VALVE ADJUSTING.

Auckland Star, Volume LXV, Issue 37, 13 February 1934, Page 16

VALVE ADJUSTING.

METHODS OF INSPECTION. DAMAGE BY BURNING-. DISTORTION XOT UNCOMMON. Eacli valve is opened liv a cam and closed by a spring; should the action of the valve become erratic; "missing" and burned seatings will follow. Should this trouble be suspected, it is quite an easy matter to remove the cover located on the side of the cylinder block in a side-valve engine, or above the head in an overhead-valve engine; the valve springs will then be disclosed for examination. If they appear to be in order the engine can be started and

run at a moderate speed, whereupon it may bo found that one of the valves is not snapping shut as rapidly as arc the others. This is commonly caused by deposits of carbon or gum on the valve stem, which increase friction.

Sticky Valves. In simple cases a euro may be effected by squirting a mixture of oil and paraffin 011 to tlio stein and into tlie carburettor intake while the engine is running quite fast. If this euro be attempted with the engine idling it will probably result in oiling-up the plugs. In bad cases it will bo found necessary to remove the faulty valve. When ."sticking" of this kind occurs, the missing which results is accompanied by a loud tapping noise as the tappet hits the valve stem. Supposing that the valves are working freely and that the springs are in order, it may be that one of the adjusters has worked loose and has been adjusting itself, so that the clearance has become excessive. An important point to remember is that one cannot check the clcaranco on any valve unless that valve is flilly shut. The best plan, therefore, is to tackle each cylinder in turn after placing it 011 tlio compression stroke. To do this one cranks the engine slowly while watching the valves of, say, No. 1 cylinder. It will be eeen that, after the exhaust has opened and closed, the inlet valve follows immediately; there is then a dwell with both valves shut and the compression of the cylinder can be felt by hand. At this stage pranking should cease. and the clearances can be checked.

Damage By Gas. Not many owner-drivers nowadays will wish to go to the trouble of adjusting all the tappets, but by inspecting them in this way it is quite easy to lind out whether or not they require attention. The clearance recommended varies between five-thousandths and ten-tliou-sandths of an inch in engines of different types—about the thickness of a visiting card. However, there are certain power units, fitted with special cams, for which much greater clearances are recommended. This mod© of inspection will also indicate whether any one of the valves is running without a clearance. Should this be the case the valve in question will not be able to close properly and the escaping gases will soon cause a great deal of damage to tlio seating. Needless to say, one must be very sure that the cylinder is actually on compression before jumping to the conclusion that there is no clearance at the valve stems. So soon as the cam comes into action to lift the tappet the clearance will, naturally, disappear. One other cause of burned valves may be mentioned in passing: this being to run the engine continuously on too weak a mixture. The result of this practice is to reduce the rate of combustion to such an extent that burning continues throughout the firing stroke and the exhaust gases are very much hotter than is usual. Tlie exhaust valves are liable to become distorted and burned.

Whenever an engine is decarbonised it is highly advisable to have the valves ground in. This process consists of removing the springs, etc., screwing the tappet heads well away from the stems, and twirling each valve to and fro on its seating with a little emery paste. Although broken valves are extremely rare, it is not uncommon to find a distorted exhaust valve. If the distortion is in the stem the valve will not work freely in its guide, or if the head is out of shape it will not close properly; in either case a new valve should be fitted.

In the course of time the valve stems and their guides become worn, and then trouble may develop owing to air leaking inwards between the two parts when the engine is on the suction stroke.

The chains or other mechanism employed to drive the camshaft seldom give rise to any trouble. In the more expensive types of car the timing gear, as it is called, is adjustable for wear; in others, the chain is simply replaced when a certain degree of wear is reached.