Hitchcocks Forum

Dennis C and others with a knowledge on spark plugs and igniting the fuel mixture... In the thread on coil currents I was looking for answers on how to get a good, strong spark delivered from another coil than the stock Lucas. I used the term hot about the spark, but that might not be correct after all. Dennis C, you suggested that a spark can get too hot pitting or holing the piston. We don't want to ruin our bikes from overdoing a parts replacement. So, as I understand it a coil can end up being too powerfull for the plug and produce a too strong spark? How can one determine the right coil for a Bullet? And the plug temperature? Can plug gab be altered for better combustion with a strong spark? Is compression a factor? Is piston design a factor? Is petrol octane a factor? Degree on plug etc.? ... looking forward to learn more about all the little things the influence the optimal running of the engine... all the best Jacob

The problem of a holed piston is not related to the coil but the spark plug. Depending on their design (ie the length of central insulator) a plug can be designed to run as a ‘hard’ (or cold) plug or as a ‘soft’ (or hot) plug. In the case of a hard plug the design allows more heat to be dissipated from the plug so that it is capable of working satisfactorily under higher engine temperatures without itself becoming too hot. A 'soft' plug retains more heat and is more suitable to use in an engine that does not run at such high temperatures. A spark plug needs to run at a sufficiently high temp (between 400 and 800 C) to ‘self-clean’ or it will ‘oil up’. But it must not run so hot that it becomes incandescent (glows) and causes pre-ignition (explosion of fuel mixture before the spark jumps the plug gap). Pre-ignition creates excessively high temperatures. This can result in 'detonation' (the mixture ignites in the combustion chamber ahead of the spreading flame) producing a resonant 'spike' (8-9 kHz, the 'ping' in 'pinking'). It is this ‘spike’ that can easily ‘hole’ a piston.

Well Jacob, Presto has beaten me with an excellent answer relating to plug type.

As far as choosing a plug and coil type, the bike designers have already done this work for you, if the engine is standard then no alterations are required, once the plug ignites the mixture the burn will proceed in just the same way regardless of the plug type, you cannot control this by using different plugs.

Increasing the voltage supplied to the plug may look to give a bigger fatter spark but increasing that voltage also reduces the plugs working life by erroding the electrodes faster, all you need is enough spark to start the burn a bigger spark will not give a faster burn and a smaller spark will not cause a slower burn, you appear to be attaching far too much importance to the size of your spark, (Ooh Err Missus).

In this connection it may be useful to put in a word for Iridium plugs. These plugs use a centre electrode with a smaller diameter than plugs with electrodes of most other materials. It follows that the tiny Iridium tip requires less 'spark jump' voltage to work efficiently – as much as 5,000 volts fewer than the normal 30,000 volts required for standard plugs. Ignition is, therefore, improved, with less electrical strain being placed on the ignition system. The (theoretic!) benefits include easier starting, better fuel economy and increased power/acceleration, together with better throttle response. I can't vouch for all those, but I've found that they do help starting in many cases.

Thank you both Presto and Dennis.. this is good usefull info. I have two reasons to ask all these questions in a forum like this. 1) Like yourself, there are so many people with knowledge about old school bikes in this forum, and people who will benefit from getting the info (me included), that I wount miss the opportunity to gather it. 2) When I got my 2003 iron barrel 500 Bullet it was so slow, I had to do something about. Slowly I'm doing the classic mods (free intake, less retricted exhaust, Mr H's tuned head, 8:1 piston etc.). The Lucas coil has all the time given me an orange spark, so I got an aftermarket coil without knowing anything about coils. Of course it went wrong (pitted points, burned rev counter), but the spark was bright blue and what a difference in performance all the way up the revs. Starting was a dream. In the end I had to put back the Lucas coil, but I miss the stronger spark. This is why I'm looking for a new coil (Bosch Blue Coil?). This time without frying anything... Making the start easier isn't all. I'm much more interested in a proper combustion of the fuel using the energy to create power at the rear whell instead of heat in the engine. So plug temp, plug gab, Iridium plugs whatever I need to know, I try to learn. I'm not into the engineering sort of knowledge; just a laymans skill to tune my own bike... Thanks again for sharing your wisdom on bike tech. Please let it stream throu this forum... Jacob

Usually pitted points are a sign of a failed condenser / capacitor. It is recommended that it be changed whenever the points are changed.

Hi Leon... I have tried out several different capacitors in combination with new/dressed points. The result is the same: pitting; only a little, but a clear visual result. Together with other little electrical issues I have located the problem to be my aftermarket coil. To swap it for another known to work coil is not that expensive, so I will give it a try and keep a close watch on points/capacitor. Right now the stock Lucas is back on the bike, and it sorted most issues. I havn't had time to look at points yet, since riding season is only about to begin here in Denmark... Jacob

Boyer preferable to points
Works for me:)