BACK-FIRING.

Press, Volume LXIII, Issue 19153, 9 November 1927, Page 6

BACK-FIRING.

CAUSES AND VARIETIES. TROUBLE LOOKED INTO. The term which is selected for the heading of this article is variously interpreted by motorists and the motoring writers. But disregarding il3 explicit meaning, we may with advantage deal with the effects to which it is applied and indicate the causes of the troubles and the remedies which may success fully be applied, says the "Auto," England. Back-firing in the silencer can only be caused by a charge of explosive, but unexploded, gas being let into the exhaust system, and tired by a subsequent red-hot exhaust discharge. The undischarged explosive gas can only enter the exhaust pipe via the exhaust valve, and it may enter in two ways. If the exhaust valve does not; seat properly.it may be forced out on the compression stroke, and may accumulate in the ports or passages until exploded by one of the discharges of red-hot gas from another cylinder- In such case the remedy is, obviously, to grind in the valve, or, if the valve spring is weak, to fit a new spring. Generally, the trouble is due to a valve which does not sit down tight. This may be due to pitting, and the need of grinding-in, or it may be due to the tappet clearance being absent and allowing the valve to sit in its stem instead of on its seat. A grinding-in of the valve or an adjustment of the tappet or push-rod clearance is the remedy. Another cause of unignited gas passing the exhaust valve is faulty ignition. The gas charge is sucked iu on the suction stroke of the piston and compressed. But at the point when ignition should take place no spark, or an ineffective spark, is made, and the gas remains inert. On the down stroke, which should be the power stroke, this charge is expanded, and in the subsequent upstroke it is exhausted, in its still unexploded form, into the exhaust manifold or the silencer.

' 'Popping Back." Another kind of back-firing may more correctly be termed popping back. It is an explosion in the induction pipe, and ia often called popping in the carburetter. It is also caused by defective valve seating or adjustment, but in this case it is the inlet valve which is the culprit. It fails to seat, and on the explosion stroke a hot flame is blown out through the inlet valve into the inlet pipe or manifold, exploding the gas which is always present there. In this case, too, the remedy is obviously the regrinding and adjustment of the valve, which •will cure the trouble. Back-fires of this description may be dangerous, and should be looked into immediately they occur, since there is a chance of the petrol in the carburetter becoming ignited, resulting in a general flare-up, only put out by an ever-ready fire extinguisher, which should be carried in a readily get-at-able position on the car.

Closely akin to the matter of backfiring is that of air leaks, which upset the operation of the engine. Air leaks .in the mixture pipe and manifolds and passages to the valve ports upset the mixture and cause mis-firing and unbalance of the .engine. Leaks in the exhaust cause noise, and the same may be said about leaks in the jointing of the cylinder head, but in this case the blowout affects the power of the engine in two ways. It releases some of the expanding gases which drive down the piston, but it also allows air to enter on the suction stroke, upsetting the mixture balance of the engine, and causing erratic running, with misfiring on one or more cylinders. Induction System. Air leaks in the induction system make for erratic running and much loss of power, and symptoms are often wrongly attributed to carburetter misadjustment. Generally, air leaks hero are accompanied by a slight hissing noise, which may be detected if the ear J

is put near to the induction joints. The joints of the induction manifold should be re-made with well varnished washers, using gold size. The thinner the washer the better the joint. But the faces must be quite fiat, and not have' been distorted by trying to open the jointwith a cold chisel or screwdriver or other pointed or chisel-shaped tool. Blow, outs in the induction manifold will necessitate new copper and asbestos washers in the case* where there are four circular ports or a flat asbestos and copper washer, manufactured to shape and supplied by the car makers, in cases where the ports are of rectangular shape and one washer is combined for all four (or three) outlet ports. When making a joint between the carburetter outlet flange and the flange on the uptake pipe (in the case of vertical carburetters), or the face on the engine block in the case of horizontal carburetters, the joint is best made with a simple soft copper washer, as it is advisable to allow the heat flow from fhe pipe or the cylinders to pass to the carburetter outlet. Copper washers are best whgn they are soft, so that they can bed down in close contact with the flanges, to exclude the possibility of air inflow which would upset the mixture. If the washers are hard they can be softened' by heating to a red heat by laying them on the top of a flat piece of red-hot iron and then suddenly quenching in cold water after they have themselves leached the red-hot state. Blow-outs in the exhaust piping should be immediately attended to, especially in the case of closed cars, or danger from partial suffocation may arise. They are easily detected by the extra sharp exhaust noises, almost like cracking explosions. The exhaust pipe should be taken down, and new copper 3nd asbestos washers fitted, and the bolts in the flanges well tightened up again with lock nuts. Spring washers are not much use under the nuts of exhaust flanges, The heat soon robs them of their spring and the joint becomes loose.

PARTS OF CAR* SIMILARITY OF NAMES. When discussing the mechanical features of the car with a repairer, or when ordering spare parts, it is highlyimportant to be specific. There are many parts of the automobile bearing similar or identical names. A car-owner might speak of having burned out or damaged the "resistance." But that docs not help the repairer nor the parts department. There iB a resistance unit in the primary circuit of the ignition system, and another resistance unit for dimming the headlights. The word "bearings" is too loosely used. Car-owners should say what they mean. Say "wheel bearing," "connecting rod bearing," or "main engine bearing," depending upon circumstances. It saves mistakes. The small iever on the steering post with which one can feed gas to the engine is not the "throttle." It is one of two "throttle controls." The accelerator is the other. That big member supporting the rear of the car is not the rear axle. It is the "axle housing." In each such housing there arc two "live axles." The panel upon which are mounted the various gauges and meters is not the "dash." It is the "instrument board." The dash is the wooden partition separating engine compartment from the driving compartment.

The practice of adjusting the brakes while the rear of the car is supported on jack 3 should be avoided, as the car is not in a general running position. This condition is especially bad should one side of the car be jacked up higher than the other. The best method is to adjust the brakes 011 the road, with the car running under the most severe operating conditions which may be encountered.