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If It Works, Don't Fix It"

- By Pete Snidal (C)2002
Let me quote from a recent discussion on the royalenfield forum: 


   "..with 9000 miles on my 500 I have yet to check the
   points or change the plug.  If it aint broke leave it
   the hell alone!!!"
Normally, I couldn't agree more, but I'd change that leave it to check it once in a while, anyway. Or better yet, learn to listen to your engine at all times, and NEVER run it with detonation! (Also called spark knock, pinking, or ping.)

When I first started riding motorcycles, more than a few of my friends were having two basic problems:

1) Colliding with cars

2) Blowing holes in pistons

The first I took care of by analyzing every accident I heard about. (And a few close calls of my own.) I soon concluded that motorcycles are less than invisible to car drivers (we appear to them as holes of opportunity in the traffic stream composed only of other cars) and that one MUST EXPECT any and all cars to cut in front of us, or run into our sides, or rear-end us, at all times. I've been riding completley paranoid since '56, and have yet to (knocking furiously on wood.....)

The second took a little longer. The popular belief at the time was that so-an-so is just really bad, and he rides so hard that the blows bikes up all the time. What a guy! This never happened to me, but only because I took the advice of the various manuals of which I was able to get ahold, and checked my timing regularly. I soon got to be able to recognize the sound of a retarded single, and of course the sound of "ping" which tells us it's too far advanced. For those who have yet to develop the ear, I'd strongly recommend checking timing on a regular basis. Too retarded will burn the exhaust valve out of the thing, and too advanced will blow a hole in a piston. Compared to the little bit of trouble it is to check your timing once in a while they're both fairly catastrophic events.

It was at first only coincidence that some of my friends were blowing their bikes up and I wasn't, but sooner or later I figured out that it had been advanced timing that had taken those pistons out, not hard driving. They had been, of course, used bikes, and the DPO's had likely sold them after tweaking timing unsuccessfully. Bummer!

So, now to today's lesson:

Checking your timing

Motorcycle engines, being small, air-cooled, and often high-output, are a little more critical when it comes to ignition timing than their automotive counterparts. This means you want to be sure that your timing is within spec at all ...... (well,..) times. (sorry!)

There isn't a lot to checking your timing - in most cases, you won't have to adjust it, and soon you'll get to know just how often you'll need to do it, and begin to develop an ear for the "snarl of a well-tuned single," a quote I still remember from an early _Cycle_ magazine - a road test of a Matchless scrambler, I think it was.

The Objective

Here's what we're trying to do: The piston comes up on the compression stroke every second time around, pushing a cloud of fuel/air mixture up into a small space under the head - the combustion chamber. The idea of the internal combustion engine is to get this compressed mix to light up, and to burn, thus expanding and bringing about the Power Stroke, in which the piston is forced back down the cylinder, making the wheels turn. The end of the power stroke is brought about by the release of this pressure by the opening of the exhaust valve on or about the bottom of the power stroke.

What lights up the compressed mixture is the spark plug, and the firing time, relative to the position of the piston, is controllable by adjustment of the spark advance - setting your ignition timing. The spark fires, in points systems, when the points open - they close soon after, and what happens during the time they're closed is another story, beyond the scope of this lecture. What's important to us just now is that each time they open, a spark is sent to the plug.

Three Possiblities:

  • Spark is too soon: The "flame front" of the expanding gasses will meet the piston while it's still on the way up. This will be signalled by a subtle sound of marbles rolling around in the motor, and is known as "ping." The preignition thus signalled will cause extreme overheating of the piston crown, and the resultant excessive pressures will work with this to blow an actual hole in your piston! This is not generally considered a Good Thing. (TM)

  • Spark is too late: The flame front has to "chase" the descending piston down the cylinder, catching up with it at some time, resulting in some burning of the fuel. The exhaust valve opening will be greeted by a burn that is still well in progress, with lots of burning left to do in the exhaust pipe, resulting in overheating of the entire engine, and signalled by a flatter exhaust note, burning of the chrome on the pipe (you'll almost always get some burning of the chrome, but only for a few inches. (I can also tell you how you'll get none! Click here) and generally poor efficiency. Retarded timing will also burn your exhaust valve, necessitating a valve job at minimum.

  • Just Right. The best, needless to say. Characterized by that "snarl of a well-tuned single," at least to some degree, the best mileage and power, the minimum pipe burning, and the longest engine life. Well worth checking once in a while to make sure you're there.

How?

I thought you'd never ask! Well, broadly speaking, there are two ways to do this - static, and dynamic, or not running, and running. The easier, and for the novice, surer way is statically - you rotate the engine, while monitoring the position of the piston, and check to see at what piston position the spark is set to go off.

Tools and Materials

All you need for tools will be a test light - a 12V bulb with wires and clips on the ends of the wires; it can be a commercial product or one you make yourself, and a "timing stick" - a piece of straight coathanger wire, which you will calibrate by marking two spots with a file or hacksaw blade. Here we go:

  • The Timing stick:

    (1) Put the bike on the center stand, or otherwise raise the rear wheel. (Milk cartons are the time-honoured way to do this for the center-standless.) Remove the spark plug, being careful first to clean the area around the plug hole to avoid ingestion of foreign object (rocks, mud, beercaps, etc.)

    2) Prepare the timing stick. Take a 6" piece of straight coathanger, welding rod, or similar material. With the bike in top gear, insert the wire through the plug hole, as close to straight down as you can get it. The rotate the rear wheel in a forward direction until the wire is forced upwards. When it gets as far up as it gets, rotate the wheel back and forth until you get the "feel" for Top Dead Center-" you'll soon figure that one out. Then, make a note of a spot on the timing stick that corresponds to some spot on the cylinder head - the top rim of the spark plug hole is a good one. Mark this with your thumbnail, withdraw the timing stick, and mark this TDC point with the file or hacksaw blade. (Or you could be using a Popsicle stick and a pencil, it's not critical.)

    3) Now, all you need to do is to mark the proper advance distance on the stick, below the TDC mark. Spark advance specs are given as before TDC, either in degrees of crankshaft rotation, or in actual piston travel distance - the spec we'll be using here. You'll need to get the distance from your manual, or a reliable source. Using a ruler or scale, mark this distance on the stick, and you're ready to check your timing. You may want to keep the stick in your toolbox for future reference.

  • Checking the point opening:

    1) For this, you'll want to use the test light. When the points are open and the ignition is on, there will be a voltage present across the points. When the points are closed, this voltage will drop to 0. So you want to connect one side of your test lamp to the wire from the coil to the points, and the other to ground (the other side of the points.) When the points open, the light will come on, when they close, it will go off. Move the wheel back and forth around TDC with ignition on and light connected, to verify that this is happening.

    2) Now it's time to check with the timing stick. Move the wheel BACKWARDS until the piston is an inch or so down. With ignition still on, push the timing stick down onto the piston crown, (vertically, remember!) and rotate the wheel forward until the light comes on. If the advance mark on the timing stick passes your reference point at that exact position, Bob's yer uncle. If not, you'll have to adjust the timing until it does. Don't forget to turn off the ignition after checking!

Adjusting the timing

1) First, examine your points while setting their gap. Manual tells you what it needs to be - .012"/,3mm should be about right. Set them when the rubbing block is in the center of the cam lobe. If they're pitted or burnt, replace them with your spare set, and order another spare set. While you're dicking around with the points, make a note of the direction of rotation of the breaker cam when the wheel is running forward.

Once the gap is right, it's time to move your breaker plate. Loosen off the holding screws so that it will turn freely - but not too freely. If the plate is too loose, it will move , once you've set the timing, when tightened, and you'll have to do it again.

2) Set the piston to the advance point. Run the wheel back past TDC, then, feeling with the stick, forward again until you reach the advance point.

3) Now to set the Breaker Plate. First, you'll need to make sure that your centrifugal advance mechanism is working. If you attempt to twist the rotor/cam shaft, back and forth, you should find that it will move a bit. This is normally accomplished by the springs working against a set of centrifugal weights behind the points plate. You need to set the timing in the full advance position - with the cam rotated full in the direction of rotation when the engine is running.

Power up the ignition and connect the light. Then, with the rotor in the advance position, (you'll have to hold it against the return springs as you have a friend rotate the rear wheel) rotate the housing in the direction of lobe rotation until the light goes on. Then rotate the housing AGAINST the direction of lobe rotation until it just goes out. This is the point at which the points will open in normal operation. Once you've done this, tighten the set screws.

4) Now, check it by rotating the engine through two revolutions in a forward direction. Pull out the timing stick, so you won't bend it, until the second time the piston is on it's way up. As the advance mark hits the spot, the light needs to come on. Keep doing it till you get it right; it'll soon be second nature. Like most things, it'll take a half a day the first time, but will soon be doable in just a few minutes.

Once you've set it spot-on, start listening to your bike. Try to develop an ear for retarded timing. As for advanced timing, read pingtime.htm on this website for a detailed rundown on setting timing dynamically - the best, but more subjective, way to do it.

No Advance Data?

Can't find a number for the advance distance? Another way to do all this is to set the ignition in the Retarded position - ie, with the centrifugal advance unit (or, in the case of manual advance bikes, the manual advance lever) at the Full Retard position. You do this with the centrifugal advance by working the rotor against the springs, so you know where it is, and then with the springs, so that it will be at full retard position (against the direction of rotation of the rotor.) For this one, position the piston at TDC. Some makes suggest a slight advance even at full retard - the Indian Enfield, for instance, calls for .8mm. Either way, this will get you to a "ball park" timing setting, so it will start, and then you can go ahead and ping-time it. (Instructions elsewhere on this website.)

One Further Complication - Two Sets Of Points

Most points-fired Brit twins have two sets of points. One set is fixed to the advance plate - the only way you can change its timing is by moving the advance plate, or by varying the point gap (yes!, that's a way! - within reason!) The second set of points is movable relative to the advance plate. So, once you've set the timing of the first pointset, tighten up the advance plate adjusting screws in their slots, and then set the timing for the movable pointset using the variablility feature - the second set can be moved relative to the main advance plate. You'll need to get the two pointsets synchronized in this way before proceeding to ping timing.

The very earliest points-fired Brit twins didn't have this variable pointset feature. For these, you have to vary the point gaps a thou or two in either direction until they both open at the same precise instant relative to piston position. Then check with the strobe. Use of a stroboscopic electronic timing light is recommended to check that both plugs show the same advance before ping-timing. Then, once you've got it ping-timed, check the timing with the light at ~1500 rpm and make a note of where the mark flashes. You can then use this reference to check your timing in future, to duplicate the ping-timing you found so successful. By the way, if you find your timing light is flashing all over the board, and it's the type that requires a 12V supply for power, use a battery other than the one on your motorcycle - they sometimes pick up spurious signals from the alternator system.

I hope this helps. All this typing was worth it if it saves just one piston! Happy riding!