HORN OF PLENTY
Kawasaki’s F9 Bighorn was a bountiful beast – probably the most powerful two-stroke trail bike of its generation
Massive grins on both road and trail with Kawasaki’s ace 250cc two-stroke dirt bike
As America entered the ’70s, its increasing ranks of recreational off-road riders were spurring on the major Japanese manufacturers to satisfy their demands for bikes. In the ’60s, the factories had paid lip service to the dirt with ‘street scramblers’ that were little more than road bikes with high-level pipes and motocross ’bars. But bikes like Honda’s CL72/77 twins, Yamaha’s YM2C and Suzuki’s TC250 proved the appetite for recreational off-road bikes was there. Then Yamaha upped the ante in 1968 with the DT1 – arguably the first true modern trail bike.
Kawasaki wanted a slice of the action, too. They had launched the first of their F-series trail bike range in 1966 – the 175cc F2R. The F-series improved rapidly, with tubular frames replacing pressed steel, and the range grew to encompass the 125cc F6, the 175cc F3 and F7, and the 250cc F4, F8 and F11 models. Then, in 1970, they went large with the 350cc F5 Bighorn, which later became the F9. And it’s fair to say that these F models were largely responsible for Kawasaki becoming a major player in the US market.
At first, their big-bore, top of the range trail bike wasn’t perfect. The 1970 F5 suffered from a catalogue of woes: wheel spokes and alloy rims broke, the clutch was prone to dragging, the carburation was less than perfect and the gearchange provided almost as many neutrals as it did ratios. On very early bikes, the crankshaft big end could seize. A new crank in mid1970 sorted that and was retro fitted to many earlier bikes under warranty. But, despite its teething troubles, the rotary-valve engine was one of the most advanced single-cylinder two-strokes of its generation. And the quality of most of the cycle parts – including the Hatta forks which provided three positions for the front wheel spindle – was second to none. Kawasaki obviously thought the Bighorn was worth persevering with, and a MKII version – the F9 – appeared in 1972.
Steel rims replaced the crack-prone alloy hoops, stronger, butted spokes were specified and a revised gear
shifter fork banished the false neutrals. And a new 30mm Mikuni carburettor, with its float pivoting sideways in the float bowl, specifically designed for use with the rotary-valve engine, solved the problem of flooding when the bike was leaned over. Small changes provided big rewards and the Bighorn was back.
After that, changes to the Bighorn were few. There were yearly colour scheme changes, indicators became standard equipment on the 1973 F9A model, and the following year conventional forks replaced the adjustable Hatta units and the redesigned silencer included a longer tailpiece. The Bighorn’s final year was 1975, after which Kawasaki’s new KE range (and later the four-stroke KLS) became Kawasaki’s contenders in the trail bike sector. The Bighorn was never sold in the UK, so not many British riders got the chance to sample what was probably the most powerful two-stroke trail bike of its generation. A shame, as the rotary-valve engine is a gem.
The key to the class-leading power of the F9 engine is the ability of the rotary inlet valve to deliver asymmetrical inlet timing. And while the shape of the F9’s rotary disc is designed to produce relatively mild inlet timing to give plenty of grunt at the bottom end, it is easily tuned to give more top end. Kawasaki’s Power Pack tuning kit included a barrel with revised porting, two-ring piston, larger 32mm carburettor, an expansion-chamber exhaust and a more radical disc valve. It pumped out a claimed 45bhp – a massive hike from the stock machine’s 33bhp, though those extra horses doubtless came at the expense of the tractor-like 28lb.ft of torque at 5500rpm.
That extra performance (and even more in the hands of top tuners) was eagerly seized upon by weekend motocross racers, flat-track riders and even road racers
(see page 52), but the Bighorn packed a significant punch straight out of the crate. It sold well, so there’s a pretty healthy supply of good used parts and enough owners to foster a cottage industry devoted to keeping Bighorns on song. Klemm Vintage in Arizona (klemmvintage.com), who have been racing and riding Bighorns for more than 40 years, are the F5/F9 gurus and a useful source of information and advice. They offer many upgrades and some – like remachined cylinder heads – may be cost effective even to committed British Bighorners.
In any case, a stock F9 model can be a pretty reliable bike anyway. If you dare to be different, there shouldn’t be many mechanical maladies to keep you from enjoying the Bighorn experience. To find out whether it’s worth it, David Silver has loaned us a 1974 F9 and given us free rein to take it out on both road and trail. It’s one of two Bighorns he has in stock at the moment – both lowmileage bikes – and this one has under 7500 miles on the clock. Apart from an ugly home-made fork brace (easily removed) and equally incongruous, aftermarket pillion footrests, it looks to be in very original condition, too. A bit of local knowledge from mechanic Tom sees me heading out towards Sizewell and a byway through the sandy flatlands that skirt the coast. First, though, getting there gives me the chance to evaluate the Bighorn’s potential on the Tarmac and I’m soon smiling at a combination of abundant midrange torque that renders the slick five-speed gearbox largely unnecessary once the engine is spinning over 4500rpm, and superb road manners from the sturdy twin-downtube frame.
The brakes are excellent, too – especially the 150mm single-leading-shoe front (the rear is the same size). Trail bike brakes aren’t usually noted for their stopping power on the hard stuff, but this bike is an exception. There’s plenty of feel at the lever and it hauls up the 120kg (265lb) Bighorn like it’s been lassoed from behind, while the rear brake is equally efficient.
There’s a compact but spacious feel to the riding position, though the seat is probably a tad too short for anything but the most occasional (and small of stature) pillion passenger. At 1397mm, the wheelbase is middleof-the-road, but the Bighorn feels nimble and stable at the same time. Despite the 21in front wheel, it turns in pretty quickly and its slim profile means it’s ideal for flicking through the tight turns and rolling undulations of the Suffolk back lanes.
At an indicated 60mph, the engine feels unstressed and more than tolerably smooth. Period road tests in the American motorcycle press put the Bighorn’s top speed at 81-84mph, so it should be perfectly capable of keeping up with traffic here. For a forty-something-year-old trail bike, it makes a great road bike.
Given that the road is where most trail bikes spend most of their time, that’s a feather in the Bighorn’s cap. But what if you want a genuine all-rounder – a bike that can handle a bit of exploring around the byways and fooling around off road? Following Tom’s directions,
‘FOR A FORTY-SOMETHINGYEAR-OLD TRAIL BIKE, IT MAKES A GREAT ROAD BIKE’
I swing off the road and follow the ‘byway’ waymarking arrow up a promising-looking dirt road. After half a mile or so, the hard-packed track gives way to a proper, unsurfaced trail. This is more like it.
Or it would be, if it wasn’t for the limitations of the ancient Dunlop Trials Universal tyres. They’re heavily worn, stiff with age and inflated to road pressures (naturally David Silver would replace them for an ‘on the road’ sale). The track’s surface is a couple of inches of loose, dry sand sitting on top of hard-packed earth. Cornering is like riding on marbles and I have to take things very steady indeed to keep upright. The front end just wants to plough straight on in corners and dialling in a bit more throttle just spins up the rear wheel too much. In a straight line, it’s great fun winding on the power and spinning up the back end, easing my weight back to hook up what little drive the rear tyre can transmit, but I long for some better tyres (and a more loamy surface) to be able to exploit the Bighorns delightfully torquey thrust.
The suspension works considerably better than the tyres as it hammers over the hard-packed bumps under the sand. I’m convinced the F9 would make a great genuine trail bike if it was fitted with some decent rubber – as well as lower tyre pressures. I love the controllable power delivery, the sweet gearbox and the feel from the brakes. I also like being able to plant a foot on the deck easily when things threaten to get a bit out of hand and I love the way I can roll off the throttle and let the engine gasp along in a high ratio to regain forward motion when the tyres give up.
Back out on the road, the high top gear makes cruising back to the Silver emporium a relaxed affair. The engine feels unhurried, the exhaust note is a muted burble and world feels like a great place to be in the late summer sun. There are probably better classic off roaders than the F9 – and there are better road bikes, too. But as a genuine go-anywhere, do-anything bike, the Bighorn takes some beating.
Turn over for the Bighorn 350 racer...