Hopping Up the Tohatsu Run-Pet
HOPPING UP THE TOHATSU RUN-PET
Getting More Horses Out of the Old Pet
As the famous French philosopher, Jacques Tati, once said, "Absolute power corrupts absolutely, but too little power will get you kicked out of Russia." Tati was talking about the Napoleonic Wars at the time, but he might just as well have been talking about the Tohatsu Run-Pet, a bike with so little horsepower it never even got to Russia. Nevertheless, loyal Tohatsu owners have more cheerful enthusiasm for their brand than a busload
of Amway distributors at a mudslide, and many owners in search of more horsepower for their aging two-stroke tiddlers have written to us, mostly with crayons on the kind of low-grade tablet paper that still has chunks of wood floating in it.
The single most limiting factor in the Run-Pet’s performance is its engine. Displacing only 49cc, the bike is at a clear disadvantage against more modern designs like the CBX or GPzllOO. Many
owners have therefore tried engine swaps, installing such powerplants as the Cummins Cargomaster Diesel or the LeRhone Radial, but these have been largely unsatisfactory. The very heavy Cummins adversely affects the handling of the delicate Run-Pet, as it has fork legs of only 28mm. The LeRhone is lighter, but its rotating cylinders tend to fling hot bean oil on pedestrians at crosswalks and bus stops, and torque reaction to sudden bursts of throt-: tie will send bike and rider cartwheeling down the street like a trash can in a windstorm.
Also, the word “swap” may be something of a misnomer in the case of the
Cummins Diesel, as few truck drivers we approached were actually willing to exchange their diesel engines for the diminutive Tohatsu powerplant. The one truck driver who was willing turned out not to be the proper owner of the truck in question and was, in fact, a hitch-hiker wanted in Maryland on six counts of mail fraud. But that’s another story. Suffice it to say modification to the existing Tohatsu engine is the best route to more power.
Intake and exhaust length tuning are the secret to more output from most twostrokes, and the Tohatsu 50 can also benefit here. Not to bore the reader with a lot of complex numbers and engineering doubletalk we ourselves barely understand, we’ve presented here a simple formula for determining optimum length:
cc est. clutch redline 3.1415927 True swept area2 GNP =lOOvds. ajillion M*
*( M being the boiling point of Velcro)
While 100 yards is the optimum pipe length for the Tohatsu, the engineer must compromise between absolute performance and riding practicality. Accepting a 97 percent loss of peak power, most tuners have therefore settled on an exhaust pipe length of 2 or 3 ft. Best length for the individual bike can be determined by a
computerized pulse-wave microanalyser or, in a pinch, using the sort of soft pine yardstick available from lumberyards with each purchase of 1 2 or more 1-gal. cans of house paint.
Another sure-fire route to more horsepower for the Run-Pet is ignition timing, without which the Tohatsu will simply sit quietly in the garage gathering dust. Normal timing for the Tohatsu is 28 BTDC (Before Top Dead Center) at full advance, and 3° JALWATDC (Just A Little While After Top Dead Center) at idle. Timing should be checked with a timing light. You can purchase one from an auto supply store, or save money and fabricate one yourself from a cast-off snapshot camera and fifty cases of flash cubes. If the engine pings (too much advance) or blows the flaming remains of your carb through the garage door (way too much advance), it’s time to hustle the bike off to a local Tohatsu ignition specialist and see what, exactly, is wrong.
Getting more power from the Tohatsu is never easy or cheap, but long months of backbreaking work, fanatical attention to detail, sweat, blood and a virtually bottomless supply of cold cash can turn that Run-Pet into the kind of bike millions of people think about owning but never do.
NEXT MONTH: Part II; Weber Carb and Overhead Cam Conversion for the Tohatsu Two-Stroke! E3
