.
• Insert the .020 feeler gauge. It should feel tight or not go in at all.
• Insert the .019 feeler gauge. It should still feel just right.
• If the adjustment fails, do it again.
• Repeat for remaining seven valves (see below).

   With This Open   Adjust This
         #8             #1
         #6             #3
         #4             #5
         #7             #2
         #1             #8
         #3             #6
         #5             #4
         #2             #7

• If the old gasket is good, replace cover using original grooves in gasket.
  
• If gasket is torn, replace with a new gasket after scraping the old one off, being careful not to let any small pieces drop onto the head where they may enter the oil galleries.
  
• If possible, replace with a Nitrile gasket (better than cork).
  

Compression Check (Step 1)
 

Interpretation of Readings (Step 2)

The MG factory never published pressure readings for compression checks. However, experience over the years has shown the following to be close enough:

   Series    Max.psi   Min.psi   Max.psi variation
 TB/TC/TD      140       100       20
 TD Mk II/TF   150       110       20

Note: if the compression ratio (directly related to compression pressure) has been raised, e.g., by skimming the head, pressures will be higher.

• Confirm by conducting a wet test.
  • Squirt a tbs. of motor oil into the cylinder.
  • Crank engine a few times to spread oil.
  • Conduct another compression test.
• Higher reading confirms bad rings. Overhaul of the engine is required. Don’t apply quick cure using STP or any other wonder additive. Modern detergent oils have eliminated problems with gummed up rings.
• No difference in readings: problem with a valve or head gasket.
• Any problems found during the compression check must be fixed before tuning carbs.

Ignition System

Continuing under Rule #4 — nothing so baffles the backyard mechanic than the ignition system. He or she generally knows what it’s supposed to do, but is often mystified about how it does it. In fact, the system is quite simple, almost crude in fact, but the job it performs is extremely exacting. It consists of the coil, distributor, high and low-tension wiring, a condenser, spark plugs and two switches—the ignition switch and the points. The only precision parts are situated inside the distributor; i.e., the camshaft, the cam itself and the points. Otherwise, it’s basic electricity 101.

A measure of the precision of the ignition system can be gleaned from the fact that at 3,000 RPM, it produces 100 sparks/second. And these sparks must be timed to an accuracy measured in thousandths of a second. Unquestionably, this calls for a high degree of precision in the manufacture and maintenance of the system.

The heart of the system relies on two basic principles of electricity: 1) when a wire is wound around a soft iron core and a voltage is induced in it, a magnetic field (flux) is set up in and around the iron core; 2) when the field surrounding an iron core collapses, a voltage will be produced in any wire that’s within the field. This is the very essence of the coils in our ignition systems.

The coil itself is made up of primary and secondary wire windings. The primary wire is the low voltage (12 volt) wire and the secondary wire is the high voltage (up to 30,000 volts) wire. The primary wire is routed from the battery through the ignition switch to the coil and on to the points which, in turn, are connected to ground. Thus, anytime the points are closed electricity flows through the primary wire and a magnetic field is induced in the iron core of the coil. At this point, nothing has happened except that 12 volts are flowing through the coil and points to ground. When the points open, the electricity flowing into the coil stops and the magnetic field collapses. Recalling the second of our electrical principles, a voltage is now induced in the secondary windings. How much voltage? The coil is actually a small transformer. The primary (12 volt) windings are relatively thick wire wound only a hundred or so times around the outer wall of the coil. The secondary winding is made up of very thin wire wound thousands of times close to and around the central core. In fact, a coil can have has much as a mile of secondary winding inside of it. Because there is so much more secondary than primary wiring in the magnetic field, when it collapses, a large rise in voltage occurs—as high as 30,000 volts. This high voltage is delivered to the top of the distributor, whence it travels to the rotor, jumps the gap to the stub and goes to the spark plug.

When the points close again, the primary field rebuilds, magnetizes the iron bar, and awaits the opening of the points when the whole process restarts—all of this at rates of up to 200 times a second. There’s an awful lot of sparkin’ goin’ on down there!

There is nothing particularly ingenious about the system, as the principles governing it have been known for more than a century. What is clever, however, is the arrangement of the distributor camshaft and cam. The engine manufacturer must design the valve train, distributor shaft and the cam to exacting specifications: the cam must be timed to open the points at the precise moment the any piston reaches TDC at the beginning of its power stroke.

What about the condenser? The input to the condenser is the same primary voltage that’s fed to the points. A problem arises when the points open (when we think we see the points opening, they’ve already been open for some time). The residual voltage across the points generates an arc. This arc causes pitting of the points, increasing their resis-tance. When that happens, the points must be replaced. The condenser’s job is to reduce or eliminate arcing across the points. Here’s how it does it. The condenser is in parallel with the points, with 12 volts going in one end and the other connected to ground. But it’s also a capacitor, which means it has a some internal resistance; moreover, because it’s a capacitor, it can store and release energy it. No voltage flows across it during the build-up of the magnetic field, because the primary voltage seeks the easiest path to ground—across the points—not the condenser. But when the points first open, the voltage bounces back to the capacitor seeking the more difficult path to ground (instead of arcing across the points). But finding no ground, it gets stored, then ricocheted back up the line to the primary windings where it enhances the collapse of the field, further increasing the voltage output to the secondary winding.

The sloppiest part of the system is the rotor, and for good reason. Have you ever noticed, when replacing the rotor, that it can be wobbled left and right a few degrees? And what about the funny shape of the top, like the thumb of a left-handed hitchhiker trying to catch a ride. First, the rotor has nothing to do with the timing of the high voltage arc. Only the camshaft, cam and the points control timing. The rotor could care less when the voltage arrives. It just sits there twiddling its thumb in circles waiting for something to happen. When it does, the camshaft makes sure the rotor is in the right position—that is, next to the correct high-tension stub inside the distributor cap. When the timing is at Top Dead Center, the cam on the cam shaft opens the points and a voltage arc crosses to the stub from the pointy of the thumb. Why from the pointy end?

You’ll remember from an earlier article in the Newsletter that the maximum degree of advance of our engines is 30 degrees before TDC. You’ll also recall from the same article why we advance the firing of the plugs ahead of TDC when accelerating the engine. Right. If the distributor is advanced say, 30 degrees, the outer casing, points and stubs—everything in it except the rotor and shaft is moved 30 degrees clockwise. Now, it just happens that the thumb on the rotor is slightly longer than a 30-degree arc around the inside of the distributor. This allows the arc to jump across further back on the thumb –closer to the big knuckle. In case you’re wondering, the few extra degrees of length in the thumb allow for retardation, too. In sum, the long thumb of the rotor allows the spark to jump regardless of the amount of advance or retardation. Hey, this is beginning to make sense!

Now that we have full comprehension of the ignition system, let’s get on with the checks.

Ignition System Checks