After you have attended to all the above easy things, if the compression is still poor, it is then time to conclude that (6, 7 and 8) the piston rings are at fault, or else (9) the valves do not seat as they should.
(6, 7 and 8) To clean, or replace faulty piston rings with a new set, take off the oil shield or crank case and remove the nuts from the bolts that hold the crank bearings in place. Turn the crank to its outer dead center and you can then pull the piston out of the cylinder. The next thing to do is to clean it with a solution of hot lye and water, then wipe it dry and wash it off with kerosene.
To take off the piston rings raise up one end of one of the rings, slip a strip of tin, about wide and 5 or 6 inches long, under it and work it around until it is midway between the ends of the ring. This done, slip another strip of tin under each one, as shown at A in Fig. 69, when you can easily push the ring from the groove it seats in and off of the piston.
Iron piston rings can be expanded and then slipped over the head of the piston onto their grooves or seats, but do not try to expand steel rings. Instead when you put one of the latter on the head of the cylinder press down on one end until it goes into the groove and then screw it around until the whole ring is seated in the groove.
(9) The two chief reasons why valves leak are because (a) carbon gathers in a film on the valve seat and (b) the valves get warped by the heat.' When a valve leaks take off the cylinder head or combustion chamber, as the case may be, and then you can lift out the valve.
The next step is to grind the valve, and this you can do by rubbing a little grinding pastes on the beveled edge of the valve seat; now set the valve in the seat and turn it back and forth a quarter of a revolution at a time with a grinding tool, which is made like a carpenter's brace, only very much smaller. See B, Fig. 69.
After you do this for a while wipe the paste from the valve and mark the beveled edge up and down with parallel lines with a very soft lead pencil, put the valve back in its seat and turn it to and fro again.
If, now, all the pencil marks are rubbed out you will know that the valve and its seat are ground true, but if some of the marks still show keep on grinding the valve until it does seat right. When you are
satisfied with the job clean of both the valve and the seat with gasoline and be sure that every particle of the paste has been removed.
When the lIngine Begins to the Trouble Lies.—There is an even dozen different dis eases that make an engine knock, and, specified, these are (1) the spark is advanced too far; (2) it needs water, or lubricating oil, or both; (3) the spark plugs need cleaning; (4) the compression is too high; (5) the bolts of the flywheel are loose.
(6) The fuel mixture is not proportioned right; (7) the bearings have too much play; (8) the piston or piston rings strike a ridge in the cylinder; (9) the clearance of the piston is too great; (10) there is carbon in the cylinder ; (11) premature ignition caused by red-hot particles of carbon and (12) the piston head is too thin.
Locating and .Removing the Cause.—The first thing to do is to (1) retard the spark by means of the spark control lever, and this will take the noc out of the knock if it was caused by the spark being advanced too far.
(2) The next best guess is that there is a lack of water in the cooling system, or a lack of oil in the crankcase or sump. If after supplying these essen tial liquids and making sure that the oil is feeding properly the engine keeps on knocking, then (3) try cleaning the spark-plugs.
Next (4) try the compression by opening the com pression cock in the head of the cylinder a little; should you find that this eliminates the knock the remedy is to put in a shinbl between the crankcase and the open end of the cylinder, so that there is more space between the piston and the head of the cylinder. This done, see (5) if the bolts of the fly wheel are loose, and if so tighten them up.
Then (6) change the proportions of the fuel mix ture and try a rick mixture, that is, one in which there is a large amount of gas, gasoline or oil and a small quantity of air, and if this doesn't help mat ters reverse the proportions and try a lean mixture.