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| Royal Enfield shocked show-goers with its four-valve motorcycle. |
Royal Enfield's line-up for 1931 made headlines at the Olympia Motor Cycle Exhibition.
"Many New Models," The Motor Cycle magazine reported, "including a four-valve single."
This was big news. Everyone recognized that a four-valve motor was the epitome of performance, prestige and, usually, price. On price, Royal Enfield had pleasant news to announce.
"A last-minute surprise was the introduction of a four-valve model of 488cc," The Motorcycle wrote. Besides the exciting four-valve motor, it had a chrome tank, and then fashionable dual exhaust ports (which gave it a fat, gleaming silencer on both sides, making it look like a twin).
"It is remarkable value for 50 pounds with a three-speed gearbox, while for an extra two pounds a four-speed box can be supplied."
Royal Enfield wasn't afraid to put its hot new machine into the hands of the motoring press. A Motor Cycling magazine correspondent who signed himself STAXTON wrote this, in April, 1931:
"I have never had a machine in the Isle of Man that would tackle the Mountain climb from Ramsey so fast as would the four-valve 488 Enfield that I rode this year. That is the chief memory that the machine left with me.
"It would bring me up to the Gooseneck... and then it would roar all the way up to the telephone box at East Snaefell Gate in a manner that no ordinary 'same as you can buy' production model would have done some little while ago....
"The Royal Enfield JF31, as this model is designated, is definitely a fast man's motor. And 50 pounds is not a lot to have to pay for it."
Royal Enfield was enthusiastic enough about its 1931 four-valve motorcycle to continue developing it in the years to follow. But development would follow a wayward road. Royal Enfield struggled to keep its muscular motorcycle affordable. This would prove difficult as the Depression deepened, and sales of all new motorcycles plummeted.
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| An improved four-valve for 1932. (From The Motorcycle) |
FOR 1932: The Model JF was replaced by the Model LF, with "Many Detail Improvements," The Motor Cycle reported on Sept. 17, 1931. The press liked that the push-rod tubes were now cast with the cylinder.
The cylinder head was modified to add "much more metal round the central sparking-plug hole, and the ports have been altered so as to obtain a rather better shape," Motor Cycling reported. The unstated reason for this would have been to reduce cracking between ports.
Prices actually went down for 1932, to just over 47 pounds. But available power was worth boasting about. Royal Enfield didn't usually release horsepower charts, but this 1932 chart was an exception.
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| Four-valve horsepower, rpm soar in 1932 chart. (From Peter Miller's book "Royal Enfield, The Early History." |
For 1933: The four-valve motorcycle was now called the "Bullet" with its 488cc motor frequently referred to as a "500." Motor Cycling got an example of it up to 78 mph in a road test. Good news, but with the economy sinking the motoring press gave most attention to Royal Enfield's less expensive models, including two-valve Bullets of 350 and 250cc.
Again, the press noted that this year's four valve "has a much strengthened head." That cracking problem still?
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| Why would Royal Enfield offer an "extra" spark plug hole? (From Motor Cycling) |
And this is very odd: The article noted that the four-valve's centrally located spark plug is a 14mm, but "provision is also made for a 19mm plug to be screwed into the side of the cylinder head in the more orthodox position, which would prove useful if the 14mm plug were to fail and another of that size be unavailable." Who ever heard of such a thing?
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| The 1934 four-valve motor featured elaborate fins. (From The Motorcycle) |
For 1934: The four-valve motor got a redesigned cylinder head with increased finning and a dramatically finned crank case. It looked great. But was the increased cooling effect of the finning yet another effort to stave off disaster?
For 1935: A real switch. The Motorcycle reported that the 488cc motor would have only three valves for 1935 -- two inlet, one exhaust. Improved lubrication, thanks to fully enclosed valves, was given as the reason.
"Justification for this head-form is provided by the performance, which is almost identical with that of the earlier four-valve engine, and by the increase in silence, reliability and neatness of the new engine," the magazine advised, on Sept. 17, 1934.
It would be called the Model LO.
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| Three-valve motor eliminated the open valves of the four-valve layout. (From The Motorcycle) |
Price was now close to 60 pounds. But not for long. The price was about to drop radically, at the cost of some performance.
By March, 1935, the three-valve had become the awkwardly named "Model LO2." The Motorcycle and Cycle Trader reported that it came with a "not so highly tuned" three-valve motor and a new price -- just under 40 pounds. Maybe performance wasn't worth paying for?
Or was it? The bargain three-valve was a one-year wonder, dropped for 1936.
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| The hot four-valve was back for 1936, with a straight-up motor. (From The Motorcycle) |
For 1936: Incredibly, the "highly tuned" four-valve was back for 1936, once again dubbed the "JF" (its original name way back in 1931). The price was back up to just under 60 pounds.
Anyone confused yet?
Previous multi-valve motors had all been "slopers," but the new JF four-valve motor sat straight up.
What about those fully enclosed valves, whose lubrication was so approved of in the LO2? Never mind. The four valves of the JF were exposed, as they always had been, and thus needed to be lubed with a grease gun.
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| The 1937 four-valve in its final form; the head would be an option in 1938. (From Motor Cycling) |
Onward: The four-valve motorcycle would continue in 1937.
In his informative article in The Gun magazine of the Royal Enfield Owners Club, Mick Lemon notes that examples of Royal Enfield's four-valve road burners of the 1930s still exist, and are appreciated by collectors.
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