BACKFIRING.

Press, Volume LXVI, Issue 19889, 28 March 1930, Page 9

BACKFIRING.

A DISTINCTIVE DEFECT. WHEN CARBURETTOR IS AT FAULT. "Popping back" is a distinctive defect. 'And, in cases of misfiring, it informs the motorist that the seat of the trouble is in the carburettor and not in the ignition system. When such a symptom manifests itself, do not imagine at once that something serious has gone wrong with the "gas main." The fault may be only in a drop of water in the carburettor. If so, it can generally be cleared by racing the engine for part of a second or so. Close the throttle and then suddenly open up fully, and as quickly close again. This should be sufficient to clear the fault, and; if not overdone, no harm will be sustained by the engine. Avoid excessivo racing, however. Similar effects will follow a partial choking of the main jet by a particle of dirt or other foreign body tliat has through from the fuel tank. If a like course of treatment does not have the desired result, the carburettor must be dismantled and cleaned carefully. Do not try to probe a choked jet in a hurried roadside job; the better way is to have a set of spare jets ready for use. The, other can be dealt within the home garage at leisure, when it should be generally cleaned up and placed in the case of spares for eventual uso when further need arises. A Leaky Intake. "Popping back," if the cause still lies in the carburettor system, m-ay also be traced to a leakage in the intake manifold. Extraneous air weakens the mixture; a weakened mixture is comparatively slow-burning, with the result that there are still burning gases when the inlet valves are opened,, so causing ignition of the gases in the inlet system. A dangerous fire may be the outcome, as is obvious. Go carefullv over all the manifold joints then, ahd make sure they are quite tight. If a joint has been loose for any

length of time, first see that the faces are perfectly clean and free, .from grit or other particles before -attempting to remake the joint. s One need'not refer at length to inevitable effects of grossly-disturbed timing, 1 whether in respect of valves • or ignition. These will become apparent long before giving rise to backfiring of a serious nature. If the carburettor must be. taken down in any way, go abont.the job with extreme care. A carburettor -is \ really a delicate motoring instrument. Rough or • ignorant handling may seriously interfere with the subsequent action of the device. ; Dismantling tho Carburettor. The float chamber is. the most read--ily get-at-able. The cover may be fastened by a few studs, or it may be held in place merely by a spring clip. Before touching this, however, see that the petrol supply to the carburettor ..is turned. off. When the cover is removed, watch out carefully for the needle valve and toggle gear. These, though small items, are. of the greatest importance.. .Lift them all from place gently, and' Jay. aside out of harm's way. Then remove the.float—which can best be. lifted: from its constrained quarters, perhaps, by means of a couple of match sticks.. Any careless handling of the " float, again) is fatal.* This' is made .of' the thinnest of brass, and is almost as vulnerable as a soap-bubble. The float chamber caii now be wiped out with a clean, soft rag, removing any residue of petrolj water, and ■ dirt that •may have accumulated. Next look to the needle valve—--particularly' its conical business end. If this is worn or ridged, refrain from any hasty impulse to-"touch it up" With emery cloth! Unless the carburettor has been flooding'as-a result; of leakage at the needle valve, "the valve'.is; best.-'left alone. If it must be ground: in, how- ■ ever, use fine crocus powder only as an abrasive, lubricated with paraffin oil. The job is advisably one for skilled hands, .but ean be tackled successfullyby anybody exercising sufficient caro and understanding.' It is vitally essential that the valve needle shall stand quito vertically in its seating while the grinding operation is in progress. The little toggle levers by means of i which the .float controls the needle valve must function smoothly and without on-, due friction. To that end see that they are not damaged in any respect, and

that they are notexcewively. worn ott their plasj tfdr itt the «pllar controlling the needle vaJye.:All-being in order, the parts can ho Replaced; and, if the original trouble was centred- in the float chamber only, it should now have been remedied and . smooth running restored.