Brit Car Tune-ups.

Several people have asked me for some basic information on tuning up MG, Triumph and Jag engines. They just want the basics-you know, stuff you can do in an afternoon so, I thought I would write something up. After some reflection, it occurred to me that one article will cover most Brit cars from the 50's to the mid 70's since all of their ignition and carburetion systems were the same so, this is sort of a "one size fits all" article. The techniques mentioned here should work for any Brit car from early 50's to early 70's. As always, use this information at your own risk. This is what I know so, don't come whining to me if I made a mistake or typo.

I have included tune up specification cards for a number of different cars:

Consult the appropriate card for information for your car.


The tune-up procedure is very routine and, unless the car is well known to you, should be followed step by step. Sure, you can omit lots of stuff when you know the car but, I recommend that you do the full routine at least annually.

The steps are:

Basic Engine Analysis

There is really no point to attempting to tune up any engine that is not in reasonably good condition. Sure, new points, plugs and correctly set timing will help any engine capable of running, run better. However, unless the engine is in good basic condition, it's never going to perform well regardless of how many times you gap the plugs.

So, your first step is to do a little bit of engine analysis. Start the thing up and let it idle. Sound smooth? No ominous rattles or clangs? Idles reasonably well? So far-so good. Let the little guy warm up and then shut it off. Take the plugs out and block the throttle open. Take a compression test on all cylinders. Screw the compression gauge into number 1 and spin the engine over to allow that cylinder to pump pressure five or six times. Record the compression reading you get and repeat this process for all cylinders.

We are not concerned with the absolute compression figures you get although they should be somewhere between 120 and 160 psi. What we are looking for is any variation of more than about 10 psi between cylinders. You can get by with a greater variation but, consider a 10-20 psi variance an "early warning" sign and, if your readings vary by 20psi or more, you should consider having the engine apart to determine the cause. A tune-up on that engine probably won't do much for it.

So let's assume the engine sounds decent and the compression readings are within 10-15 pounds of one another, what's next?

Check the carburetor spindles for free play. These things wear and, when worn, allow the carb to suck outside air into the engine through the spindle holes. Badly worn spindles make it nearly impossible to adjust the carbs at idle or low speed. Wiggle the spindle ends up and down. A little play is OK but, if they move noticeably up and down, they must be replaced and/or the carburetor must be bored for oversize spindles or rebushed. I've done this myself but frankly, it's more trouble than it's worth. Get a machine shop to do it and log back on when the carbs have been rebuilt.

Snap off the distributor cap, remove the rotor and attempt to move the shaft from side to side. If you can see movement, that's not good. You will need to rebush the distributor. Although you can certainly do this at home, its probably another machine shop job.

Now, the preliminary checks are over, let's get into the tune-up.

Valve Adjustment

The old mechanics I knew used to adjust valves with the engine running. Set it on a slow idle and slide a box end wrench over the rocker arm nut, loosen it up a bit and then slide the feeler gauge in between the arm and the valve stem. The gauge should slide out smoothly when the valve is closed. Takes a little while to get the hang of it but it works.

For the more conservative, there are a number of other methods to use on an overhead valve engine. The simplest one is to rotate the engine until the #1 valve is fully open. Then rotate the engine a complete turn and set the #1 valve clearance. Rotate the engine until #2 valve is fully open and then another complete revolution. Keep doing this until you have set them all. This is a failsafe method but requires lots of spinning of the engine.

Wiring and Ignition

Pardon this rather clumsy drawing but, I think it captures the essential simplicity of the ignition systems we are dealing with. The ignition switch, coil and points make a series circuit with both the points and ignition switch acting as switches. The ignition switch is activated(closed) by a turn of the key while the points are opened by the cam on the distributor. They close by spring pressure. Assume that the ignition is "on" and the only switch in this circuit is the points. When they are closed, current flows from the battery, through the ignition coil and through the points to ground(or from the ground to the battery, depending upon polarity). This current flow generates an electromagnetic field in the coil which suddenly collapses when the points are opened. The collapse of this field induces a very high voltage, low current electrical pulse in the winding of the coil connected to the distributor rotor and ultimately, the spark plug. That's it-that's your ignition system-full stop.

Sure, there are some other nuances to your car's ignition such as the various advance systems built into the distributor but we'll cover those later. For now, this is enough. Let's go back to the ignition coil for a moment. All other things being equal, the more current that flows through the low voltage or primary side of the coil, the more energy the coil has to induce a fatter spark in the spark plug. Since your car only has a 12 volt system, current must be quite large in the primary side of the coil to allow it to induce a usable spark at the plug. Primary current is on the order of 2 amperes in most systems and since this current flows through the points it is imperative that they be clean and offer a low resistance to the current. It is also imperative that all of your primary side wiring be in good condition and the connectors be clean and solidly connected. Pay particular attention to the wire connecting the coil to the distributor. This wire flops around a good bit and is subject to the vibration of the engine. Check it to make sure the individual strands of wire haven't broken where they enter the connectors on either end of the wire. Perform the same inspection on on the wire connecting the coil to the ignition switch.

Visually inspect all of the secondary circuit wires(they are the fat ones that go to the spark plugs, Ralph). Look for bulges and cracks in these wires and replace any offenders. NOTE: Replacement of secondary wires requires some thought. They are secured into the distributor in several ways. You need to inspect the distributor cap before you start madly trying to pull the old wires out. Generally, caps that have the wires coming in to their sides secure the wires with small pointed screws which may be loosened from the inside of the distributor cap. Unscrew the screw and pull the wire out. Fit the replacement wire into the hole and screw the pointed screw all the way into the cap. The point on the screw pierces the wire outer cover thus making electrical contact with the inner copper wire and also holding the wire in the cap.

Other types of distributor caps use screw in connectors which accept the secondary wire through a hole in the connector body. The outer covering of the protruding end of the wire needs to be trimmed back about 1/4 inch, a small brass disk is then fitted over the 1/4 inch of bare copper wire and the strands of wire are splayed out over the disk. This assembly is then screwed into the cap until it seats. Again, it will make a secure mechanical and electrical contact.

The latest types of distributor caps use toothed brass spring fittings which hold on to the wire by virtue of their teeth and grip the inside of the cap hole by spring pressure. Although the teeth are designed to pierce the wire insulation and make contact with the wire core, I generally strip about 1/2 inch of insulation off the wire, fold the stranded copper conductor back over the insulation and then install the brass clip.

Unclip the distributor cap and inspect its inner surface for moisture, cracks or lines that look like pencil marks. If you see any cracks or lines on the inner surface-replace the cap. Next, pull the secondary wires out of the cap(one at a time please) or, unscrew the fitting or remove the holding screw and inspect the holes where the wires locate. The holes should be clean and free from corrosion. You can clean them up with a small round file and pencil eraser but, I recommend that you replace the cap. Finally check the little round carbon rod that sits in a hole in the middle of the distributor cap. This rod is pushed out by a small spring and should freely move up and down in its hole. Push it up and let go. It should pop back out. If it doesn't, replace the cap.

OK. You have done the easy work. Now it's time to get to it. Take out the spark plugs but, before you do, number each plug wire with its cylinder number. Masking tape works fine. Locate the timing marks on the engine. These are different between engines. Refer to your specifications card for the exact location and meaning of the marks on your car. Spin the engine by hand until number 1 cylinder is at Top Dead Center(TDC) on its compression stroke(both valves for that cylinder are closed) and the rotor in the distributor is pointing toward the terminal on the cap for #1 cylinder.

Take off the distributor cap, unhook the vacuum line and remove the low tension lead. Scribe a line on the distributor body in line to the tip of the rotor and remove the distributor. Take that little guy to your bench and remove the rotor. You will notice that, as you turn the bottom of the shaft, the cam turns and, as the lobes on the cam pass under the rubbing block on the points, the points open. Hold the bottom of the shaft firmly and twist the top of the shaft in the direction of normal rotation of the distributor. The top of the shaft should move about 15 degrees and then snap back against spring action. If it doesn't behave this way either, the top of the shaft is rusted to the bottom portion or the centrifugal weights that control spark advance are corroded and frozen. In any event, you will have to disassemble the distributor in order to fix that. This is a common problem with Lucas distributors and results from lack of periodic lubrication.

If the cam portion of the shaft is rotates as described and the weights are free, you are in luck and can skip the next section. If not, you have a fiddly little job ahead of you.

The fiddly little distributor job

Clamp the bottom portion of the shaft in a vice, pull out the little felt washer that rests in the top of the shaft and squirt some WD-40 down the hole where the felt rested. Remove the screw that you saw when you removed the felt from the top of the shaft. Remove the points, condensor and the plate the points are mounted upon. Now, pay attention to the next part of these instructions, failure to understand this bit has caused a lot of people a lot of grief. You will notice that there is a gear on the bottom portion of the distributor shaft and, on the end of the shaft are the drive dogs for the oil pump. These dogs are not aligned on the center line of the shaft which means that, although the shaft gear may be meshed with the cam gear in any one of a number of different positions, the off center oil pump dog requires the shaft to be located in only one position. So what? You might say. Well, when you finally free up the upper part of the shaft and remove it for a thorough cleaning, you will find that it will reassemble very nicely either in its original position or 180 degrees away from its original position. It's a 50-50 proposition and most people seem to loose that bet. You can wind up with timing 180 degrees out which creates lots of interesting pops and bangs from the engine and no usable output, no oil pressure or a broken distributor body resulting from trying to jam the distributor back into its hole and seat it on the engine block(it wont go all the way in the hole unless that dog meshes).

So, mark the position of the top portion of the shaft against the bottom portion. Remove the two springs that run from posts on the top portion of the shaft to posts on the bottom part of the shaft. Break the top portion of the shaft free by gradually turning it back and forth with a pair of pliers and work it upward and off the inner shaft. Remove the top shaft and weights and clean them up. Reassemble the weights, springs and top of the shaft, lubricating the weights and shaft with a little light grease. Replace the point plate and the job's done.

Replace points and condensor with new items and gap the points at .015". Smear a little grease on the cam and squirt a few drops of oil on the felt that fits in the top of the shaft. Replace the distributor in the engine so that your scribed marks on the case line up with the rotor.

Bolt down the distributor but do not tighten the bolt. Connect the low tension cable and vacuum line. Set the ignition timing by connecting a 12volt test lamp from the electrical connection(low tension cable) to the engine block(ground) and switch on the ignition. Rotate the engine backwards about 20 degrees and then slowly rotate it forwards. Stop rotating exactly when the lamp lights and note the relative position of the timing marks. If the lamp lights when the crankshaft timing mark has gone past the pointer or mark on the engine block or timing case, the ignition is restarted. Turn the distributor slightly clockwise and recheck the timing. Continue doing this until the light goes on when the timing marks are exactly aligned. Secure the distributor. You have finished your ignition work and the spark is properly timed. The next step is carburetion.


SU carburetors require three adjustments. Float levels must be set, then airflow through both carburetors must be matched and, finally, air/fuel mixture must be adjusted. Float levels should best be set with the engine cold since you will probably spill some gasoline. Remove the covers from the float bowls and locate a rod of the diameter indicated in the appropriate specification card for your car.

Turn the float covers over so that the fork bears down on the needle valve and attempt to slide the gauge rod under the float fork. Adjust the tab on the fork as necessary in order for the gauge rod to just slide under the fork when it is resting on the needle valve.

The next step is to clean the carburetor suction chambers and pistons. Remove the suction chambers and slide the piston out of them. Unscrew the damper valve and remove it. Thoroughly clean the interior of the suction chamber and the periphery of the piston.

Remove the needle and roll it on a flat surface to determine whether it is bent or not. Any bent needle must be replaced. While you are at it, check the needle model against the specifications for your car.

Replace the needle in the piston making sure the little step on the needle's shank is in line with the face of the piston. Reassemble the piston, spring and suction chamber assembly to the carburetor base and check for main jet alignment. Many earlier SU carburetors have a main jet that may be moved axially in relation to the needle. The hole in the jet and the needle must on center and the jet may be moved in relation to the needle to accomplish this. Screw the main jet adjusting nut all the way in(raise the jet as high as possible) and then lift the piston up and let it fall of its own weight. The piston should fall quickly and hit the jet mounting with a sharp "click". If it hangs up, your mainjet is not centered.

To center the jet, loosen the jet locating nut and allow the piston to fall all the way down. Tighten the locating nut and lift and allow the piston to drop again. Repeat this process until the piston strikes the jet mounting with the aforementioned "click".

OK, let's tune these beasts. Fit some new spark plugs, refit the valve cover, top up the dashpots with 30 weight and reinstall the secondary ignition wires. Remove the air cleaners. If you centered the jet as described above, back the adjusting nut off by 12 flats of the nut. Start the engine and let it warm up to operating temperature. Switch off and loosen the interconnecting couplings between the carburetors.

Locate a length of rubber hose of about 1/4" diameter. Stick one end in the carburetor mouth and the other end in your ear. Listen to the hissing sound as the engine runs and increase or decrease the idle speed on one carb until the hissing sound is the same in all carburetors. Switch off the engine and tighten the shaft clamp bolts. Start the engine and set all idle adjusting screws the same amount to give you an idle of about 6-700 rpm. Then lift the piston on the front carburetor by just a little(1/16-1/8") and listen to the engine. If the engine runs faster and continues to run faster-that carb is too rich. Turn the adjusting nut up one flat and try again. If the engine speed slows when you lift the piston-the mixture is too lean. Richen the mixture by turning down the adjusting nut by one flat. You want the engine to momentarily speed up and then return to it's idle rpm-that shows your mixture is correct.

Since most Brit cars have the carbs feeding a single manifold rather than directly to the cylinders, mixture adjustments on one carb will affect the mixture on the other(s). I generally leave the front carb slightly rich and then work on the rear one. When I get that one leaned out correctly, the front one is quite close to optimum. In any event, if your carbs are in good condition, you should wind up with both adjusting nuts the same number of flats away from full up. Replace the air cleaners and recheck your mixture using the lifting pins on the carb bodies. Many cars I have had went rich by about 1/2 or 1 flat when the air cleaners were installed. Some stock cleaners are pretty restrictive, I guess. Set the idle at about 6-800 rpm and you are done.