Tuning A.J.S. Motor Cycles for Competition Work.
By T. G. ROOK.
ALTHOUGH the majority of riders of these machines are quite satisfied with their excellent performance as turned out, readers of “Motor Sport” belong to the class of motorists who are never content with any performance however good, while there remains a possibility of improving it by their own efforts ; and it is for such as these that the following tips, the result of some years ownership of these machines, are intended.
First, I will deal with the side-valve and G.6. O.H.V. models as these are similar as to the crankcase, bearings, timing gear, etc., and it is only by careful attention to these parts, before touching the upper part of the engine, that the best results can be obtained.
The crankshaft is carried in bronze bushes in the crankcase and the first task is to split the crankcase and by the use of a bearing scraper, make these bushes a perfectly free fit on the journals. No play should be detected if this is done properly, but all traces of tightness must be eliminated.
The same remarks also apply to the cast iron bushes in the timing gear. However, it is unlikely that much work will be required here ; the important point to watch is that there is no tightness due to lack of endplay when the timing cover is screwed down tight. A paper washer is fitted between timing cover and case and should not be omitted. All timing case and main bearings are lubricated by oilways cut along the top of the bush. In the case of the main bearings these only extend a little way along the bearing from the inside of the crankcase and I have found it advisable to con
tinue this oilway to within a It” of the outer end of the bush. This modification is specially desirable in the case of the older type crankshafts on which the mainshafts are plain: on the latest models the mainshafts have spiral oilways cut on them and therefore the lubrication should be sufficient for the hardest work.
The big end consists of a single row of rollers on these models and will be found quite O.K. without any further attention. It should be possible to rock the small end of the connecting rod sideways quite appreciably, but when a definite up and down play can be felt, it should be renewed. The flywheel alignment should be very carefully
checked, if for any reason the flywheel assembly has been dismantled ; this should not be done unless ab solutely necessary.
Many amateur tuners think that there is some magic in valve timing by which experts can achieve marvellous speeds, and it is therefore advisable to point out that this is not so, and that no good will be done by altering the valve timing.
The actual timing of the opening and closing points makes very little difference to the performance of an engine, compared with the shape of the opening and closing curves of the cam itself, which are settled on by the makers as the result of their extensive racing experience.
The sports side-valve and O.H.V. models have different cams, each being most suitable for the particular engine, and should be kept to, also the standard timing which is clearly marked.
The piston should be run in carefully at first, and any high spots removed. This process being repeated two or three times. Each period of running in should be faster than the last, the piston being examined each time for high spots, and being taken down with a smooth file where necessary.
The 1922 side-valve machine was fitted with a light cast iron piston, and it will be found an improvement to replace this with the latest type alloy piston, now fitted as standard. I have not found any rebalancing necessary when this is done. The side-valve piston is made in more than one height of dome and the highest type should of course be obtained when purchasing an alloy piston to replace the cast-iron one. All the sports models have very narrow rings which, although best for speed, wear fairly quickly and should be replaced as soon as they show signs of getting “tired,” new rings being carefully lapped in with rouge.
The G.6. O.H.V. model is supplied with a low compression piston giving a ratio of 5.5 to 1. For speed work the high compression piston giving a 7.5 to I compression ratio should be obtained. This is quite satisfactory for ordinarry use with a 50-50 Benzol mixture, and this may be used for short speed events. However, for best results with this ratio, and especially for long distance work, Discol grade P.M.S.II should be used.
When tuning the carburettor for Discol a jet some 40 per cent. to 6o per cent. larger than that used for petrol will be required. As the price of P.M.S.II is only Is. Iod. per gallon it cannot be objected to on this score, though the consumption is noticeably greater than with Petrol, It has the great adv antage of cool running, this making for reliability in long distance racing, though it is by no means essential as the speeds of this years T.T. races shewed. However, it is probable that some of the speeds would have been higher had fuels not been restricted.
The cylinder head and ports should of course be polished, and any irregularity in the ports cut away.
However, the standard finish of A.J.S. heads is such that not very much work will be required. With the latest type O.H.V. head nothing need be altered and the valve guides should not be cut down to the level of the port, unless they are to be renewed very frequently: otherwise they will wear excessively causing damaged seatings in the head and possibly a broken valve, and as the extra speed gained is very small, if any, it is not worth the risk.
With the old 1924 type head with the thinner valve stems, breakages were not unknown when racing, and in the case of anyone trying to go really fast with one of these it is worth while having a new exhaust valve made with the stem the same diameter as the latest type, i.e. a”, by some firm who specialise in this work.
The latest type valves cannot be used for this purpose, as they have smaller diameter heads. With regard to fitting special valve springs of increased strength to obtain higher revolutions, this should be done with discretion. For all ordinary work, and for all long races, the standard springs are quite sufficient, the only time that stronger ones could be used is for sprint events, where engine speeds of 5,500 to over 6,000 r.p.m. are attained. As only very well tuned engines in experienced bands peak at anything much above 5,000 r.p.m. a definite loss of power is the only result of over revving. A gear should be selected that allows the engine to run at the peak revolutions in short events, and in long distance work it should be geared as high as possible consistent with maintaining sufficient speed, and this must be estimated as experience dictates. Many races have been lost solely through over or underestimating this speed ; in the former case by breaking up through going faster than necessary and in the latter case by being too slow to do better, than ” a place.”
A power curve of the engine in racing tune is invaluable for working out these points, but unfortunately the amateur tuner rarely has facilities for obtaining one, and must learn by “hit and miss” methods, what is the best gear for any given conditions.
The G. 6 model is supplied with a 21 tooth engine sprocket which is most suitable for ordinary work and short flat speed events, but for long distance work a 22 tooth sprocket or even higher can be used, and of course, for hill climbs a lower gear must be used, depending on wind, gradient, length of flying start, if any, etc., and again experience is the only sure guide. The side-valve models are supplied solo, with a 20 tooth engine sprocket and a definite increase in speed
can be obtained by fitting a 19 tooth sprocket giving top gear of 6.1. An aluminium exhaust valve cap should be fitted as this will aid cooling, a point where the side-valve by the nature of the design is necessarily weak compared with the O.H.V.
The standard exhaust valve is liable to distortion, due to heat, this resulting in a loss of power and the need of constant grinding in. Also it is rather heavy in the head for high revolutions. The best course is to get a pair of light tulip pattern racing valves made, but in case this is considered too
expensive it should be noted that a 1923 O.H.V. exhaust valve of the tulip pattern can be fitted to the sidevalve without any alteration to springs, etc., as it has a
similar cotter fixing. This valve being of special steel will only require very occasional grinding in, and the engine will not lose tune. The standard inlet valve is quite satisfactory apart from its weight, but the exhaust should certainly be changed. Standard valve springs are quite strong enough. I have obtained engine speeds of considerably over 6,000 r.p.m. without a trace of misfiring, using
standard springs on light valves which is much easier on the timing gear than using heavy valves and extra strong springs.
When driving the side valve really hard give lots of Castrol R. A pump every 3 or 4 miles will not be too much as it will help cool the engine.
For ordinary work H.S. 3 K.L.G. plugs are 0.K but for racing I find a K.L.G. 246 best for the sidevalve and the 180 for the O.H.V. Those who prefer other makes can always obtain the necessary information from their plug makers.
A sound ignition timing for speed is full advance, piston 13 m.m. before T.D.C., and with the O.H.V. 16 m.m. This of course must be varied a little to suit different compression ratios and fuels.
A. J. S. machines are now all fitted with 2 jet Binks carburettors which are very easy to tune. For short events and fast road work increase the main jet till the engine takes full air, when all out. Then increase the pilot jet till the throttle can be opened right up from corners, etc., without choking.
For long distance racing on P.M.S. II, increase the main jet beyond the above setting as much as the machine will take without actually slowing. This will keep it cool. The following settings for petrol will do to start, but of course require modification to suit different conditions.
Side-valve small carburettor. Pilot jet 2, main jet 8. Sports model Binks No. 305. Jets 3 and Do.
O.H.V. pilot 3, main 8-1o.
O.H.V. using Discol. Jets 4 and 16.
The above remarks should also apply to the G. 8 500 c.c. O.H.V. model except that racing cams can be obtained from makers instead of the touring ones fitted.
The G. R. 7. and 0.I0 models which are the racing 350 c.c. and 500 c.c. machines differ from standard in having roller main bearings and ball-bearings for the camshafts, and the G. lo frame is somewhat shorter than the 0.8. Both engines have slightly different heads with larger exhaust ports, otherwise the machines are almost standard. The G.io is still somewhat experimental and is only produced in small numbers as yet, but the G.R. 7. is fully in production.
85 m.p.h. should be fairly easily obtained from the G.6., and I have been timed over a flying quarter-mile on several different occasions at 75 m.p.h. on a sidevalve A. J.S. using 19 tooth engine sprocket giving engine speed in top just under 6,000 r.p.m. The engine must not be more than just warm when starting, as when it gets really hot it will settle down to a maximum sustained speed of just over 6o m.p.h. on the level, though down hill about 8o m.p.h. can be obtained all right, e.g., coining down Snaefell in the I.O.M.
HAVE YOU SEEN IT?
(The interesting front wheel drive racing car seen above, was intended for the R.A.C. Grand Prix, but owing to an unfortunate miscalculation when completed, it had a wheelbase of only 22 inches I In reality it is a scale model built by one of our subscribers, and has a working 3-speed and reverse gear-box, genuine brakes, and is propelled by an electric motor.)
THE big twinMcf,A.roy, by reason of recent notable successes at Brooklands, and on the Continent, is a machine that is rapidly building up a reputation for itself in a class of which there are only too few representatives. To survive for more than a few months on the market a 1,000 c.c. motor-cycle must be really good, since the demand for this type comes only from a select few of the most hypercritical enthusiasts, who, while willing to pay a high price, at the same time are extremely particular as to their requirements. For such as these Mr. McEvoy has produced the Anzani (or more correctly British Vulpine) engined model, and at the price of &3o, which is considerably below that of several other machines in its class, it would be difficult to find a more interesting machine. To begin with, the McEvoy, as one would expect, is fast ; with regard to definite maximum speeds, no recording instrument was fitted, no timing was performed, and readers will appreciate the difficulty of fully trying out a machine of this kind on ordinary country roads, however, the following information may form some slight guide to its actual capabilities. On second gear we had not the slightest difficulty in passing a friend on a wellknown sports 31 whose Bonniksen speedometer registered a steady 64 m.p.h., we were probably doing some 70 m.p.h. On another occasion we passed the same
single ” when it was doing a good 68 m.p.h., this time we were on top gear and we left the other machine as though it was stationary. Our speed must have been between 8o m.p.h. and 85 m.p.h., and this on just over half throttle and with the machine still accelerating when the throttle was finally shut for one of those bends which usually appear quite straight on a more sedate vehicle.
A Fast Machine.
We have no doubt that given a clear road 95 m.p.h. would be well within the capabilities of the machine, without any special tuning. The actual model tested was brand new and so far as we know had not been run in except for about 50 miles of fairly gentle touring. With a machine of this type, of course, speeds up to 6o m.p.h. do not harm the new engine and so it is not surprising that we were quite unable to provoke any signs of distress during our test. During the course of a recent week-end we were called upon to assist in the