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Where Do The HT Leads Fit?


We've all done it - we've removed the HT leads without noting which one went where. The last time we made the mistake we said that we would mark the distributor cap to make it easier next time but did we? No, of course not.

Of course we can look up the firing order and since we can see which way the rotor arm moves we can fit the leads in their proper place provided we know which distributor cap terminal corresponds to No.1 cylinder. The RM Riley is a particularly awkward engine as it has no timing marks at all but being a true Classic Car it does have a starting handle. First of all remove all the sparking plugs. You may as well clean and regap them while they are out. Remove the distributor cap but leave the rotor arm in place. Now use the starting handle to turn over the engine a few times so that it turns easily. While you are doing this note which way the rotor arm turns, clockwise or anticlockwise. Early and late models go in different directions so check this carefully. As you turn over the engine put your thumb or a fat finger over No.1 spark plug hole. As the piston rises on the compression stroke you will feel the pressure rise in the cylinder. The compression stroke occurs once for ever two turns of the crank shaft. Having found the compression stroke, stop and then turn the engine about 1 turns so that the piston in No.1 cylinder is about to come up on its compression stroke. Hold a medium/large screw driver vertically in the spark plug hole so that it touches the top of the piston. Slowly turn the engine and the screw driver will be pushed up by the piston until the piston is at top dead centre after which it will start to go down again. We want to get the piston at top dead centre on the firing stroke. At the first attempt you will probably go past top dead centre so carry on turning the engine for just under two turns and this time stop turning as soon as you feel the screw driver stop rising.

The spark plug for number cylinder 1 "fires" just before the piston reaches top dead centre but this is near enough for our purposes. Now look at the rotor arm and note which way it is pointing - say 2 o'clock. Now refit the distributor cap and you will see an HT terminal also at 2 o'clock. Since the rotor arm is pointing to that terminal with No.1 spark plug firing, that must be the terminal to which No.1 HT lead must be connected. If you have a distributor cap with side entry terminals you will have to look inside the cap and trace which lead hole goes to the No.1 terminal. Having found No.1 terminal just follow around the cap terminals in the direction which the rotor arm moves and connect the remaining leads in the firing order given in your Manual. For RM Rileys the order is 1-2-4-3. Don't forget to refit the spark plugs!

If you have also removed the distributor completely and hence lost its position too the procedure is similar. Once again set No.1 piston to top dead centre on its firing stroke and replace the distributor having slackened the clamp screw so that the distributor body is free to turn in the clamp plate. Turn the distributor body until the heel of the moving point is right on the top of a cam so that the points are wide open. Now slacken the clamp screws on the fixed point and set the correct gap for the points (12 or 15 thou.) and retighten the fixed point's clamp screws. Remembering which way the rotor turns, twist the body of the distributor the same way until the heel of the moving point just contacts the cam. Find the place where the points are just about to be opened and clamp the distributor body in the clamp plate. Done carefully this will be near enough to get the engine running but if in doubt set the timing to top dead centre by checking electrically when the points are about to open. The HT leads can now be reconnected as described above.

Setting The Ignition Timing

If the spark timing has been set to top dead centre, the engine will run safely but will be retarded. The Riley RM engine has no timing marks and the Manual talks of setting the timing by measuring piston displacement from top dead centre. This is not very accurate (sines, cosines and all that) nor is it likely that the timing which was right for a new engine running on 1940/50 petrol will be right for your engine today. Far better is to get the engine running by setting the timing to top dead centre and manually tweaking it from there. To do this slacken the clamp on the distributor body a little. Run the engine at tickover and turn the distributor a little in the opposite direction to the rotor arm. This will advance the spark. As you do this the engine will speed up a little and run more smoothly. As you continue turning it will start to run rough and slow down. When it reaches this point it is over advanced so turn the distributor back (the same way as the rotor moves) to retard it a little. As you turn the distributor back and forth you will find a "sweet spot" where the engine runs fast and smooth. For safety, retard the spark a very small amount form the sweet spot. Leave the distributor there. Turn off the engine and tighten up the clamp screw. Now release the clamp screw holding the advance/retard manual control cable and set the dashboard knob to the mid way position. Reclamp the cable.

You are now ready for a road test. Start the engine. It may help to retard the spark a little on the hand control when doing this. Now take the car for a run and during the run use different positions for the manual advance/retard. You will quickly find the optimum position for it so leave the control in that position. When you get back, release the clamp on the advance/retard control wire and, without letting the distributor twist, set the control knob to the mid way position and reclamp the wire. Your engine should now run sweetly on the road with the knob in the mid way position leaving you free to retard or advance the spark as conditions dictate.

Although the above used the RM Riley as an example the same procedure will work on all Classic Cars. All you need to know is the correct setting for the points and the firing order. All right, you win - it won't work on your diesel!