PHUTT, PHUTT! BANG!

NZ Truth, Issue 1144, 3 November 1927, Page 13

PHUTT, PHUTT! BANG!

Back-firing and Remedies Explained

When a noise like unto gunshots emanates from your car, don't get panicky and think another war has broken out — the engine is back-firing, that is all, and for the benefit of our motoring friends we will explain its causes and remedies.

BACK-FIRING m the silencer can only be caused by a charge of

unexploded gas being let into the | exhaust system and fired by a subsequent red-hot exhaust discharge. The undischarged gas can only enter the exhaust pipe via the exhaust valve, and it may enter m two ways. If the exhaust valve does not seat properly, it may be forced out on the I compression stroke, and may accumul late m the ports or passages until exploded by one of the discharges of redhot gas from another cylinder. In such case the remedy is, obviously to grind m 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 m, or It may be due to the tappet clearance being absent and allowing the valve to sit on its stem instead of on its seat. 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 m 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 m the subsequent upstroke, it is exhausted m its still unexploded form, into the exhaust manifold or the silencer. Another kind of back-firing may more correctly be termed popping back. It is an explosion m the induction pipe, and is often called popping m the carburettor. It Is also caused by defective valve seating or adjustment, but m 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 re-grinding and adjustment of the valve, which will cure the trouble.