IGNITION TIMING

IGNITION TIMING

Here is a step-by-step procedure for setting the ignition timing on your HONDA CB550.  It is the same procedure for all 4-cylinder HONDA CB’s (350, 500, 550 and 750) from the 70’s. I have provided a few explanations in layman’s terms (the only ones I understand) along the way to help understand why we do what we do when we set/adjust the ignition timing.

WHAT IS IGNITION TIMING TO BEGIN WITH?  IGNITION refers to igniting the air-fuel mixture in the combustion chamber, i.e. cylinder, by means of a spark generated by a spark plug.  TIMING refers to the moment the spark is sent to the combustion chamber to ignite the air-fuel mixture.

WHY DO WE NEED TO SET THE IGNITION TIMING? The ignition timing (the moment of sending a spark to the combustion chamber) must be relatively precise or the engine will run poorly and in some cases may backfire and blow the head gasket.  We certainly don’t want that to happen. For the HONDA CB bikes from the 1970’s, the spark is sent to the cylinders at 5 degrees before top dead center (BTDC) when the engine is idling and the timing becomes more advanced (the spark is sent earlier) when the engine is in high revs.

HOW DOES IGNITION TIMING WORK?  The objective is to generate a spark and to generate it at the exact time it is needed.  The spark is, of course, generated by the spark plug, but a lot must happen to make that spark plug fire.  Here is how it works:  When the ignition key is turned on, electricity from the battery goes to the ignition coil.  The ignition coil actually has two coils of wires in it called the primary wire and secondary wire.  They are wound around a soft iron core, which is made from thin iron sheets stacked on top of each other.  The electricity flowing through the primary wire exits the coil and goes to a  contact breaker (essentially an on-off switch), and from there to ground.  Cycle complete.  As the electricity flows through the primary coil, it generates a magnetic field in the iron core.  The more electricity flowing through the primary wire the greater the magnetic field’s strength in the iron core.   If the contact breaker is now thrown into OFF position, the flow of electricity is interrupted.  The magnetic field in the iron core can no longer be maintained and it collapses.  As it collapses, it moves through the secondary wire.  As described in the ELECTRICAL SYSTEM BASICS post, a magnetic field moving in and out of a coil of wire generates electricity in that wire.  Because the secondary wire consists of thousands of windings of thin wire, it acts as a transformer and the electricity generated by the collapsing magnetic field reaches 20,000 V or so.  This newly generated electrical potential is now “desperate” to get to ground.  By design, the only way to ground is via the spark plug.  But there is an air gap in the spark plug terminals. Air is an excellent insulator making it very difficult for the electrical potential to go across so it puts out a huge resistance.  Resistance in an electrical field generates heat (which is why incandescent light bulbs glow) and so the tremendous 20,000 V electrical potential starts to heat the air.  The temperature rises instantly to thousands of degrees and the heat causes the air molecules to become electrically charged.  Now, a medium with charged particles is electricity’s best friend.  So, figuratively speaking, the high voltage builds itself a bridge by turning the inert air molecules into electricity-friendly ions and runs across this bridge to ground with a spark and a bang.  Exactly the same phenomena as lightening and thunder.

So, by opening and closing a simple on-off switch called contact breaker, we can send sparks to the combustion chamber(s).

This contact breaker consists of two small discs (called points).  When the discs are in contact with one another (it is said that the contact breaker points are closed) we have continuity and electricity flows (creating a magnetic field in the ignition coil).  When the points are open, the continuity is broken (and the magnetic field collapses).  Obviously, if the points never opened or never closed, there will be no spark and the bike won’t run.  So, a relatively precise gap between the points must be maintained and that is precisely why this gap must be checked and adjusted if needed.

ADJUSTING THE POINTS GAP:

Remove the points cover (the round cover on the right side of the crankcase) to access the points plate which looks like this:

There are two sets of points on that plate: the points for the coil that fires the spark plugs in cylinder 1 & 4 are on the left side of the image with the blue wire connected to them, and the points for the coil that fires the spark plugs in cylinders 2 & 3 are on the right side of the image with the yellow wire connected to them.

Here is a close-up of one of the two sets of coils:

To adjust the points gap, rotate the crank clockwise using the nut in the middle of the points plate (a 23 mm socket or wrench is needed) and watch the gap between the points (say, for the points connected to the blue wire; it doesn’t matter which set of points is adjusted first).  Once this gap reaches maximum size, stop. There isn’t a magic moment where and only where the gap is at its  maximum; the gap will remain at max value for some time, i.e for a few degrees of revolution of the crankshaft.  Loosen the screw in the middle of the image above just a little and using a flat-head screwdriver positioned in the notch, turn the plate until the gap is 0.012″ – 0.016″ (0.3 – 0.4 mm).  Tighten the screw and re-check the gap.  Adjust again if necessary.  Repeat with the other set of points.

ADJUSTING THE IGNITION TIMING:

This is best done using a test light (a.k.a “circuit tester”).  The ignition timing for cylinders 1 & 4 is adjusted first and then the timing for cylinders 2 & 3.  Connect one of the test light’s leads to the blue wire’s terminal and the other to the engine case.  Turn the ignition switch on.  Both the HONDA and CLYMER manuals say that you should now turn the crankshaft until the light comes on leaving you with the impression that there is only one magic spot where this will occur.  So if you, like me, expected the light to be off at first and only come on at that one magic spot as you turn the crankshaft, you could have an “Oh, sh..” moment when the light came on as soon as you switched the ignition on.  So, you turn and turn a little more and the light stays on!  What the hell is going on? Am I doing the right thing?  Where did I screw up?  No need to panic.  Remember that electricity should be flowing from the battery and through the points for a considerable length of time in order to  generate sufficiently large magnetic field in the iron core in the ignition coil.  (The duration of time, or degrees of angle, for which the points remain closed is called a Dwell Angle and for 4 cylinder engines that is around 46-49 degrees; the points remain open the rest of the time.) Keep turning and the light will go off.  Continue turning and eventually the light will come on again.  When that happens, stop.  You can check the timing now, but it’s better to make one more revolution to get that moment when the light comes on again a little more accurately.  Turn almost a complete revolution and then very slowly continue turning until the light just comes on.  Stop.  Look though the view hole at 12 o’clock on the timing plate.  This is what you should see if the timing is perfect:

The F 1.4 mark (F is for FIRE) should line up with the notch in the engine case.  If the alignment is not what it should be, loosen the three screws that hold the timing plate to the engine’s case and turn the whole plate a little. Just a few degrees.  Tighten the screws lightly and repeat the above exercise. Keep going until the F mark and the notch on the engine case are aligned perfectly. Tighten all 3 screws.

Repeat the exercise for the other set of points until the F 2.3 mark lines up perfectly with the notch on the crankcase as shown below.

There is no need to rotate the entire points plate for this adjustment, only the plate associated with those points as shown below.  Loosen the two screws identified by the two arrows and move the plate a little.  Recheck the ignition timing for cylinders 2 & 3 and keep adjusting until perfect alignment between the F 2.3 mark and the notch is achieved. (Ignore the “323” mark shown in the image below through the view hole.  It is not related to the ignition timing adjustment.  The plate just happened to be in that position when the photo was taken.)

So, it seems to work like this:

As the crankshaft completes half a rotation (180 degrees), the points connected to the blue wire close and open once causing the coil connected to them to fire the spark plugs in cylinders 1 & 4.  For the next half (180 degrees) of the rotation of the crankshaft, the points connected to the other coil open and close causing the spark plugs in cylinders 2 & 3 to fire.  And so on for every subsequent rotation.

This would seem to mean that both cylinders 1 & 4 are fired at the same time and then a little later cylinders 2 & 3 are fired at the same time.   But, wait a minute.  HONDA tells us that the firing sequence in the CB 550 engine is 1-2-4-3 meaning cylinder 1 is fired first, then cylinder 2, then cylinder 4 and finally cylinder 3.  Both statements are correct!  The explanation is that the HONDA CB550 ignition system is designed to have a “wasted spark” meaning that as the spark is sent to both cylinders 1 & 4, for example, only one of them is actually fired because it is almost at the end of the compression stroke and the other receives a spark as well, but is not fired (the spark is wasted) because it is almost at the end of the exhaust stroke.

QUESTIONS?

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