Wow, my kind of bike! Let's see a photo of that Cooper thingy.
I've built a whole bunch of bitzas over the years, some horribly unsuccesful like the XL Honda powered Montesa King Scorpion or the Triumph powered M11 Bultaco, and some extremely successful like my current batch of custom framed Yamahas and my DT1 from Hell.
The DT1 started life as a pile of unrelated parts and was built on a challenge to build a competitive DT1 racer from my mate Geoff Eldridge. The heart of the bike is the frame which started life as under an RT1. Initially I started with the usual 2'' lowering of the subframe but early in the piece a friend donated a badly rusted and weather worn OSSA Stilleto roller that I soon noticed was of similar frame design to the Yamaha. Before long I'd cut the OSSA frame in half and grafted the front section to the Yamaha frame. The frame is OSSA from the front engine mounts up, including the steering head to the top rail under the tank. By a stroke of luck the OSSA top rail inside diameter was the same as the Yamaha outside diameter so the OSSA graft slipped over the Yamaha rail like a sock over a cock. It was then noticed that the rake was too far out, a bit like a chopper so I used the well respected old boilermaker raking technique of heating the frame top rail immediately behind the steering head to a cherry red colour and rolling the bike with considerable gusto into a brick wall until the rake looked 'about right'. I then gusseted the area to prevent any further involuntary rake change! The first series DT1 'square' swingarm was lengthened 2".
To simplify the build I used the OSSA tapered roller steering bearings, triple clamps and Betor forks to which I fitted OSSA Phantom dampeners and Ceriani springs. The one mistake I made at the front of the bike was to use the absolutely awful and heavy OSSA front wheel
because it looked cool and had shiny S/S spokes and a nice Akront mudcatcher rim. I tried everything to get that bloody OSSA brake to stop..all to no avail. The tank and seat are stock DT1 and the front mudguard is a genuine Yamaha GYT high mount alloy guard, the rear a fibreglass replica of a stock DT1 'guard I'd bought at a swapmeet. The side panels are from a '75 Maico, painted candy apple tangerine to match the stock '69 tank colour. Rear shocks varied between alloy Konis or Boge-Mulhollands. The rear wheel was originally from an MX250 but I later went over to one from a Mk8 Bultaco Pursang. Despite it probably being ilegal, nobody ever protested it. The engine is where the bike excelled, the cases are early series DT1 fitted with YZ250A gears, DT2MX crank and clutch and DT400 CDI ignition. The cylinder was donated to the cause by this forums Yamaico Pete who informed me years later it'd been ported by road race legend Warren Willing. The GYT head was revamped to YZ250A specs by 'Professor Pete' Reynolds, a porting guru who now builds amps for rock bands. The carby is a 34mm Amal taken straight from a Weslake speedway bike using the stock Weslake settings and jetting. It was planned to alter the jetting to suit later on but it worked perfectly on methanol right out of the box and the carby never had the bowl off after the first ride.
Initially I used a GYT pipe fitted with a Supertrapp muffler but as we developed the bike "Professor Pete' built a sweet 'low-boy' fatty up pipe that crossed over the top of the engine into a muffler made by yours truly. Looking back, the pipe is what really produced the horses from the bike but in today's stricter rule environment it'd probably be considered politically incorrect....sort of like a pre '70 version of Diamond Lil, for those who are aware of that bike.
The bike performed extremely well, far better that the original expectations, often winning the pre 70 class on both motocross and dirt track with various riders including myself. Kevin Flood rode the bike to two runner up Nationals spots behind Boagies CZ in an era of two complete grids of pre 70 bikes when the top riders competed in the class. After a nasty crash caused by a stuck throttle at Pacific Park in which I ended up with a broken elbow, broken sternum, two broken ribs and other cuts and bruises I rarely raced the bike until I took it out for its last blast at the final Condo meeting in 1999. It now rests partially dismantled in the garden shed awaiting a comeback one day. That wonderful engine is to be utilised in my upcoming Hindall DT1 special however, only this time out with a 36mm Mikuni and Avgas.
Looking back without the rose coloured glasses, the bike was in reality an over powered heavy pig of a thing that tried to kill nearly everyone who rode it (
and nearly succeeded on a couple of occasions, hence the name, christened by the late Tony Kirby). However the bike packed a very high cool factor and despite its high state of modification it still looked like a fairly standard DT1....the main aim of the exercise. Prior to building this bike I had little interest in DT1 Yamahas other than an admiration of their place in our sports history but owning this bike changed all that. Today, I've got a mini DT1 fetish thing happening with a restored '68 stocker and four DT1/RT1 based specials languishing in various states of build. I truly believe that a DT1 or RT1 is the cheapest way of building a truly competitive pre '70 class bike. The DT1 is the 55 Chevy of the motocross world.....infinitely hot roddable and way cool.