DON’T MESS WITH TX
The rot set in about 1973 during a ride on a Yamaha TX750 owned then by someone who may not be willing to admit that now.
My initial impression was one of mild surprise – it was good compared with contemporary offerings – the ergonomics, and that word didn’t even exist then, worked.
It fitted me, turned, stopped and went such that the Honda CB750 felt like a cart in comparison, in everyday use at least.
That little germ of interest festered quietly in the rearmost neurons until the late 1980s when one came up in the paper for $80. It wasn’t running and was unloved as most of them were by then. My intention was to race it. Superficially foolish, you say?
The original engine was built with not much scientific panache. I didn’t know much about race engine building then.
For two years I tinkered, exploring the lubrication system by using a die grinder in a set of terminally damaged cases. This deplorable concoction was eventually all blocked off and replaced by external oil lines with a conversion to wet sump because the scavenge pump is bigger than the feed, making it the supply pump instead.
Using this enhanced oil-feed system must have worked. The original bearings are still there in excellent condition after 17 years of part-time competition. That whole motor is still quite serviceable, and the only real failure it had internally was a crank that had been reclaimed by welding, predictably breaking at the edge of the weld. Once the ignition became electronic, it actually ran well.
Standard performance stuff like porting, cam, compression, carbs and exhaust happened because that’s what racers did. In fact, the entire bike developed along the ‘TLAR’ principle – that looks about right.
The new engine is much more sophisticated, using all the lessons learnt in ‘Version one’ plus proper porting, an altered primary drive ratio, eight-plate clutch, Nikasil bores, Vernier cam, 76-degree crank based on the Phil Irving principle to control vibration, and the intake/exhaust systems refined and matched on the dyno.
The dyno is the cheapest horsepower I ever bought, however the extra neddies forced a front brake upgrade from twin standard disc to cast-iron rotors and sintered pads. The only hiccups in this version are limited to an ignition system failure, some broken (stainless) rear spokes and a dropped valve head from an experimental small-stem exhaust valve. That was a big ouch. This one otherwise thrives in the 5-7500rpm bracket with safe over-rev to 8000rpm.
The new crank morphed from a standard one, cut in half and rejoined by Phil Baughan in Adelaide and wears Carillo rods. It still shakes a bit, but the whole vibration profile up to 8000rpm is not unpleasant. The small rocking couple, however, breaks, of all things, plastic number plates.
Jim Hanlon of Superbike Performance is a porting guru and a few dollars spent in his shop reaped rewards. This motor gained Porsche rocker adjusters which are much kinder to the valve stem tips. I kept the same cam profile since it did what I liked, but the exhaust had to be redeveloped entirely. Using Jim’s in a stepped diameter pipe for each cylinder giving a lovely torque curve for lots of mid-range punch.
Both engines are metho burners attempting to keep them cooler. However, across both motors, the clutch is not Yamaha’s finest creation. It copes – just. Modern oils and friction materials help. The primary drive being sped up reduces torque loads but means that the back sprocket is the size of a dinner plate for the hungry.
Chassis-wise, Koni dampers dressed the rear end, followed by YSS later. Roy Bognor painted the tank in 1991 and polished aluminium supported some 10mm, high-density foam to represent a seat. Weight loss came from a diet of wrecking yard parts, from dirt bikes mostly.
The brakes were awful at the start courtesy of a lousy surfacing job, the jetting so far off it was all but unrideable, and I am sure there were plenty of uncharitable comments about my presence on the track.
Despite many years on road bikes, a stack of worn tyres and collapsed footpegs, I had not the faintest clue about track craft, lines, braking points and basic racing manners.
Eventually the refinements came, civility emerged, and I won the odd trophy if the good guys stayed home. Racing a 750 in the unlimited class (to 1300cc) is hardly competitive, but it did raise eyebrows and comment. There is no other out there racing in Australia that I know of.
Along the way, it has been to Lakeside, Broadford, Philip Island, Mac Park lots, Mallala, Wanneroo and a multitude of now defunct street circuits in country Western Australia, scoring a State Championship in Unlimited P4 early in the piece before big name riders and classy machines of large capacity started to dominate the otherwise amateur scene. I don’t claim to be a consummate competitor but I do enjoy a spirited ride.
Only the frame with swingarm, forks, seat, tank and battery box remain of the original incarnation. Everything else has been updated, but it is still a TX750 Yamaha, and reliable to boot.
Looking back, the ancillaries have given more grief than internal engine parts that create motive power. I’m happy with that.
Quite by default, I seem to have become a repository for knowledge of TX innards along the way.
Finally, the spares on the shelf allowed for the restoration of a standard street bike (see pictures) with some of the technology incorporated, and there’s still enough for at least half another bike.
The complete story of Nick’s adventures with the TX750 along with everything you need to know to build and ride a reliable TX750 is available for $27 including postage, via firstname.lastname@example.org or by post at PO Box 403, Littlehampton, SA, 5250.