HINTS ON MAINTENANCE.
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How to Get the Best Out of a Vehicle, to Secure Reliability and to Avoid Trouble.
CONTRIBUTIONS are invited for this page from fleet managers; drivers, garage foremen, and mechanics, works staff and draughtsmen,
and will be paid for on a generous scale. Every system, make, and type of commercial motor vehicle will be dealt with, and the matter should be. written with a view to the disclosure of workshop and garage practice in the maintenance of a vehicle—practices
which„ whilst they may be quite normal, are peculiar to the particular vehicle and may not be generally known to those responsible for its running. Expedients and suggestions for overcoming roadside and other troubles are covered in the following page,. dealing with letters from our driver and mechanic readers. Conummicetioes should be addressed to " The Editor; The Commercial Motor, 7-15, Rosebery Avenue, London, E.C.1."
291.—Improving Fan on Tylor Engine:
in Hint No. 62 we gave details of a method of locking the fan support on Tylor engines. Normally the fan is spring-mounted, but this method of mounting has proved. unsatisfactory. At the same time, fixing the fan support permanently allows adjustment of the Whittle belt only by removing a complete link, and this may be rather too much, in which case the belt will be too tight. Another contributor sends us a method of holding the fan, which enables the fan belt to be adjusted without removing any links. The existing fan uprights are screwed for their full length, preferably with a B.S.F. thread, and the two bushes pressed out of the spindle holder, or adjusting table, as it is sometimes called. Now, either make four special nuts, or turn down four ordinary nuts, so that a portion of each can fit into one of the holes from which the spindle-holder brushes have been removed. These nuts must be tapped to suit the uprights and the reduced portion _turned to ;.1 diameter, to suit the holder.
Now screw one nut on to each pillar so far as it will go, place the spindle holder in. position, and screw on the two remaining nuts, locking these tightly in position, when the correct tension of the belt has beers obtained, by unscrewing the lower nuts.
M.—Gap Adjustment on Plugs.
This is a point to which the majority of drivers do not attach sufficient importance, with the result that so long as an engine will run and is not sluggish in a way which calls for the lowest gears, a driver will be content with his second gear on roads where he ought to he able to carry his load on top gear. The
normal gap, it should be understood, should not exceed 1-32 in. If it does, there is not only a ten• dency to intermittent missing, hut the magneto armature is strained, so that in many cases the insulation breaks down, necessitating complete rewinding of the whole armature. Gaps increase through the burning of the plug electrodes, and in resetting a gap which has become too wide in this way, it is necessary to allow a little less than 1-32 in. in view of the countersinking effect-, caused by a hot spark eating into the electrode. .Some. drivers .enlarge the gap' to prevent failure in anengine..-which-• showsa tendency to oilup its _plugs. Vh.er overoiling is chronic; it is eer
• tainly advisable to resort to the normal and recognized methods of _curing oiling troubles rather than to endanger the magneto by -using excessive gaps in spark plugs and to persist with an engine which, is constantly' inefficient and wasteful 'of fuel' and equally wasteful of lubricating oil. It is highly inadvisable to resort to jumping the spark over excessive gaps outside the cylinder from the terminal to the plug, as is done in some cases when testing plugs, and 'care should be taken to see that plug wires do not 'come adrift and act in the same way on the cylinder block.
293.—Engine Tuning after Overhauling.
During the tuning of an engine after an overhaul, the tester encounters many peculiar little happeningswhich eguse considerable trouble, and need a lot of investigation before the causes can be traced.
We will assume that an engine timing is correct and other usual adjustments j.n order, and proceed to enumerate various small items, which, although they cause trouble, yet are apt to be overlooked, because of their insignificance:— (1) Valve tappet. clearances.—The usual standard clearance for valve tappets is .006 in. for inlet valves and .003 im for exhaust. When adjusting tappets to get these clearances, the engine should be turned slowly by an assistant at the' starting-handle ; meanwhile, the feeler gauge must be kept moving between valve stem and tappet, starting from the time the valve closes until it con mences to open again. By this means any unevenness of the backing of he cam is discernible.
A Worn or slightly bent camshaft may show the correct clearance, or even more, at one spot, yet at another it may be high, and even slightly lifting the tappet and valve, thereby putting the cylinder coneerned out of action. The symptoms of this trouble ' are uneven running, similar to misfiring, in the case of the exhaust valve, with popping back in the carburetter in, the case of the inlet valve.
(2) Bent, valve stems, or valves sticking in their .guides, produce symptoms similar to the above, (2) Air leakages -at carburetter and induction pipe joints also cause considerable trouble. Broken gaskets, or those of uneven thickness, should be unfailingly renewed, as the slightest leakage will upset the running.
(4) Choked jets.—Never hesitate to examine a jet; do not think that because you removed a jot and blew
through it a few moments beforehand that the jet is surely clear. One should never blow through a jet with the mouth ; it is best to hold it up to the light, so that any obstruction can be seen.
Forcing air through a jet by means of the mouth may cause particles of food, or even stray pieces of tobacco from pipe or cigarette, to lodge in the jet, yet anyone who has just blown through a jet would naturally think it clear.
(5) Misfiring.—Examine sparking plugs. The gaps may be wide or the plug may have become oiled by the amount of oil put in the cylinders when reassembling these. Examine high-tension wire terminals on magneto, as straggling strands of wire may cause shorting. (6) Oiling.—Run a newly overhauled engine steadily for the first few hours. Then drain off the oil from crankcase sump and fill with new oil. The oil drained off can be used again at a later date, if allowed to stand and settle, and then carefully poured off, leaving the sediment, which almost invariably contains grit, particles of metal, etc.