In 1954 Herb Gilroy asked me to fit a Ford Model T rear axle into an ex dirt track midget chassis, which was being fitted with an Austin A40 motor. I disliked the thought of fitting the axle in the usual way because of the large amount of unsprung weight it would give to a small car. Therefore I decided to try some Mercedes -Benz ideas which would provide more of a challenge and the possibility of results interesting enough for me to feel that the job was worth doing.
The right hand side axle housing was cut off and a universal joint fitted as close as possible to the differential. The axle on that side was left open with just the wheel bearing, forward radius arm and a mounting for a transverse leaf spring above on the end. The differential housing was supported underneath by a pivot, the axis of which was inclined to line up with the left side, radius arm anchor point.
This in effect gave a low pivot swing axle similar to that used on Mercedes Benz saloons, but with some errors in geometry on the right hand side because a sliding joint was not used on the universal. Engine torque would press down the left wheel and lift the rear of the car, but this effect did not seem to be too noticeable. Rear wheel adhesion was very good and transmitted the power well and demonstrated what Hec. Green had told me about the importance of low unsprung weight.
Although the original midget car beam axle in front could hardly have helped, Herb did some very spirited driving when he became used to the way the back of the car stuck to the road!
After all the foreign machinery, the tuned A40 motor seemed a pleasure to work on, with plenty of inexpensive parts available over the counter and shell bearings instead of the integral white metal type. Stock motors are much more robust than they used to be and we fitted this one with a Hec. Green camshaft, larger valves, and twin Amal carburettors.
The carburettors were mounted on an independently supported plate and connected to the inlet ports by rubber tubes. This arrangement insulated them from motor vibration and allowed the intake length to be quickly changed by altering the length of the tubes.
When all was ready the car was taken to Muriwai beach for testing and we managed to find another 1,000 r.p.m. by experimenting with intake lengths, jets and ignition timing. After seeing 7,300 r.p.m. on the clock Herb decided that the A40 Special was going well enough and we called it a day. This motor continued to perform well and required little attention.
Herb later changed the A40 gearbox for one out of a Bugatti. The closer ratios were a great help in achieving better acceleration times, enabling him to get down to 16.2 seconds for the standing quarter mile on one occasion.
However I was not really happy with the idling, as smooth running could not be obtained, even after a balance pipe between the carburettors was tried. This was also the situation with an Austin Seven fitted with twin Amal carburettors. The uneven and overlapping induction strokes, of a siamised port four-cylinder engine, may be the cause of this problem.
One carburettor per cylinder I found always worked well and needed no balance pipe. They cost only a little over five pounds each in the 1950 period and provided an economical four carburettor set up, that was not too difficult to adjust.
The size of the port is not a guide to carburettor size and the size of an Amal should be selected by making comparison with motor cycle engines of similar cylinder size and r.p.m., or by calculation, to give a choke velocity of 450 ft per sec. Four 1 3/32 inch Amals fitted to the Standard Vanguard motor of a Morgan Plus Four, transformed the performance and installations on Rileys also worked well.
Although the cables supplied with the carburettors can be used to couple the throttles to the accelerator pedal, a shaft mounted on good bearings with four arms and adjustable links is very much better. The Amal throttle stops are then not used, idle speed adjustment being made via the links, so as to ensure that all throttles open exactly in unison. Float chambers were usually modified by fitting SU tops, with lever action floats, so as to prevent trouble with flooding due to vibration, or difficulties with pressure fuel feed.
I once gave Malcolm Gill some assistance overhauling an L type MG motor and have always regretted that we did not fit it with six Amals as this would have resulted in a very impressive set up.
A number of porting jobs were done at various times and Merv. Neil once brought along a Speedway J.A.P. head and told me stories about how tough dirt track racing was in England. Passing on the outside when coming out of the corners was not done and any innocent new riders who tried it were promptly pushed into the fence and Merv. had a damaged thumb to prove the point !
When Merv. again went over to England, he took a Goldfinch tuned J.A.P. engine with him and with this fitted the bike would jump off the start line and give him an advantage. It went well, until one day when wiping the carbon out of the head, Merv. noticed some cinder marks on the valve seats so he gave the valves a grind in.
The motor never went the same again and he sadly lost his edge at the starts. If one aims at perfection, it becomes necessary to touch up the streamlining round the edges of the valve and seat every time the valves are ground in. A convenient way to do this is with aid of a hand operated fly cutter, using a tool bit which can be ground to any angle required.
Merv's stories would always leave me in the right frame of mind to work happily away trying to get the best out of an engine.
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