- by Pete Snidal (C)2001

The Flame Front

The Flame Front is the term used to describe the beginning of the cloud of rapidly expanding gasses that is the basis of the internal combustion engine. When the fuel mixture is ignited by the spark plug at the top of the compression stroke, the burning occurs outwards, moving towards the piston, and it is the combustion of which this flame front is the beginning that makes the power to get us from A to B. Obviously, we want this combustion to be as efficient as possible - to give us the most bang for the buck, the most piston movement and power for the least amount of fuel.

Ignition Timing

The basic idea is that the flame front has to meet the piston at exactly the right place. If the piston's still on its way up, it's obviously going to be an ugly situation - flywheel inertia is forcing piston up, expanding gasses from burning fuel are trying to push it down. It gets over the top, but accompanied by pinging (spark knock), power loss, and pretty soon serious engine damage (holes in pistons, for instance -the piston crown gets so hot after enough of this madness that it semi-melts and gets a hole blown in it. Ugly. So excess spark advance - firing too soon - is to be avoided at all costs.

On the other hand, retarded spark must also be avoided. This is when the piston has gotten over the top, and the flame front actually has to chase it down the "power" stroke, shortening the actual length of time the piston is being driven by the combustion. Needless to say, you're not going to be developing the best power this way, and the incomplete combustion results in poor mileage, and exhaust valve and general overheating - from fuel burning in the exhaust port and pipe, where it shouldn't, because it didn't burn well enough in the combustion chamber, where it should - remember, the combustion is "stopped" at the end of the power stroke by the opening of the exhaust valve, and if there's still lots of unburnt fuel in the cylinder, power and mileage losses are inevitable, as are futile burning in the port, pipe, and muffler. So ignition timing is all- important.

In the early days of driving, designers recognized that the best way to control timing was to leave it up to a properly-trained driver. Assuming the driver has the situation in hand. Since most these days don't, and since it's cheaper to build, manual advance has been replaced by centrifugal advance units in the points drive, which advances ignition timing with rpm.

Crude, but ball-park effective. Points drives built this way have an "advance curve" which is set by such things as the strengths of the springs in there and the size of the weights.

Advance curves can vary from machine to machine, and with time. For this reason, setting the timing "static," ie, with the machine not even running, is far from preferable to setting it at the actual rpm range within which you intend to be operating. So the obvious ideal is to find some way to check the timing in this rpm range, and an ideal way to do this is called Ping Timing.

Ping Timing

Ping - the name given to the sound made by pre-ignition , or detonation , also called spark knock, sounds like marbles rattling around in your engine. It appears, if at all, when you open the throttle, especially at low rpm, and disappears when you back off. It is a sound of major danger, but it can also be the sound you need to tell you that your ignition is just about right - when you can't hear it unless you're trying. Once you get there, there's just one last step - to retard it just a tad more, and you're home free!

Personally, I have never relied on timing lights. I set the timing first of all in the shop for fastest idle, then retard it just a touch. Then I take it out for a test ride, wrench in hand, and adjust the timing on the ride. I'm looking for just enough advance to make it ping when I'm lugging it - putting on more load at less rpm than I would normally drive it. I wind it on it harder than I normally do on acceleration, or leave it in a higher gear for longer than I normally would, and I expect to hear just a little pinging. Then, I back off on the throttle, or drop it down a gear, and if the ping goes away, I know I'm right on the edge of pingland - that's where we want to be for the final stage.

I believed for many years that this was the ideal spot. But a new e-friend responded to an earlier version of this article with some hot news. He's had experience tuning race cars on a chassis dyno, and wrote to tell me that the actual power peak is a bit below the spot just below the ping threshold - he couldn't tell me how to get to the actual right-on sweet spot without a dyno, but he said that it's more than just a bit below the PT. So, since then, I've started tuning this way, and I believe I'm getting more power, and I know I'm resting easier with the larger safety margin afforded by the greater distance from the dreaded detonation. So that's where I do the final tightening down of the points plate, distributor, or whatever. And much thanks to Mr. Starkey!

I've also started to use the timing light, which I have on hand anyway for trouble-shooting misses in multi-cylinder engines. Once I get the timing right, I check it with the strobe, and make a note of it so I won't have to go through all of the above every time I want to check my timing. The Triumph twins have a mark on the rotor, and a removable plate in the primary cover to make this simpler - it can be done on the Enfield, too. You can make your own timing mark on the rotor, but you'd have to drain the primary oil and remove the chaincase cover to check your timing. You'd also have to provide a reference marker of some sort - the Tri's use a pointed stud threaded into the outer case sitting beside the rotor ,but Enfield owners might be able to drill and tap a similar device into the inner cover - or just fix a pointer under one of the rotor mounting nuts. Either way, at least you've got that wonderful single-bolt outer cover, and the O-ring gasket, both of which make the cover re & re a not-insurmountable chore.

It's up to you which is simpler, but you should check your timing regularly, whichever way you choose. If you find it never varies between checks, extend the interval. But, just as with checking valve clearances, the little time it takes can pay big dividends in major maintenance.