TUNING THE 7HP AUSTIN ENGINE FOR BEST ALL-ROUND PERFORMANCE

PART 1

Click here for Part 2    -    Click here for Part 3

 

From the Austin Magazine

 

 

SECTION 1.—HOW TO TUNE FOR BEST ALL-ROUND PERFORMANCE.

This section will be the longest as it will contain quite a lot of work which need not be repeated in the second two sections.

If the best performance is to be obtained from the engine, one of Abe most essential features is interior cleanliness, and with that in mind we will commence by removing the sump and tho­roughly clean­ing out the base chamber and oil filter.

 Remove the Sump.

The sump should be refitted, taking care that the joint is in a perfectly sound condition if necessary, renew it.  This packing joint should be greased on both sides before refitting.

 Next—the Cylinder Head.

After draining the water from the engine we remove the cylinder head, the exhaust manifold and induction pipe and the carburettor, and lower our tappets.

 What the Carbon Deposit Tells.

It is as well here to take a note of the quality and the quantity of the carbon deposit in the engine. The carbon that is there should he of a dry hard nature. If of a soft and cheesy type, the oil consumption is evidently too heavy. If the carbon is of a powdery or sooty nature, the carburation is imperfect.

 Piston Clearance

One must bear in mind that it is no use attempting to obtain the best performance that an engine is capable of if there is wear in the pistons, rings, or cylinder bores.  One should feel for any undue slackness in the pistons, but must bear in mind that with the aluminium type of piston the clearance must be at least .004 in.  This clearance, however, should not be more than .005 in. to .0055 in.  It is impossible, of course, to judge the exact clearance without removing the cylinder block, but from the condition of the carbon and from the feel of the pistons it is quite possible to judge whether it is necessary for renewal or not.

 Decoke.

Finding the pistons to be in good condition, we now clean the top of the cylinder block and the pistons and valve ports.  Scrapers are the best tools to use for this purpose.  Next wash the cylinder top with petrol and clean the tops of the pistons with metal polish.  Emery paper should not be used for this purpose.

 With Special Attention to Valve Ports.

The valve ports should be very carefully cleaned of carbon. Any carbon in the valve guides should be carefully cleaned out. The carbon in the guides can be loosened by dipping the valve stem in paraffin and working the valve up and down in the guides. The paraffin should afterwards be washed out with petrol.

 Examine the Valves, Guides and Seatings.

Now examine the valves and the valve guides for wear, and renew any that are necessary. Carefully note the valve seatings in the block. If any of these are burnt they should be recut, but a valve seating cutter should never be used unless absolutely necessary and then with the utmost caution.

 Cleaning Valves and Valve Stems.

In cleaning the valves a wire brush is a very handy tool and in finishing the stems a very fine emery paper may be used but the stems should be rubbed along their length and not round their circumference.

A Practical Test for the Valve Seatings.

Each valve should now be very carefully ground in, and an easy and good method of testing the seating is to mark the valve seating with a pencil in about eight places round its diameter, drop the valve into its position and make about an eighth of a turn with a screwdriver.  Remove the valve and examine the pencil marks—if perfectly seating all the marks should be obliterated on that portion of the valve head coming in contact with the valve seating.

 Check the Valve Springs.

The valve springs should be checked and if found to be weak, replaced, otherwise we shall get valve bounce.  The valve tappet heads should all be examined and if recessed they should be ground down.

 How to Test Cylinder Head for Truth.

Now thoroughly clean the cylinder head, not forgetting to remove all traces of carbon around the entrance of the sparking plug holes.  It is advisable to check the cylinder head on a surface plate as it is quite possible for this to distort slightly, thereby causing difficulty in getting a watertight joint.

 If No Surface Plate is Available.

If a surface plate is not available, place the cylinder head, after cleaning, on to the cylinder block minus the cylinder head

 Reassembly.

Now refit the valves and springs, setting the valves with a .oo6 in. clearance.  The cylinder head can now be replaced. Unless you are perfectly sure that the old gasket is in a sound condition, a new one should be fitted. The gasket should, of course, be fitted copper side down.  Care must be taken in bolting down the cylinder head and the centre nuts should be tightened first, then working from corner to corner and side to side of the block, down towards the centre nut again.  Pull them down a little at a time and keep going over them in the same direction until thoroughly tight.

 Dismantle and Clean Sparking Plugs.

Before fitting the sparking plugs, clean them.  Don’t be satisfied with cleaning the points with a wire brush, but dismantle them and thoroughly clean the insides.  Set the points with a gap of .018 in. to .020 in.  There is a very efficient tool on the market for dismantling sparking plugs, which is especially useful when this operation is being carried out on the road and a vice is not available. The tool is marketed by Messrs. KLG.  It is a simple, yet efficient tool, which will loosen and tighten the nuts of the sparking plug with no danger whatever to the porcelain.  If the electrodes of the plugs are burnt, renew the plugs.  (It is advisable to renew sparking plugs at any rate after 15,000 miles.)

 Carburettor.

Whilst the carburettor is off, thoroughly clean it, not forgetting the filter at the petrol inlet.  On the pump type engine make sure that the unions on the pump and the pipe line are quite tight.  Unscrew the thumb screw at the top of the petrol pump, remove the top and clean the gauze.  Refit and make sure that the washer is intact, and don’t forget to tighten down the thumb screw properly.  Check the screws around the body of the pump and see that they are all tight.

 Exhaust and Induction Manifolds.

Now refit the exhaust and induction manifolds, using new washers.  Next check the delivery of the pump.

 How to Check the Petrol Delivery Pump.

To do this, remove the float chamber of the carburettor and hold it just below the level of the body.  Then get someone to turn the engine over with the starting handle.  About 10 revolutions of the handle should fill the float chamber to its correct level.  Bear in mind it is possible for a petrol pump to over-deliver.  This causes the carburettor to flood.  If the carburettor floods make sure that the needle is seating properly.  The carburettor, of course, will only flood when the engine is running and it mostly happens when the engine is ticking over after being revved up.  The pump should deliver at approximately 3-lb. pressure, but this is not an easy thing to check, and so in the case of any defect it should be returned to the nearest A.C. Service Station for attention.