Wednesday, August 20, 2008
Velocette data from a variety of sources....Velocette Lucas automatic advance unit information...
When Veloce Ltd introduced the "alloy" MAC in a spring frame in 1953, they fitted Lucas K1F magnetos to their engines and continued with this until Lucas ceased supply of magnetos in the late 1960s...the exception being the scrambler and some Clubman Venom and Viper Clubmans models ( 1958-62) which had manual advance TT BTH magnetos..likely some old stock held by Veloce.
They also utilised the new Lucas automatic advance unit on these magnetos.
Prior to this, in 1937, in collaboration with BTH, the first automatic advance inits were introduced to the MOV,MAC and MSS range.
Illustrated is one from the Bob Burgess "Red book" on Velocette.
Most Velocette riders assume there is only one Lucas automatic advance unit type used on the engines mentioned above, however there are three... for years I also was ignorant of this fact.
But between 1952-54 on the "alloy" MAC and the newly introduced "alloy" MSS, Lucas automatic advance unit 47522A was fitted...it had an advance range of 16½ ° to 18½ °, which is 33° to 37° at the crankshaft.
This meant that if you timed the ignition at the recommended full advance, in the case of the MAC, 38° BTDC, you had only 1° to 5°BTDC at full retard when you were starting the motorcycle. Worse still the MSS had 36° full advance BTDC so if you were lucky you had 3° of advance BTDC fully retarded or even 0° or less.
Velocette's have a low kickstarter ratio, so when kicking, the engine often didn't easily start. When pushed, and so with a faster engine speed and the auto advance unit starting to advance you had more ignition advance, the bike would start.
Many a carburettor was blamed for the poor starting.
In 1955, Lucas introduced the 47545A unit with 11° to 13° span or 22° to 26° at the crankshaft.
This meant at full retard you had around 15° plus BTDC advance and starting was easier.
You do need a minimum advance to easily start a Velocette....
Both these units used the same advance springs, and regretably I cannot find any information on their characteristics.
With the Venom and Viper introduction in 1956, Lucas introduced another unit, type 47576A with the same advance range as the replaced unit 47545A, but different advance unit springs.
Again there appears to be no information on this spring characteristics.
All we can surmise is that the advance curve varied between these units.
Problem is all the units look identical.....
You would need to discover a brand new, old stock unit in its box, with the part number on it and then you could have the spring tension measured.
Back to the BTH advance unit, and again I have no information on it, but it appears to suffer from wear and often has up to 50° advance span, so timing it on full advance, means for sure the unit will fire after TDC and the bike will be a pig to start.
A Venom I had, fitted with such a unit caused me so much anguish... I tried three carburettors, including a brand new Amal Monobloc before I looked at the auto advance unit, a BTH and removed it , checked its advance span...it was around 40°, so I had no hope.
A replacement Lucas made a better starter of it, but now I utilise fixed ignition timing, with a bronze gear, 34° advance all the time.
The bike starts OK providing you leave the throttle alone and fully closed, allowing the pilot system only to work.
Finally the details of these units...
The teeth are 92 teeth, 32D.P. ( diametrical pitch), 16° helix angle. ( Veloce used other angles but these were in the early 1930s).
The magneto taper...another piece of information which eluded me for years is.. a 1 in 5 taper.
Left click on images to enlarge for better viewing.