TUNING THE 7HP AUSTIN ENGINE FOR BEST ALL-ROUND PERFORMANCE
PART 3
Click here for Part 2 - Click here for Part 1
SECTION 2.—TUNING FOR BEST CONSUMPTION.
One is sometimes asked to tune up an engine for economical running at the sacrifice of power and speed. With the 22 F.Z. type of carburettor one can do this fairly simply, but it is a more difficult job with the 26 V. type. This carburettor is already so economical that the sacrifice of speed would not improve the consumption materially, and one cannot expect much more than 50 m.p.g. After carrying out exactly the same work to the interior of the engine as specified in Section 1, this can be attained by careful adjustments of the slow-running jet. The latter can be cut down to 50 in some cases. On the 22 F.Z. type carburettor the best setting is choke 15, main 70, comp. 70, consumption 52. Another setting giving the same consumption, but which roughens the engine slightly, is: choke 15 main 65, comp. 65. When carrying out the internal attentions to the engine special care should be taken that the valves are a good fit in the guides, particularly the inlet. It is, of course, hopeless to expect economical running unless the pistons and rings are in perfect condition.
SECTION 3.—TUNING FOR MAXIMUM SPEED.
We now come to the section dealing with tuning for maximum speed. To get a higher speed than that we already attained we have to find more engine revs, and we cannot get sufficient by carburettor or ignition adjustments. We have to get them from inside the engine.
Polish Valve Ports.
We must go back to the commencement of our article, and in addition to removing the cylinder head, we must remove the cylinder block so that the valve ports can be polished. This is an operation that requires care and patience. Take care to remove an equal portion of metal from each. Open the exhaust manifold outlets to correspond with the outlets in the cylinder block. On the engine fitted with the 22 F.Z. carburettor do the same with the induction pipe where it joins the exhaust manifold. Now polish the head. A small flexible head emery wheel is of great help on this work. If this is not available, files and emery cloth will have to be used.
For Early Types of Cylinder Block.
If the cylinder block is of the earlier type, in which the valve guides protrude into the ports, shorten them. Cut off that portion which protrudes. The guides can be knocked out of the cylinder block and cut off with a hacksaw, but when refitting it will be necessary to recut the seatings again. Very carefully grind in the valves.
Aero-dual Valve Springs.
Now fit a set of Terry’s aero-dual valve springs. Where the 22 F.Z. type of carburettor is fitted it will be necessary to drill out the induction pipe to 1 in. diameter. Now reassemble the engine, and for the benefit of those keen amateurs who are, perhaps, refitting the cylinder block for the first time, don’t forget to see that your piston ring gaps are equal distance apart.
Refitting the Cylinder Block.
You will find the refitting of the cylinder block a simple operation if you use piston cuffs. You can obtain a set of these from the Service Department of the Austin Motor Co. at the price of is 1/- each. (Of course, you can make some of tin if you so desire.) Take care that the faces are clean, and the packing joints are new. If you do this, gold size or jointing solutions are unnecessary. Now for a run to see the results. With the 20 V. carburettor no change of jets will be necessary, except perhaps the slow running. This may require an 80. We find the engine opens up quickly with plenty of zip. Third speed 38 m.p.h. Good! Now top—56 m.p.h. We will try a little more ignition—the maximum 2 in. When running with the ignition set at this point it should be used when driving and retarded at the lower engine speeds. Off again! Third speed—40-41. Good! Now top—57-8-9-60. That’s all we can ask of a little engine like this, with the standard gear ratio and no special lubrication or camshaft. The best setting for the 20 F.Z. carburettor is choke 16 main 80, comp. 75. This should give us the same speed of 60. Now for a final word. It is the little things that count when tuning an engine. Perfect valve seatings on all valves, good joints, and cleanliness inside and outside of the engine.