plug leads and rev counter

[jesfirth]

i fitted magnecor leads to my griff 500 and it has caused the rev counter to become erratic. refitted the oe leads and the rev counter is fine. what is going on?

thanks

[anonymous-user]

in a word Noise !

assuming your rev counter is connected to the coil negative switched lead, then the new leads either have:

a) a different resistance to the original leads (2 sorts of leads, "resistance" type which use a high resistance carbon stranded conductor to carry the HT to the plugs, and "copper" leads, using normal low resistance copper wires. On top of that, 2 sorts of plugs exist as well, with higher resistance types having an extra resistance built in to the plug top) If the lead resistance is different, the plug firing current (and dV/dT) will be different, resulting is the coils primary flyback voltage being different, and hence possibly affecting the rev counters input clamping

or

b) the new leads have a "arc" somewhere (check they are firmly and correctly fitted at both ends)
(in this case, the high voltage is finding another route out of the leads somewhere, before enough voltage has built up to actually jump the gap in the spark plug. All sparks create a wide band electromagnetic interference, but normally the arc is within the metal combustion chamber which acts as a faraday cage. Any externala arcing (including silly things like dirty spark plug ceramics etc) will cause significant EMC emissions that may affect the rev counter.

Frankly, more likely to be b)

[blitzracing]

The trouble with some types of Magnecores, is the HT resistance path is to low, (they use inductive reluctance to the control spark pulse, completely different to straight DC resistance used on stock plug leads) so the decaying ringing voltages from the coil primary gets picked up as more than one trigger pulse by the rev counter, not so much radio interference. You can get rid of it by adding more resistance to the HT line, typically resistive plugs or even just a normal resistive HT lead between the coil and distributor. Mind you fitting Magnecors (Blue or Red) is a waste of time in the first place, decent resistive leads work perfectly well on this application. Normal mistake of thinking fitting low resistance or "race" leads will produce more power, when it wont. I would like to add the information on the Magnecore site is valid and honest, you just need to understand what its telling you. Unfortunately some re-sellers are not so clued up.

Edited by blitzracing on Friday 30th December 20:42