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Talking Technical: Tuning for Need, Part 3 (Part 2 Part 1)

Paul Friday has plans to make his banger sizzle. This time he explores the theory of ignition and sets a strategy for some bodging...

Right, so I've found some electronic ignition modules that will hopefully let me build the ignition system of my dreams (see parts 1 and 2 of this saga). But first I need to have proper dreams. I need to work out a plan for which types of module I will need.

Real Mart has a fantastic collection of old spark plugs.My bike engine is a big single, so is the simplest form of motive life. Providing my one spark plug lit up at the right time, that was it and Robert was my father's brother. But that would be a little unfair to those of you who have strayed from the true path, and have more than one cylinder. So what I needed was a strategy that would cover the options.

Bike engines are simpler than car engines in ignition terms, or they used to be. The new fully-mapped fuel-injected jobs are much the same, however many wheels they drive. But the older ones, the sort of thing we might want to fit a new ignition system to, they are diferent.

Cars will typically have a single trigger for the ignition, and use a distributor to get the spark to the right cylinder. Bikes used to have one trigger per cylinder - anyone remember timing a two-stroke triple with three sets of points?

Cars will typically fire each cylinder in turn, while many more modern bikes use a spare-spark system. This fires all the cylinders that are at top dead centre. Those that are on the exhaust stroke will not fire again, so the spark is wasted. The reason for doing this is to remove the need for a distributor.

Let's assume that your bike is an easy one to fit an electronic ignition to. Singles are good, as are parallel twins with 360-degree crankshafts. Flat twins are OK. V-twins aren't recommended for these units, as they don't cope will with the uneven firing intervals if you try to use a spare-spark system. Fit separate units for each cylinder though, and you can make potato noises to your heart's content. You can also add electronic ignition to a four-cylinder engine without a distributor, but you will need to fit two systems - one for each pair of cylinders that rise and fall together.

You can even retain the original points and centrifugal advance if you must, but what would be the point of fitting electronic ignition? It would be the electrical equivalent to a DOHC sidevalve engine.

The Programmable Ignition Timing module can provide up to 45-degrees of advance. This is probably enough for all but the most tortured of engine designs. Remember that this is 45 degrees on top of whatever the static advance is, so what the unit is providing is 45 degrees of advancement - not a maximum advance of 45 degrees BTDC. It was designed to work directly with the High-Energy Ignition system, but the kit includes a diagram of how to fit it to the Multi-Spark CDI.

So, which one to use?

The main difference is in their size, and the sparks they make. Both units will manage the charging rate of the coil to stop it overheating, and both will stop charging the coil if you leave the bike with the ignition on but the engine not running. These features are meant to protect the coil form the extra power it will be handling. The good news is that both units work best with normal coils. Indeed, they warn against the use of special racing coils.

The following table is a feature comparison of the two systems:

Bet you weren't expecting a caption here, were you? These captions - or ALT tags - are supposed to fill in the details for people whith graphics turned off, or who are using a screen reader to access the site. If you are using a screen reader and are having trouble finding your way round the site, please let us know.

Here are some of them, from a Kawasaki and a VW.They are pretty well matched, but the decider is probably size - the CDI is probably too large to be installed neatly on a motorcycle. Also, the pair of Programmable Ignition Timer and High-Energy Ignition together cost about the same as the Multi-Spark CDI on its own.

So, I flexed the credit card and a neat little parcel arrived in the post.

The Jaycar kits are nicely packaged with almost everything you need. They do contain all the bits to make the two kits, but the timing unit has no box to mount it in. The circuit board is small enough that it could probably be squeezed into the same box as the ignition module. However, I didn't feel that happy to be poking the keyboard with 300-volts lurking nearby. I paid a visit to a local branch of Maplins Electronics and bought a little plastic box and some bits of wire for under three quid. Sorted.

This was my first time for building anything electronic, but it's quite straightforward. The igntion module is the one to start with, as there are not many components and they are well spaced out on the board. The programmable timer is small and crowded and needs care. Even as a novice, the ignition module took me one evening to build, and the timer two. My only advice is that you need good colour vision and a magnifying glass to decode the coloured bands on the resistors. Oh, and don't scratch your ear when you're holding a soldering iron.

The other thing that was missing was one of the most important parts: the trigger or sensor. The kit listed a Hall effect sensor made by Siemens, but they are as rare as an un-bodged Classic these days. This was the key part that I most needed - I wanted to get rid of all the nasty mess of points and bobweights, and have a simple non-contact and non-wearing sensor. One route would be to use a crankshaft position sensor from a car. If you are going down this route, take yourself off to a scrapyard and peer down at the bottom of a car engine, at the ends. The sensor will usually be mounted at one end of the crankshaft, where it can be close to a rotating part like a flywheel. It will sense a notch in the flywheel, or sometimes a
He keeps the boxes too, you know.protrusion. The joy of Hall effect devices is that they can do either. What controls them is a small magnet on the back of the sensor. Depending on whether the south or north pole of the magnet is against the sensor chip, it will either sense a notch or a tooth in a ferromagnetic material (mild steel, to thee or me). You may have to butcher the plastic casing to get at the magnet to turn it round. You will probably also have to try all nine combinations for connecting the three wires, unless you have a very good wiring diagram for the car of choice.

If that sounds a bit too Blue Peter for you, you can still buy Hall effect gear tooth sensors - RS Components list several. As I wanted reliability, I bought a new sensor.

(As an aside, the reason I thought reliability might be good is that a pal at work built an electronic ignition system for his old car. It broke when he was driving in the fast lane of a motorway).

So, if your bike has an electric starter, you could buy these two kits and a sensor, spend a few happy evenings soldering, and make sparks.

Why an electric starter? Well, there's a catch... The Programmable Ignition Timing module does not fire the ignition on the first pulse from the sensor when you start the engine. It waits for the fourth pulse, so that it can accurately calculate the revs. No bother at all for the car engines this unit was designed for, or even these modern bikes with electric feet. Potential big bother indeed for me, trying to start a single-cylinder engine with a kickstarter. Sure, I could kick it over four times to get it to fire, but people would think it was knackered.

But I had a cunning plan.

Tune in next time to learn what happens when theory meets practice...

People To Speak To:

The High-Energy Ignition System and the Programmable Ignition Timer are both available from Jaycar Electronics in Australia. They are kit numbers KC5247 and KC5202 respectively. The pair of kits together cost Aus$140 with postage, or 55. Customs and Royal Mail then add another 13.

  • See

    The Hall effect gear tooth sensor I used was bought from RS Components. The part number is 235-5706. The bad news is that it will cost 30 with delivery. There are other cheaper versions listed, but this one has a really nice sealed plastic body with a mounting lug.

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  • Tuning classics. What do you think?


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