George McKenzie produced motorcycles from 1921 to 1925.
1921 The McKenzie was produced as an ultra lightweight/featherweight, in an attempt to provide a machine suitable for everyday transport. It weighed a mere 80lb and had an open frame, unsprung forks, direct-belt drive and a 170cc two-stroke Hobart engine.
1923 As the model was a success and large numbers had been sold, the basic model continued into that year. A new two-speed model was now added, with Druid forks.
1924 That new model was uprated in a new design and given a new 147cc Aza two-stroke engine, Albion two-speed chain-cum-belt drive, clutch and kick-starter. The open-frame miniature also had a two-speed gear. The business faltered during that year.
1925 After restructuring the company, George McKenzie secured the goodwill and continued production with the same two-model range, fitting the lightweight with a larger Aza engine. It was the last year of manufacture for the McKenzie.
The McKenzie by John McVey
Following World War One, Mr G. McKenzie marketed an "ultralight motor cycle" as the type was then known. From an engineering point of view, there does not appear to have been anything new about the McKenzie design, the pre-first world war Lady's Humber in the Coventry Museum of Transport being virtually identical. McKenzie planned to sell his bikes through franchised cycle shops, and bring cheap motorised transport to the masses. To quote Andy Barber "He saw himself as the Henry Ford of the motorcycle."
The engine is also of Hobart make. Bore and stroke, unusually for that time, are both 60mm, giving a capacity of 170cc. With the whole machine weighing only 75lbs you might expect a lively performance but a road test of 1923 mentions "the maximum being about 28mph. At the legal limit, the McKenzie is very comfortable and can be ridden 'hands off'."
The deflector type piston is cast iron with three rings in two grooves: two in one groove at the top. It has plain bronze bushes for small end, big end and mains. The crankshaft is most unusual, being in two parts. The left hand part drives a large flywheel with integral belt pulley, which, by means of a leather belt, drives the back wheel. The right hand bob weight has an oblong slot that engages with flats on the end of the crank-pin, thus drive is taken to a 3 gear train that operates the 'Baby Fellows' magneto that is mounted behind the engine.
This is the first engine I've come across that was actually made with a broken crank, but of course many engines have a cantilever crank with only one flywheel, such as the Power Pak and the Piatti scooter, but at least on those engines some attempt is made to provide support by having a long shaft with bearings as far apart as possible. Not so the McKenzie, which has only a bronze bush about 2.5 inches long. My engine has worn out main bearings while the barrel and piston seem near perfect. The carburettor is a 'Wex' lightweight but I do not have any data on this instrument.
Andrew Barber's McKenzie is the early type with fully open frame as introduced in 1921. These seem to have had a weakness in the frame as it was redesigned and strengthened by 1923. The engine was lowered and an extra horizontal strut was inserted. Also in 1923, a counter-shaft clutch was offered and a two-speed gearbox for £5 15s and £7 15s respectively. These extras were fitted between the engine and the front wheel, with a chain primary drive and a belt to the rear wheel as before. Most of these components were also used in the McKenzie Roadster, which had a conventional motorcycle frame with a round tank. The weight had crept up to 120lbs by this time, so the performance couldn't have been too frightening.
When the McKenzie Popular was introduced in 1921, Hobart was said to have 38 years of experience of making cycles.
What exactly became of the alliance of Mr McKenzie and the Hobart Cycle Co I do not know, but I have here an advertisement for a Hobart Universal light motorcycle at £21, which is virtually identical to a (26 guinea) McKenzie. Let's face it; it is a McKenzie. I'm told that both firms ceased trading at about this time but have no written source.
This was probably the last incarnation of the same old 60 - 60 two-stroke engine. This time in a motor cycle frame it was known as a Wee MacGregor or Wee Mac. 
It was made at the Coventry Motor Cycle Co of Wellington Street, Coventry by Barbary and Downes, former employees of the Hobart Motor Cycle Co. They ceased trading around 1924 or 1925 and that, as they say, was that. 
McKenzie's last design appears to have been intended for the 1924 season. This was a 147cc lightweight described by 'Motor Cycle' magazine as a "thoroughly practical miniature" in October 1923. This was clearly an all new design with a new engine and it looks very neat and 'late vintage' with a large wedge shaped tank. A loop frame lady's model is mentioned but not illustrated and I wonder if any were actually made.
1. In France they were marketed as the Mac Gregor.
2. Coventry Motor Cycle Co, aka Coventry Bicycles, continued to trade well into the '30s.
Sources: icenicam.ukfsn.org, Graces Guide, et al.
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