Ignition Timing on th Mkviii KTT, by Peter Miles
The published informationon the Mkviii gives an ignition setting of 30 degrees BTDC. The figure for a Mkvii is 32 degrees on alchohol or petrol \ benzole but with 30 degrees with pool petrol.
I have found these settings to be too retarded and have used 36 degrees. This was applied to a motor with a 'pool' piston but modified to give a compression ratio of around 10 : 1. If using the machine for competition, some further experimentation would be adviseable to suit modern fuels.
The theoretical optimum is to have as little advance as possible, consistent with maximum power out put. Hence, in an ideal world, a session on a dyno would be useful to arrive at the correct setting for your motor.
When fitting the magneto drive sprockets ,fit the magneto one first and tighten it. Then, having set the magneto points in position and using a light spring to remove the slack on the magneto chain, fit the lower sprocket and tighten. If the timing needs re-adjusting, the lower sprocket can be taken off by loosening the nut and tapping the drive shaft backwards into the casing - there is enough space internally to avoid the need for an extractor. Don't try this if you are 'blessed' with the additional scavenge pump !
The advance \ retard lever fitted by Veloce is needed on the Mkviii, a little retard helping starting from cold.
Adjustable Magneto sprockets, made by Ian Johns November 2006
Ian Johns has made a batch of adjustable magneto sprockets. They are made in two pieces, the hub taper fitting onto the magneto armature and fixed with a 3\8" BSF nut, as per normal. The sprocket is spigotted onto the hub and held in place by four screws. The fixing holes in the sprocket are elongated to give twelve degrees of movement. If the sprocket is set with the scews in the centre of the slots, it will allow six degrees of movement in either direction - enough to correct any normal innacuracy when tightening the unit onto the magneto.
They will also be very useful to those who wish to experiment with timing as it will be quick and easy to make adjustments.
These sprocket assemblies are on the spares pages, along with other useful parts the Ian makes for KTT's
Note : the sprocket is reversible on the spigot to give correct alignment for either KTT's with platform magnetos or KSS's with flange mounted magnetos.
On the subject of ignition timing, after some experimentation, I arrived at exactly the same conclusion - that 36 degrees advance is about right. My motors have compression ratios of 10.5 : 1 using the domed Mkviii piston and I run the on a 50 : 50 mix of Avgas and super unleaded. ( I suspect that it is not necessary to use so much Avgas ) The motors run sweetly with no pinking or knocking and with a reverse cone on the mega, carburate cleanly all the way through.
Mk VIII KTT
IGNITION TIMING BY MEASUREMENT FROM TDC
These are my calculations for converting degrees before TDC on a KTT into a linear measurement from TDC. I prefer to use a degree disc rather than measurement.
Conrod length 165.10 Crank throw 40.50
|Crank rotation (degrees)||Increments (mm)||Dist below (tdc)|