Two-Stroke Tuning Basics for Yamaha RD350 Motorcycles

Changing the power band of your dirt bike engine is simple when you know the basics. A myriad of different aftermarket accessories is available for you to custom-tune your bike to better suit your needs.

The most common mistake is to choose the wrong combination of engine components, making the engine run worse than stock. Use this as a guide to inform yourself on how changes in engine components can alter the powerband of your bike’s engine. Use the guide at the end of the chapter to map out your strategy for changing engine components to create the perfect power band.

Two-Stroke Engine Principles for Yamaha RD350 Tuning

TWO-STROKE PRINCIPLES

Although a two-stroke engine has fewer moving parts than a four-stroke engine, a two-stroke is a complex engine because it relies on gas dynamics. There are different phases taking place in the crankcase and in the cylinder bore at the same time. That is how a two-stroke engine completes a power cycle in only 360 degrees of crankshaft rotation compared to a four-stroke engine which requires 720 degrees of crankshaft rotation to complete one power cycle. These four drawings give an explanation of how a two-stroke engine works.

1) Starting with the piston at the top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close. At about 90 degrees after TDC the exhaust port opens, ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.

2) Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The gasses travel up the back side of the cylinder and loop around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke.

It is critical that the burnt gasses are scavenged from the combustion chamber, in order to make room for as much unburned gasses as possible. That is the key to making more power in a two-stroke engine. The more unburned gasses you can squeeze into the combustion chamber, the more power the engine will produce.

Now the loop of unburned mixture gasses has traveled into the exhaust pipe’s header section. The gasses aren’t lost because a compression pressure wave has reflected from the end of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the port. This is the unique super-charging effect of two-stroke engines.

The main advantage of two-stroke engines is that they can combust more volume of fuel/air mixture than the swept volume of the engine. Example: A 125cc four-stroke engine combusts about 110cc of F/A gasses, but a 125cc two-stroke engine combusts about 180cc of F/A gasses.

3) Now the crankshaft has rotated past the bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The compression wave reflected from the exhaust pipe is packing the unburned gasses back in through the exhaust port as the piston closes off the port to start the compression phase.

In the crankcase, the pressure is below atmospheric, producing a vacuum and allowing a fresh charge of unburned mixture gasses to flow through the reed valve into the crankcase.

4) The unburned mixture gasses are compressed and just before the piston reaches TDC, the ignition system discharges a spark, causing the gasses to ignite and start the process all over again.

Cylinder Porting Techniques for Yamaha RD350 Two-Stroke Performance

CYLINDER PORTING

The cylinder ports are designed to produce a certain power characteristic over a fairly narrow rpm band. Porting or tuning is a metal machining process performed on the cylinder ports (exhaust & transfers) that alters the timing, area size, and angles of the ports in order to adjust the power band to better suit the rider’s demands.

For example, a veteran trail rider riding an RM250 in the Rocky Mountain region of the USA will need to adjust the power band for more low-end power because of the steep hill climbs and the lower air density of higher altitudes.

The only way to determine what changes will be needed to the engine is by measuring and calculating the stock engine’s specifications. The most critical measurement is termed port-time-area. This term is a calculation of a port’s size area and timing in relation to the displacement of the engine and the rpm.

Experienced tuners know what the port-time-area values of the exhaust and transfer ports should be for an engine used for a particular purpose.

In general, if a tuner wants to adjust the engine’s power band for more low to mid-range, he would do the following things:

• Turn down the cylinder base on a lathe to increase the effective stroke (distance from TDC to exhaust port opening). This also retards the exhaust port timing, shortens the duration, and increases the compression ratio.
• Narrow and re-angle the transfer ports with epoxy to reduce the port time area for an rpm peak of 7,000 rpm.
• Re-angle the rear transfer ports so they oppose each other rather than pointing forward to the exhaust port. This changes the loop scavenging flow pattern to improve scavenging efficiency at low to mid rpm (2,000 to 5,000 rpm).

An expert rider racing motocross in England would want to adjust the power band of an RM250 for more mid to top-end power. The cylinder would need to be tuned radically different than for trail riding.

Here is an example: The exhaust port would have to be raised and widened to change the port-time-area peak for a higher rpm (9,000 rpm). For either of these cylinder modifications to be effective, other engine components would also need to be changed to get the desired tuning effect.

Cylinder Head Adjustments for Yamaha RD350 Two-Stroke Power

CYLINDER HEAD

Cylinder heads can be reshaped to change the power band. Generally speaking:

• A cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of the bore area), combined with a compression ratio of 9:1, is ideally suited for low to mid-range power.
• A cylinder head with a wide, shallow chamber and a narrow squish band (35–45% of bore area), and a compression ratio of 8:1, is ideally suited for high rpm power.

There are many reasons why a particular head design works for certain types of racing. For example, a head with a wide squish band and a high compression ratio will generate high turbulence in the combustion chamber. This turbulence is termed Maximum Squish Velocity (MSV), rated in meters per second (m/s).

A cylinder head designed for supercross should have an MSV rating of 28 m/s. Computer design software is used to calculate the MSV for head designs. In the model tuning tips chapters of this book, all the head specs quoted have MSV ratings designed for the intended power band changes.

Crankshaft Tuning Options for Yamaha RD350 Two-Stroke Engines

CRANKSHAFT

There are two popular mods hop-up companies are doing to crankshafts: stroking and turbo-vaning.

Stroking means increasing the distance from the crank center to the big end pin center. There are two techniques:

• Weld old hole and re-drill a new big end pin hole (labor-intensive).
• Install an offset big end pin (cheap, non-permanent, and quick to change).

In general, increasing the stroke of a crankshaft boosts mid-range power but decreases the engine’s rpm peak.

The term “Turbo-Crank” refers to a modification where scoops are fastened to the crank to improve volumetric efficiency. Every decade, some hop-up shop revives this idea with trendy names and overhyped promises.

These crank modifications can cause oil to be directed away from the connecting rod, and the vanes may detach at high rpm, causing catastrophic engine damage. My advice: don’t waste the $750!

Carburetor Tuning for Yamaha RD350 Two-Stroke Powerbands

CARBURETOR

In general:

• A small-diameter carburetor provides high velocity and good flow for low to mid-rpm power bands.
• A large-diameter carburetor works better for high-rpm power bands.

Recommended sizes:

• 125cc engines: 34mm for supercross/enduro; 36mm or 38mm for fast mx tracks.
• 250cc engines: 36mm for low to mid-power bands; 39.5mm for top-end power bands.

Recent trends include airfoils and rifle boring for carbs, designed to improve airflow at low throttle openings. Some companies sell carb inserts to change the diameter. Typically, a carb is over-bored (e.g., from 38mm to 39.5mm), and inserts allow restriction back to 36mm or 38mm as needed.

Reed Valve Tuning for Yamaha RD350 Two-Stroke Efficiency

REED VALVE

Think of a reed valve like a carburetor: bigger valves with large flow areas work best for high rpm power bands.

• 6+ petals: used for high-rpm engines.
• 4 petals: best for strong low-end and mid-range power.

Other factors to consider:

• Angle: 30° for low to mid rpm; 45° for high rpm.
• Material: Carbon fiber (stiff, resists flutter at high rpm); Fiberglass (low spring tension, responsive at low rpm, but flutters at high rpm).
• Petal thickness: Thicker for high rpm, thinner for low rpm.

Boyesen Dual Stage reeds use a large thick base reed with a smaller thin reed on top. This widens the rpm range: thin reeds respond to low rpm, thick ones handle high rpm without flutter.

Boyesen RAD valve redistributes gas flow evenly in bikes with offset carbs (e.g., single rear shock models). It improves overall power delivery.

Polini Super Valve (Italy) uses vertically stacked mini reeds instead of horizontal ones. Excellent for enduro due to improved throttle response. Tests show superiority during power shifting, though peak power remains similar to conventional valves. Imported to the U.S. by Moto Italia in Maine.

Exhaust Pipe Design for Yamaha RD350 Two-Stroke Power

EXHAUST PIPE

The exhaust pipe harnesses the energy of combustion pressure waves. The diameter and length of its five main sections are critical to shaping the power band:

1. Head pipe
2. Diffuser cone
3. Dwell
4. Baffle cone
5. Stinger

Aftermarket pipes typically shift the power band higher in the rpm range. Most are designed for stock cylinders — not tuned ones.

Companies like MOTOWERKS use custom computer design to fabricate pipes based on cylinder specs and target power bands.

Silencer and Flywheel Tuning for Yamaha RD350 Two-Stroke Bikes

SILENCER

Silencers vary in shape and size:

• Long, small-diameter silencers: Enhance low to mid power by increasing bleed-down pressure in the pipe.
• Short, large-diameter silencers: Provide optimal bleed-down pressure for high-rpm engines.

Too much pressure at high rpm increases piston crown temperature and may cause seizure.

FLYWHEEL WEIGHTS

Flywheel weights improve engine tractability at low to mid RPMs. Two types:

• Weld-on: Permanent; cannot come off (e.g., A-Loop).
• Thread-on: Attach to flywheel’s left-hand threads (e.g., Steahly). Only usable if threads are in perfect condition.

External rotor flywheels have greater inertia (smoother power delivery). Internal rotor flywheels offer quicker throttle response.

Ignition Timing Effects on Yamaha RD350 Two-Stroke Performance

AFFECTS OF THE IGNITION TIMING

Changes in static ignition timing affect the power band as follows:

Advancing the timing: Makes the power band hit harder in the mid-range but fall flat on the top end. It allows the flame front adequate time to build pressure ("hit"). However, too much advance causes high cylinder pressure, leading to pumping losses and pinging at high rpm.

Retarding the timing: Results in a smoother mid-range and more top-end over-rev. Less pressure rise at mid rpm prevents detonation. At high rpm, it shifts heat from cylinder to pipe, increasing wave velocity and improving synchronization between returning compression wave and piston speed — effectively extending the rpm peak.

Adjusting Ignition Timing on Yamaha RD350 Two-Stroke Engines

HOW TO ADJUST THE TIMING

Rotate the stator plate relative to the crankcases to change timing. Most stator plates have three marks near mounting holes:

• Center: standard timing.
• Rotate clockwise (with flywheel rotation): advances timing.
• Rotate counterclockwise: retards timing.

Never rotate more than 0.028 in (0.7 mm) past the standard mark.

Kawasaki and Yamaha stators are marked. Honda stators have a riveted sheet metal tab limiting position. To adjust Honda timing, file the tab with a 1/4" rat-tail file.

Aftermarket Ignition Upgrades for Yamaha RD350 Two-Strokes

AFTERMARKET IGNITIONS

The latest innovation: internal rotors with bolt-on discs that act as flywheel weights. PVL (Germany) offers these for modern Japanese dirt bikes, with interchangeable discs to tune inertia for track conditions.

MSD makes ignition systems for CR and RM 125/250 models, featuring adjustable advance/retard curves. These systems offer performance tuning at lower cost than OEM equivalents.

Big Bore Tuning Tips for Yamaha RD350 Two-Stroke Cylinders

TIPS FOR BIG BORING CYLINDERS

In the mid-90s, European electro-plating services expanded to America, enabling precise over-boring and plating. Tuners now push displacement to class limits or reclassify engines.

When changing displacement, consider these factors:

• Port-time-area
• Compression ratio
• Exhaust valves
• Carb jetting
• Silencer
• Ignition timing

Port-Time-Area: When increasing bore size, transfer ports effectively drop in height and retard timing. The exhaust port narrows. An unmodified over-bored cylinder produces excessive low-end power. Use tools like TSR’s Time-Area software to calculate precise port adjustments.

Cylinder Head: Must be modified for larger piston:

• Bore enlarged to final size.
• Squish band deck height set for correct clearance.
• Squish band may need narrowing due to increased turbulence.
• Chamber volume increased to prevent detonation or flat top-end.

Exhaust Valves: Check piston-to-valve clearance, especially on engines with close-clearance exhaust valves. Failure to adjust may result in collision and engine damage.

Carb: Larger piston diameter increases intake velocity, enriching jetting. Plan to lean the mixture after over-boring.

Ignition Timing: Retard timing slightly to improve over-rev and counteract flat top-end tendency.

Pipe and Silencer: No need for longer pipe — but a larger center section helps. FMF’s “Fatty” pipes work well. Some riders use shorter silencers with larger outlets to manage backpressure for larger engines.

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