Engine timing

It’s worth making sure your engine is timed correctly. Here, it’s explained and exampled on an engine which is all a motorcyclist really needs (according to some, anyway…) – the single cylinder four-stroke.

It has more than once occurred to me that since all proper engines contain the same basic components – flywheels, con rod, piston and valve gear – they ought to be more similar than they actually are. In fact, some are lovely and some not so, but I am certain that optimising the timing of any engine will see it at its best.

One of the most useful tools in the workshop is a timing disc. It is the principal method by which we can optimise an engine’s performance. Time and money spent on new engine internals during a rebuild might bring little benefit if the valve and ignition timing are not correct to the manufacturer’s specifications. The most accurate way to confirm this is in degrees of the flywheel.

Setting the timing disc

It is said ‘that a job well started is a job half done’. Good preparation here will be well rewarded later. Take the trouble to devise a robust mounting for the timing disc and attach it to an engine main shaft, the drive side is usually the more convenient. The mounting should allow the disc to be rotated into position and then firmly locked. Make a good pointer with a small range of adjustment. A piece of wire really is not adequate. See picture one.

I have found that there is real benefit in marking on the disc the timing events specific to the engine being worked on, it saves referring to a separate sheet and makes it impossible to time the ignition on the wrong side of TDC (Top Dead Centre) firing stroke.

Accurate setting of the timing disc to

TDC is crucial and best achieved using the ‘positive stop’ method. An arrangement to stop the piston at about 40 degrees before

TDC has to be made. With the cylinder head on, this can be an old spark plug body, threaded, with a piece of stud and a lock nut. With the cylinder head off, the cylinder studs can be utilised. See picture two.

First, set the piston at TDC (by eye or feel) and lock the disc at zero degrees

(TDC). Rotate the flywheels backwards and forwards until the piston meets the stop and adjust the pointer until an equal number of degrees is showing in either direction. This guarantees that TDC on the disc will be TDC on the engine.

In contrast, using a drinking straw or pencil down the plug hole is almost futile as a method to determine TDC. Typically, a rotation from five degrees before TDC to five degrees after will involve about

.005in vertical movement. Thus, unless one has fingers with the sensitivity of a Latin lover, a discrepancy of up to 10 degrees is virtually unnoticeable.

Valve timing

At this point, much depends upon the construction of the engine. There is little point in discovering a discrepancy if there are no means to rectify it. Many engines have cams fixed into gear pinions and the main shaft gear located by a key. If a cam is pressed into a gear of 36 teeth, each tooth will represent 10 degrees, or 20 degrees at the flywheel. Unless there is an idler gear with a ‘hunting tooth’, the minimum adjustment of one tooth will be 20 degrees of the flywheel.

A hunting tooth is where there is one extra tooth on the idler gear to ensure that the work of opening the valves is not done by the same two or three mating teeth but is distributed around the idler. As an example, in a Velocette pushrod engine, the main shaft gear is 46 teeth and the cam gear 92.

The idler gear has 93 teeth, ensuring that the load is distributed evenly around it. There is another benefit which will be touched upon shortly.

However, timing marks on old engines are frequently indistinct and often ambiguous. For instance, Matchless singles use the same cam in different engines, so have single or double centre dots and sometimes a scribed line on the same cam. Fortunately, there is a point of symmetry common to nearly all engines, in that at TDC on the exhaust stroke both valves will be equally open. Some engines have an advance of five degrees on the inlet but if the movement of one tooth is 10 or 20 degrees, then this is of no consequence.

This happy circumstance creates the opportunity to set the timing on an engine for which no data exists and also provides a simple check without recourse to a timing disc (apart from setting the piston at TDC exhaust stroke). The lift will be small, so it is easy to compare the valves. On a sidevalve engine, it can be done by eye or with a straight edge. Likewise for an OHV on the tappets or by pointers on the rockers. By moving the engine gently backwards and forwards, it is easy to see the tappets or valves rocking as the piston passes TDC. On an OHC engine it is easy to compare the two valves in the combustion chamber. Moving the cam or idler gear will quickly establish the position where both valves are equally open.

For engines where there is a ‘hunting tooth’ arrangement, it will be possible to set the valve timing with a much greater degree of accuracy. To revert to the Velocette pushrod engine : rotating the idler gear by one tooth will advance or retard the valve timing in four degree increments. The gears in this engine are particularly fine but

the same effect can be achieved with any hunting tooth. Other methods of varying the timing include a plain taper on one of the timing gears and verniers.

Checking the valve timing with a disc

With the valve opening and closing times clearly marked on the timing disc and the valve clearances set to the manufacturer’s checking clearances, a start can be made but the engine needs to be checked for backlash in the timing gears first and excessive play adjusted out if there is provision to do so. If not, care must be taken to ensure that any backlash is taken up in the approach to a valve event–it will change direction, of course as the follower goes to the opposite flank of the cam.

I have found it to be helpful to replace the valve springs with light ones, something a little stronger than a throttle spring. This allows the engine to be rotated by hand through a complete cycle of valve events without causing undue backlash or stress.

Rotate the engine forward until the clearance on the exhaust valve is just taken up, noting how close it is to the correct position, then, if fitted with light springs, continue to rotate forwards until the valve closes and the slightest clearance can be felt. Check and record the closing time in degrees. Repeat the process for the inlet opening and closing times.

The Velocette engine in the photographs has a hunting tooth arrangement that means that each revolution of the flywheel in relation to the cylinder head advances or retards the valve timing by approximately 16 degrees. There are slots in the crown wheel which allow the timing to be adjusted exactly.

If the engine is capable of adjustment, a calculation of how many degrees the engine must be advanced or retarded should be made. Make the adjustment and check again until the timing is as close as can be obtained to the marks on the disc.

Once the valve timing has been optimised, the tappets need to be reset to the manufacturer’s running clearances. Timing marks are often indistinct on old engines and is the principal reason why I always check the valve timing using a disc. When I am satisfied, I paint new timing marks. Nail varnish is very good for this, comes in many interesting colours and with its own brush. See picture three.

Ignition timing

Many medium performance engines will tolerate a variation to valve timing fairly well and with no real harm, but ignition timing really should be right, especially as it’s not difficult to get it so by using a timing disc. Ignition should always be set at full advance.

Preparation

For the purpose of illustration, I will assume that the engine is fitted with a manual magneto. In preparation, the taper on the pinion or sprocket should be lapped onto the magneto spindle to obtain an even matt grey finish on both parts and then thoroughly cleaned and degreased. The sprocket or pinion should then readily stick onto the magneto taper with very little force. Ensure that the magneto is set in the fully advanced position. If chain driven, use a wire hook and elastic band to take up the slack on the back run. If gear driven, eliminate any backlash. Ensure that the magneto points are clean and set to the correct gap. Have an extractor to hand to remove the sprocket or pinion.

The traditional method of determining when the points are open is to use a cigarette paper or .002in feeler gauge. There are now available electronic indicators where a light comes on when the points open. They offer a simpler and more accurate determination.

Method

From TDC (Top Dead Centre) firing stroke, rotate the engine backwards well beyond the ignition mark on the disc and roll slowly forwards to the mark. Fit the sprocket or pinion lightly onto the magneto spindle and start the nut. With the points closed, insert the cigarette paper or attach the crocodile clips of the device to each side of the points. Rotate the magneto points until the light comes on or the paper is released. With a tube and a small hammer, tap the sprocket or pinion onto the taper. Lightly tighten the nut.

Rotate the engine backwards and then forwards again until the light comes on – see photograph four. Check the pointer on the disc. If correct, tighten the magneto spindle nut, if not , release the sprocket/pinion and repeat the process.

Conclusion

It’s not possible, or desirable to try to cover every type of engine and circumstance – there are even engines with more than one cylinder but the same principles apply. Some manuals give comprehensive guidance, some merely advise the owner to ‘align the dots’. Tuning for Speed by Phil Irving probably presents the best insight and overview. It is clearly written and worth referring to.

An engine that is carefully assembled with particular attention paid to the valve and ignition timing correct will not only go better, it will be more fuel efficient and much nicer to ride.