Many of the troubles that are likely to arise have already been referred to in previous chapters, but the following additional notes may be found useful.

1. Engine refuses to start.

Care must be taken to observe exactly what happens, and one cannot do better than ask oneself mentally some of the following questions.

(a) Is the ignition “on”?

If a magneto is fitted the earth connexion should be open, but if a coil and accumulator are fitted the earth connexion should be closed.

(b) Is the petrol reaching the carburettor jet?

Before removing the jet for the purpose of examining and cleaning it, it would be advisable to ascertain whether the petrol was reaching the float chamber. Provided there is a reasonable amount of petrol in the tank and the tap is turned on, there must be a stoppage either in the petrol filter, the petrol pipe, or the bottom portion of the float chamber. Examine the filter and float chamber before disconnecting any pipes.

(c) Is there a good compression in all the cylinders?

If there does not appear to be any compression in any of the cylinders, it is probable that the carburettor throttle is closed and no air or gas can enter the cylinders. If there is a good compression in some cylinders and a poor one or none at all in others then—

(1) One or more of the valves may be held off its seat by dirt, by distortion, or by some derangement of the valve gear. Examine the valve gear externally, turning the engine slowly to watch its action. Afterwards remove valve caps and inspect valves if necessary.

(2) One or more of the sparking plugs or valve caps may be short of its washer. In this case the blow will be heard as the engine is turned round by hand.

(3) A piston may be cracked or broken or a cylinder cracked.

(4) A cylinder may have got badly worn and the rings on the piston jammed so that they no longer keep it gas-tight.

(d) Is the engine very stiff to turn over?

Stiffness is due as a rule to lack of oil on the cylinder walls, caused by absence of oil in crankchamber or the film of oil on the cylinder walls having been washed off when priming the engine with petrol in attempting to start it. If a connecting rod is bent, or the crankshaft distorted or a piston ring broken, stiffness will also be noted. Very often by removing the valve caps and pouring a teaspoonful of oil or paraffin into each cylinder the engine may be freed by vigorously turning the starting handle by hand until the cylinders and pistons are well lubricated.

(e) Is there any sign of an attempt to fire the charge such as an occasional puff of smoke from the exhaust or inlet, or an occasional jerk round of the engine as you turn the starting handle, or an occasional “bang” in the exhaust box?

If the ignition is “on” and the carburettor jet clear, the compression good and the engine quite free, yet there is no sign of a “fire” from any of the cylinders, it is possible that air is leaking into the induction pipe through a faulty joint or any one of the following ignition troubles may have occurred:—

(f) Defective sparking plug or plugs. This may arise from water or oil or dirt between the plug points; or from faulty insulation in the body of the plug. To test whether the plugs are at fault an easy method is to take a screwdriver with a wooden handle and place the metal blade on the terminal of the plug, letting the point come about one thirty-second of an inch from the metal of the cylinder or any of the pipes; when the engine is turned by hand the spark will be seen to pass across this improvised gap if the magneto and leads are in order.

(g) Defective electrical connexions.

The high tension cables may be broken, or disconnected, or short-circuited. The earth wire may be short-circuited (i.e., in electrical contact with some other wire or metal fitting). There may be a short-circuit in the ignition switch.

(h) Defective magneto or coil.

The low tension contact breaker lever may be jammed so that the make and break is inoperative, or one of the carbon brushes may have got broken. Occasionally one finds the magnets of the machine have lost their power; or there is some electrical defect in the armature or condenser. The battery may have become exhausted. The trembler blade may be stuck up. Water may have found its way on to the high tension electrode or into the safety spark gap.

2. Engine starts up fairly well, runs a little, and then stops.

Take care to notice the manner in which the engine runs and stops. Note whether it runs regularly or irregularly and for how long a time.

If the engine runs regularly with all cylinders firing, then probably the exhaust is choked or the petrol supply fails. Failure of the petrol supply may be due to the use of too small a jet in the carburettor, too low a level in the float chamber, or to partial stoppage in the pipe line. Another cause of this trouble of intermittent running would sometimes be loss of battery power when using coil ignition, i.e., batteries want recharging.

If the engine runs irregularly the trouble is probably due to too much oil in the cylinders causing the plugs to misfire, the presence of water or dirt in the petrol, a defective valve, a broken carbon brush, or poor electrical contact somewhere in the magneto, the low tension contact breaker (coil), or high tension distributor (coil).

To ascertain whether the engine is firing regularly on all cylinders, or to detect which cylinder is misfiring, the best procedure is to open the compression taps in turn while the engine is running and in each case speed up the engine while you have the tap open. Cylinders which are firing well give a sharp cracking noise, those which are not firing merely give a hissing noise. If no compression taps are provided, each plug must be short-circuited to the frame in turn by the screwdriver method given above. The short-circuiting process causes a reduction in engine speed except on that plug which is already not firing. The method is not so good as the compression tap process, because the plugs often get oiled up during the short-circuiting process and the difficulty is accentuated.

3. Timing the Ignition.

My colleague, Mr. Oliver Mitchell, has pointed out to me that it is often impossible to tell directly when the piston is exactly at the top of its stroke, and he recommends a study of the accompanying Valve Setting Diagram (Figure 71). From this it will be seen that it is sufficiently near to bring the engine first of all to such a position that the exhaust valve has just closed; then make a chalk mark on the flywheel and give the engine one complete turn round; the piston will then be in the firing position if the flywheel is turned a shade backwards. Another method would be to retard the ignition fully and time it so that the spark occurred one complete revolution after the inlet valve had just commenced to open. When either valve is closed its tappet can be felt to be free, the amount of freedom depending upon the clearance between the tappet head and valve stem.