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176 enough, or are dirty inside or outside, or the passages leading to them are clogged. When exhaust pressure is adopted for feeding the burners with petrol, the pressure-valve sometimes refuses to act and lets the pressure out. Remedy for this:—Grind the little valve or change the spring, and see that its lift is just one millimetre. Perhaps there is oil on the sparking plugs, or the battery is run down, or the timing is not correct, but we are here trespassing on to the province of 'Ignition' (see Chapter VIII). If the root of the difficulty is elsewhere, perhaps

B. The compression is poor (see 'Motor will not Start') or

C. The carburation is not good:—

Cause 1.—The proportions of air and gas are not well adjusted.

Cause 2.—The petrol is stale.

Cause 3.—Petrol cannot get free access to the carburator (see 'Fits and Starts').

Cause 4.—The gauze through which on some cars the air is sucked is blocked with dust, or the gauze which is sometimes fitted into the induction pipe is dirty, or that fitted between the exhaust and the pressure valve (in cases where a branch of the exhaust is utilised to maintain pressure in the petrol or lubricating tanks) is foul.

Cause 5.—A pipe-joint is loose or has a hidden crack through which an excess of air enters.

See also 'Carburation,' pp. 165 and 166.

D. Too much lubricating oil is used, causing (i) valves to stick; (ii) a deposit on the sparking plugs; and (iii) an unhealthy charge in the cylinder. Excess of oil reveals itself in the form of smoke issuing from the exhaust.

E. One cylinder does not work at all. If one cylinder misses fire regularly, it is probable that

i. The exhaust or induction valve has given up the ghost, or

ii. The nozzle for supplying that cylinder with spray is blocked.

F. If, however, the miss-firing is irregular, and none of the defects aforementioned are found, we must look to less common sources for the difficulty.