Most Common Two-Stroke Engine Bike Myths Debunked for Yamaha RD350 Owners

Two-stroke engine bikes like the Yamaha RD350 have a cult following, but they’re also surrounded by myths that can mislead even seasoned riders. From break-in misconceptions to wild tuning claims, these tall tales persist in garages and forums. Let’s bust the most common myths with straight facts, so you can keep your RD350 running true without falling for old-school folklore.

Two-Stroke Engine Break-In Myths for Yamaha RD350

BREAK YOUR ENGINE IN FAST AND IT’LL ALWAYS BE FAST
The grain of truth here is that if the piston rings are never seated against the cylinder walls by a proper break-in, they won’t seal and the engine will never develop full power. On the other hand, how fast should break-in be? Do you take out your new zero-mile bike up the interstate? No. A normal break-in, as described in the maker’s manual, and performed with understanding, is all that’s needed.

No matter how fine the surfaces produced in manufacturing on cylinder walls and crankpins, they are like the Alps in comparison with the much finer profiles that proper break-in will create. Break-in is the final machining operation. The oil films that will support moving parts in operation may be as thin as 1.5 microns (.00006 inch), so to avoid piercing these’ films, the Alps of manufacturing must be scrubbed down to an even lower height by the process we call break-in.

A normal break-in calls for a period (usually 500 to 1000 miles) of controlled operation in which the engine is never steadily, or heavily loaded. You would not, for example, climb long hills at full throttle and low rpm. The idea of break-in is to impose short periods of various loads, separated by recovery periods. While the Alps are at work, knocking each other down, wear particles and heat are produced. The recovery periods allow the heat to dissipate and allow the particles to flush out from between surfaces and be swept away to the oil filter. Once the break-in is complete, the engine oil and filter are changed. That’s it.

Cooling Myths About Yamaha RD350 Two-Stroke Engines

ALL ENGINES NEED MORE COOLING
This one is history. Remember the guys with all the scoops and ducts, the black paint, and the extra, welded-on fins? Back when most engines were air-cooled, there wasn’t enough cooling to allow continuous full-power operation without burning up, so savvy tuners over-jetted, using the extra fuel’s heat of evaporation as an internal engine coolant, and to limit combustion flame temperature. Jetting to the chemically correct mixture (at which every molecule of oxygen in the air charge i1-reacted with hydrogen or carbon from the fuel, leaving no extra fuel unburned) gives maximum power and maximum heat release, but on that jetting, poorly cooled engines would run hard for a couple of laps, then cook. That heat would cause the intake of air to expand and lose density, thereby causing power to fall off. Over jetting, by limiting this heat, allowed engines to lose less power. This is why. even today, air-cooled 500 MX engines sound so rough; they are over-jetted to the point of misfiring when cold so they’ll keep more of their power when they’re hot. More cooling would help these engines.

Modern liquid-cooled engines can be overcooled to the point where they lose power simply because too much heat is being absorbed by cool metal surfaces in the combustion chamber, leaving too little heat in the combustion gases to generate pressure to act on the piston. Tuners learn by experimenting with what coolant temperature gives the best performance. This is why you’ll often see race bikes with their radiators partially taped over when going out for practice on a cold morning. Conversely, you’ll also notice that Yoshimura has decided its Suzuki 750 Superbikes need more cooling than they have, and has provided two extra oil radiators.

Performance Myths for Yamaha RD350 Two-Stroke Bikes

GOTTA SCREAM YOUR ENGINE ALL THE TIME TO GET REAL PERFORMANCE.
The fragment of truth here is that the rpm of the torque and horsepower peaks is usually higher than it’s appropriate to use in most street or highway situations; you use less power than you have. Yes, a racer keeps his tach in the region of peak torque and horsepower to get the most from his engine. But some street riders (and even a few racers) assume that if a little is good, too much ought to be just enough. You hear them winding their poor, suffering engines far into the red zone, above the torque and horsepower peaks, above the good performance-just to hear the noise. And they are going slower than the rider who knows where the peaks are.

To make power at higher rpm, the engine must be given the ability to breathe up there with longer cam timings, refined porting, perhaps bigger carbs, and a suitable exhaust system.

Clearance Myths in Yamaha RD350 Two-Stroke Tuning

BUILD IT REAL LOOSE. THERE’S BIG POWER IN BIG CLEARANCES.
It’s true that racing engines often have larger clearances than street engines, but it’s wrong to jump to the conclusion that this is done to cut friction, A racing engine is on the full throttle more than a street engine, so its piston temperature is higher and its pistons expand more. Therefore they need some extra clearance- Street engines must be quiet, which calls for low-expansion cast pistons to run at close clearance. The extra stress put on a race motor often calls for the extra strength of forged pistons, but forging alloys expand more with heat than do casting alloys, and so require more clearance. At operating temperature, racing pistons fit as closely as street pistons: they must in order to present their rings squarely and stably to the cylinder walls. A loose, rattling fit is an invitation to the loss of the ring seal. Also, pistons are cooled by close contact with the cylinder walls; a loose fit means hotter-running pistons.

Torque Myths for Two-Stroke Engines Like Yamaha RD350

THE FEWER THE CYLINDERS, THE GREATER THE TORQUE.
This one derives partly from Number 6 above, and partly from the different riding qualities of different engines. A big single seems to have impressive torque when you ride it, but on the dyno, a twin or four of the same displacement almost always has as much or more. It’s just that a big single feels so torquey. The feeling comes from the flywheel mass, and from the ability of a slow-turning engine to produce thrust without rpm. A single needs big flywheels to idle, and in rough going, those heavy flywheels may carry it through where a twin or four might bog. A single usually has moderate to small valve sizes, so its torque is given at low rpm. That being so, it also usually has very conservative cam timings-timings that give full torque at some wonderfully low, putt-putt speed like 3500 rpm. On a four, snapping open the grip at 3500 produces nothing but a cloud of fuel fog from the carbs and a sickly drone from the exhaust.

Sensibly designed engines take advantage of their natural strong points. A multi-cylinder engine has a lot of potential valve area, and so is usually designed in such a way that its torque and power are given at higher rpm. A single, with far less cylinder-head real estate in which to set valves, is, therefore (usually, but not always) designed to deliver its torque at the low speeds that small valves favor.

Flywheel Myths in Yamaha RD350 Two-Stroke Performance

MAN. I CUT MY FLYWHEELS PAPER-THIN, SO NOW MY ENGINE REVS UP FANTASTIC.
It’s true that during acceleration, extra power is consumed in speeding up moving parts. Between two otherwise identical bikes, the one with light flywheels will usually have some edge in acceleration, but not in top speed This process can be taken too far. When Honda raced its RC166 six-cylinder 250 back in the mid-1960s, and again when it tried to race its NR500 oval-piston four-stroke in the early 1980s, crank mass was reduced practically to nothing. The result? These engines were tricky to ride because they could Stall between downshifts, or indeed any time the clutch was pulled.

Recently some enterprising engine fanatics decided to build a V-twin out of car-engine components, but they left out the flywheel. Result? Their engine had such violent variations in crank speed-owing to a lack of rotating inertia to smooth it out that it often stalled on the dyno, or tossed its valves. Although it had the potential for nearly 100 horsepower, it developed only a third of that in tests.

Any piston engine needs a flywheel capable of storing enough energy at idle speed to compress the charge on the next cycle without stalling. If you plan to run at higher rpm, you can get away with less flywheel, because energy storage in a flywheel increases with rpm. But you can go too far-as Honda once did.