There is something you can do to keep exhaust emission levels down to a minimum and that is to make sure your engine is in tune and stays that way. A regular tune-up, along with new spark plugs and possibly high tension wires, will not only save money on fuel bills, it could also save you some pretty expensive repair bills.
You may of heard the phrase ‘Tuning by Ear’ which basically means adjusting the ignition and carburetor until the engine sounds right. It’s a phrase you can forget, for tuning by ear is about as accurate as setting a quartz watch with a sundial – it just ain’t right! To do it correctly, you’re going to need some instruments.
If your car is equipped with breaker points (more of which later), you’re going to need a meter at least capable of reading a dwell angle and engine speed. It would be best to get one that read volts, amps and ohms as well in fact, a multi-meter.
You are also going to need a stroboscopic timing light, commonly called a strobe, of which there are two basic types – those with a neon tube and those with a xenon (pronounced zenon) tube. The neon versions are cheaper but produce a much poorer light, which may be all right in shaded conditions but difficult to set in normal daylight. Many neon versions also have a longer flash duration, which instead of pin-pointing the timing marks, will make them appear as a broad band.
It would be most unwise to attempt adjusting the carburetor mixture control on any modern car without using an exhaust gas analyzer, although Gunson’s Colortune is equally good for obtaining the correct air/fuel mixture ratio. When setting the carburetor you would also need a tachometer (rev-counter – often included with a multi-meter) unless the car is already equipped with one among its instruments.
Finally, the mechanical condition of the engine can affect what is issuing from the exhaust and, although to some extent this can be checked with a tachometer in a cylinder balance test, it would be wise to carry out a compression test before attempting to tune an engine. Ideally, the valve clearances should also be checked. Ignition Tuning
As the setting of the contact breakers can affect the timing, it’s essential that this is checked and, if necessary, re-set before any attempt is made to time the ignition.
The contact breakers (points) are nothing more than a simple mechanically operated switch and, as such, are the Achilles Heel of conventional ignition systems, being subject to wear and electrical erosion, which in time affects both the quality of the spark and its timing. At high speeds, they also have a tendency to bounce and cause the engine to misfire.
Ideally, they should be renewed every 6,000 miles, even though they may last over 12,000, and cleaning them with a fine file or emery cloth generally does more harm than good. An even better thing to do than renewing them every 6,000 miles is to throw them away and fit a contactless electronic ignition system. Even an electronic system which retained the contacts is better than leaving things as they are. The points would then last at least twice as long.
There are two basic methods of checking/setting the contact breaker – static, using feeler gauges, or dynamically using a dwell meter. Of these, the dynamic method is better,as explained later, except that when renewing a set of points, it can sometimes be advantageous to set them with feeler gauges intially and then check them dynamically.
The exhaust gases from a well-tuned and mechanically sound engine are made up primarily of relatively harmless nitrogen, water vapor and carbon dioxide. The actual pollutants, comprising mainly hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), amount to only around 2 per cent of the total.
These pollutant levels are related primarily to the air fuel mixture strength, especially CO emissions which are very low with weak mixture levels but rise steadily as the petrol content increases. HC and NOx levels are also dependent upon the ignition timing, with the hydrocarbon content showing an increase with both advanced and retarded ignition, whereas the NOx levels rise with ignition advance.
All pollutants would show a marked increase if the quality of the spark or any other factor caused a misfire. This would be most likely to occur at high speeds and may go unnoticed by the driver.
To check the points gap, using the static method, first remove the distributor cap and turn over the engine until the contacts are as far apart as possible, which will be when the fiber heel of the moving contact is on the lobe of the cam.
Now insert a feeler gauge of the specified size between the two contacts, taking care to insert it straight and ensuring it doesn’t move the spring-loaded contact. If the setting is correct, the feeler strip should just slide neatly between the contacts.
If incorrect, slacken the screw securing the contact set anchor plate slightly and move the anchor (with the fixed contact) to achieve the specified setting, then tighten the securing screw and re-check.
This brings up one of the disadvantages of using this method in that the check is carried out only on one of the cams and they may not all be the same. Although it would be possible to repeat the operation on all of the cams and average out any differences, it would be something of a hit and miss effort and would not eliminate the other major disadvantage, in that any play in the distributor shaft bearings could make even the most carefully measured setting incorrect.
Checking the points setting dynamically involves measuring the period when the contact points are closed in relation to distributor shaft rotation. This is referred to as the dwell period and may by quoted as either an angle or a percentage (see box).
Checking the dwell period is a simple operation and involves connecting up the dwell meter according to the instructions supplied with the instrument. This normally amounts to a couple of connections to the car’s battery and one to the distributor or coil, after which start the engine and check the meter reading. A reading other than specified relates to the points gap as follows: Large gap – small dwell Small gap – large dwell
Unfortunately, although most of the time an engine is running, it is doing so at speeds of anywhere between 2,000 and 4,000 rpm, most car manufacturers still only specify the timing setting at idle speeds and getting it right at that speed is no guarantee that it is still right at, say 3,000 rpm, when the total (initial and mechanical) advance could be around 30 degrees.
However, that’s the specification given and, without knowing the advance curve for any particular engine, it would be difficult to check at any other speed. In any case, it’s very seldom that any other markings are provided on the flywheel or crank pulley, although there are ways to overcome this as explained elsewhere, or by using a strobe with a delayed flash such as Gunsons Supastrobe.
If your car is fitted with an electronic ignition system, which eliminates the contact breakers, then unless it has been physically disturbed, there is no way the ignition timing could have altered, so there should be no need to check it. Indeed, many new and fairly new cars are fitted with computer controlled engine management systems, where the ignition timing cannot be altered without loading a different program (timing map) into the computer.
Although most car manufacturers quote a dwell angle, some specify a dwell percentage. This in fact makes more sense, as a percentage reading is independent of the number of cylinders. For example a typical 60 per cent dwell would represent 54 degrees in a four cylinder engine, 108 degrees in a two cylinder engine and 27 degrees in an eight cylinder engine (with eight lobed cam in distributor). We say that with electronic systems there is no need to check the timing, but this assumes that it was timed correctly in the first place and that there is no other problem, such as an incorrectly fitted cam belt, or one that has jumped a tooth, which would upset both the valve and ignition timing. Oddly enough a recent survey found around 17 per cent of cars fitted with electronic ignition were timed incorrectly.
Basically there are two methods of checking the timing, one with the engine stopped and the other with it running. As with the contact breakers, these are termed static and dynamic. With some older cars, only the static timing will be given, whereas with most modern models only the dynamic will be given at a set engine speed. Obviously, the dynamic method is preferable as the static method doesn’t take into account any play in the camshaft or distributor drives, which could easily result in an error of 3-5 degrees.
timing marks are, what they represent and what the manufacturer’s recommended settings are.
On most cars, these timing marks will be on the crankshaft pulley with a corresponding mark or pointer mounted on the engine block, adjacent to the pulley. Alternatively, it could be just a Vee notch in the pulley and a series of marks on a plate fixed to the block.
On some cars, however, the marks are on the flywheel and can be seen through a window in the flywheel housing. With some of these, such as the Mini, the use of a mirror will help provide a better view of the marks.
If the timing is found to be incorrect, it can be corrected by releasing the distributor clamping screw a little and turning the distributor body. Turning it in the direction of rotation of the rotor arm will retard the spark whereas turning it against the rotor will advance it.
Some older distributors may have a knurled wheel type adjustment provided.
Connect a test lamp (or voltmeter) between the distributor input terminal (coil to distributor lead) and earth. If this is difficult at the distributor input terminal, make the connection at the distributor side of the coil. Then, with the ignition switched on and the distributor cap removed, turn the engine in its normal direction of rotation until just before the timing marks align and the rotor arm is approaching number one segment (if the cap were fitted).
The test lamp will light up exactly when the points open, which is when the spark occurs. So if the engine is turned over a little further very slowly, the lamp should come on as the marks align.
If the timing is incorrect, the easiest way to remedy the situation is to align the timing marks, then slacken the distributor clamp screw and rotate the distributor body a little in the direction of rotation of the rotor, then bring it back slowly until the test lamp comes on. The setting will now be correct, or at least as correct as it can be using this method.
Regardless of which type of strobe is used, simply connect it up according to the instructions supplied, start the engine, run it at the specified speed and point the strobe at the timing marks, with its trigger (xenon versions) pressed.
If the timing is correct, the stroboscopic effect should make the appropriate marks align and appear static.
In the majority of cases, this check should be carried out with the vacuum advance pipe disconnected at the distributor end and plugged.
Additional Ignition Timing Marks
If you wish to include additional timing marks on the crank pulley so as to check the mechanical advance, or indeed to re-time the ignition, when using unleaded fuel, first accurately measure the diameter of the pulley, then cut out a piece of cardboard to the same diameter.
Now work out its circumference using the formula 3.142 x D, where ‘D’ is the diameter. Divide the answer by 360 and multiply the result by the number of degrees you wish to mark out.
The final answer will be the measurement around the circumference of the card from the datum point (TDC) to your ignition advance point. So make a mark for the TDC point, then measure the required amount around the card and make a further mark. Finally, offer up the card to the pulley, align the TDC marks and transfer your advance mark from the card to the pulley.
Don’t forget that any mechanical advance quoted will be in addition to the initial advance.
An alternative, and possibly easier, method on some is to use the existing markings as a yardstick. For instance if the pulley is marked with TDC and a 10 degree BTDC point, you can carefully measure this and extend it to 20, 30 or even 40 degree marks and, in each case, you could split each into two, giving 5 degree markers.
The best advice we can give about making any carburetor adjustments is not to. All too often the carburetor is blamed for poor engine performance when, in fact, the fault lies elsewhere.
Before attacking the carburetor, first look for any other possible cause of the trouble. Check out the ignition, look for any leaks and/or obstructions in the intake and exhaust systems, inspect the air cleaner and make sure all the carburetor controls (accelerator and choke linkages) are intact and free with no excessive backlash or play. It may also be necessary to check the valve clearances and carry out a compression test.
If all this proves satisfactory, only then suspect the carburetor.
Ideally, you should have an exhaust gas analyzer or Gunson’s Colortune for correct setting up of the carburetor, especially if your car was made later than 1976. This was when emission control regulations required that all manufacturers ‘fine-tuned’ the carburetors on the assembly line and then fit anti-tamper seals over some of the adjusting screws, primarily that controlling the mixture strength.
Most carburetors have two basic adjustments – one regulates the mixture strength, as we’ve already said, and the other the slow running, or idling speed. On fixed jet carburetors, the mixture screw will only affect the mixture ratio at idling speed and just above. From around 1,300rpm on, the air/fuel ratio is a function of the main fuel circuit in the carburetor.
This is not the case with most variable jet carburetors, of which the SU and Stromberg are the most common. With these, altering the mixture strength at idle will affect it at all engine speeds – this doesn’t mean that it will be correct at all speeds, for the needle profile is also a factor to be considered. For example a worn needle (or jet) will result in a rich mixture, regardless of the initial setting.
Prior to attempting any carburetor adjustment, it is vital that the engine is at its normal working temperature. Once it is, avoid leaving it idling over for long periods as it can then become even hotter and any adjustment of the mixture control would probably result in a weak mixture at the engine’s normal running temperature.
In the space allowed for this feature we couldn’t possibly cover all the various carburetors, so we’ve just given a general description of a typical procedure to follow when adjusting any carburetor. Don’t forget you may have to remove any tamper-proof plug on emission carburetors and on some you may need a special tool. Incidentally, in some European countries, we understand it is a legal requirement that the tamperproof seals are in place.
The mixture control screws on most modern fixed jet carburetors are screwed anti-clockwise to enrichen the mixture. This is because the majority are, in fact, a volume control screw which, when unscrewed, increases the fuel flow through the idle circuit.
Certain older fixed jet carbs may have an airscrew adjustment and turning these anti-clockwise will increase the airflow, so weakening the mixture. In general, this type of adjusting screw will be fairly high up on the carburetor.
As far as the variable jet or constant depression carburetors are concerned, on the SU HS-type and pre-emission Stromberg carburetors, the mixture controls (underneath) are turned anti-clockwise to enrichen the mixture. Later models are adjusted using a special tool through the top of the dashpot, which is turned clockwise to enrichen. On SU HIF models, the mixture screw is towards the base of the carburetor and is turned clockwise to enrichen the mixture.
There may be a specific procedure laid down by some manufacturers for carrying out any carburetor adjustment, but a typical method would be as follows: 1. Run the engine until it reaches its normal operating temperature. 2. Check that it is running at the correct idling speed. If not, correct it by adjusting the idling control. 3. Turn the mixture control screw until the fastest idling speed can be obtained with consistent smooth running, then back the screw off slightly to give a small reduction in engine speed. 4. If possible, check the exhaust gas CO content (or mixture strength with Colortune), making any slight adjustments necessary. 5. Reset the idling speed.
With some carburetors, it is recommended that if the correct setting is not achieved within 30 seconds or so, the engine should be run at a higher speed for a short time to clear any possible excess fuel build up.
Combustion knock, detonation or pinking are all terms used to describe the spontaneous combustion of part of the fuel/air mixture in the combustion chamber, which results in the engine giving off a metallic tinkling sound, usually more pronounced when accelerating or climbing a hill.
What happens is that, when a spark is produced at the plug in a well designed and tuned engine, a flame front spreads out from the plug and moves progressively to the far corners of the combustion chamber.
However, as this front moves out, a pressure wave builds up in front of it, which further compresses the unburned gases in the farthest reaches of the combustion chamber. At the same time this ‘end gas’, as it is called, is being heated even more, again by the flame front.
In our well-tuned engine, this pressure and temperature rise will not reach a critical level and the flame front moves smoothly through the whole combustion chamber.
In other cases, the end gas will self-ignite. It will, in fact, detonate or explode, creating a shock wave within the combustion chamber and producing that tinkling sound.
If the spark is produced early, the speed of the flame front is reduced and the end gas absorbs more heat than it would otherwise do, resulting in detonation. This in turn creates even more heat in the confined area of the combustion chamber, which, in severe cases, could burn a hole in the piston.
This condition should not be confused with pre-ignition, although the resulting noise and possible damage could be the same. Pre-ignition is when some overheated part of the combustion chamber, or the spark plug itself, glows incandescent red and fires the mixture before the spark occurs, or indeed fires the end gas afterwards, obviously in this latter case it wouldn’t be caused by an overheated spark plug.
Both conditions are self generating, in that both cause overheating, making matters even worse.
In fact, detonation can lead to pre-ignition.
Pre-ignition can also result in the engine running on after being switched off.