Rd350, yamaha, specification, information, details, parts, modification…

15 Фев 2015 | Author: | Комментарии к записи Rd350, yamaha, specification, information, details, parts, modification… отключены

Changing the power band of dirt bike engine is when you know the basics. A of different aftermarket accessories is for you to custom tune your to better suit your The most common mistake is to the wrong combination of engine making the engine run worse stock. Use this as a guide to yourself on how changes in engine can alter the powerband of bike’s

Use the guide at the end of the chapter to map out your for changing engine components to the perfect power band.


Although a two-… has less moving parts a four-… engine, a two-… is a engine because it relies on gas There are different phases place in the crankcase and in the cylinder at the same time. That is how a engine completes a power in only 360 degrees of crankshaft compared to a four-… engine requires 720 degrees of crankshaft to complete one power cycle.

four drawings give an of how a two-… engine works.

1) with the piston at top … (TDC 0 degrees) ignition has and the gasses in the combustion chamber are and pushing down the piston. pressurizes the crankcase causing the valve to close. At about 90 after TDC the exhaust port ending the power ….

A wave of hot expanding gasses down the exhaust pipe. The phase has started and will end the transfer ports open. The in the cylinder must blow-down to the pressure in the crankcase in order for the mixture gasses to flow out the ports during the scavenging

2) Now the transfer ports are uncovered at 120 degrees after TDC. The phase has begun. Meaning the unburned mixture gasses are out of the transfers and merging together to a loop. The gasses travel up the side of the cylinder and loops in the cylinder head to scavenge out the mixture gasses from the power ….

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

Now the loop of mixture gasses have into the exhaust pipe’s section. The gasses aren’t because a compression pressure has reflected from the end of the exhaust to pack the unburned gasses into the cylinder before the closes off the port. This is the super-charging effect of two-… The main advantage of two-… is that they can combust volume of fuel/air mixture the swept volume of the engine.

Example: A 125cc four-… combusts about 110cc of F/A but a 125cc two-… engine about 180cc of F/A gasses.

3) Now the has rotated past bottom center (BDC 180 degrees) and the is on the upstroke. The compression wave from the exhaust pipe is the unburned gasses back in the exhaust port as the piston off the port the start the compression In the crankcase the pressure is below producing a vacuum and a fresh of unburned mixture gasses is through the reed valve the crankcase.

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


The ports are designed to produce a power characteristic over a narrow rpm band. Porting or is a metal machining process to the cylinder ports (exhaust that alters the timing, size, and angles of the ports in to adjust the power band to suit the rider’s demands.

For a veteran trail rider an RM250 in the Rocky mountain of the USA will need to adjust the band for more low end power of the steep hill climbs and the air density of higher altitudes. The way to determine what changes be needed to the engine is by measuring and the stock engine’s specifications. The critical measurement is termed

This term is a calculation of a size area and timing in to the displacement of the engine and the rpm. tuners know what the values of the exhaust and transfer should be for an engine used for a purpose. In general, if a tuner to adjust the engine’s power for more low to mid range he would do the things.

Turn down the base on a lathe to increase the … (distance from TDC to port opening). This retards the exhaust port and shortens the duration and increases the ratio. Next the transfer should be narrowed and re-angled epoxy to reduce the port-time-area for an rpm of 7,000 rpm.

The rear ports need to be re-angled so oppose each other than pointing forward to the port. This changes the scavenging flow pattern of the ports to improve scavenging at low to mid rpm (2,000 to 5,000 rpm). An rider racing mx in England want to adjust the power of an RM250 for more mid to top end power.

The would need to be tuned different than for trail

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


heads can be reshaped to change the band. Generally speaking, a head with a small and deep combustion chamber, and a squish band (60% of the area). Combined with a ratio of 9 to 1 is ideally suited for low to mid power.

A cylinder head a wide shallow chamber and a squish band (35-45% of area) and a compression ratio of 8 to 1, is suited for high rpm power.

are many reasons why a particular design works for certain of racing. For example; a head a wide squish band and a compression ratio will high turbulence in the combustion This turbulence is termed Squish Velocity, MSV is rated in per second (m/s). A cylinder designed for supercross should an MSV rating of 28m/s.

Computer software is used to calculate the MSV for designs. In the model tuning chapters of this book, all the specs quoted have MSV designed for the intended power changes.


There are two mods hop-up companies are to crankshafts; stroking and turbo-vaning. means to increase the distance the crank center to the big end pin center. are two techniques for stroking crankshafts; old hole and re-drill a new big end pin hole, or by an off-set big end pin. The method of and re-drilling is labor intensive.

The pin system is cheap, non-permanent, and can be quickly. In general, increasing the of a crankshaft boosts the mid range but decreases the engine’s rpm peak.

The Turbo-Crank refers to a modification to the of a two-… engine, whereby are fastened to the crank in order to the volumetric efficiency of the engine. decade some hop-up revives this old idea and it a trendy name with promises that it can’t up to. These crank modifications oil to be directed away from the rod and often times the vanes detach from the crank at rpm, causing catastrophic damage.

My advice, don’t the $750!


In general a diameter carburetor will high velocity and a good characteristic for a low to mid rpm power band. A diameter carburetor works for high rpm power bands. For 125 cc a 34mm carburetor works for supercross and enduro and a 36 or 338 mm carburetor best for fast mx tracks. For 250 cc a 36 mm carburetor works best for low to mid bands and a 39.5 mm carburetor best for top end power bands.

Recently there has been a in the use of air-foils and rifle-boring for carbs. innovations are designed to improve air at low throttle openings. Some sell carb inserts, to the diameter of a carb. Typically a set of is sold with a service of boring the carb. For example; a for a 250cc bike (38mm) be bored to 39.5mm and two inserts be supplied.

The carb can then be to a diameter of 36 or 38mm.


Think of a reed valve a carburetor, bigger valves large flow-areas work for high rpm power bands. In reed valves with six or petals are used for high rpm Reed valves with petals are used for dirt that need strong low end and mid power. There are three factors to consider when a reed valve. The angle of the valve, the type of reed and the petal thickness.

The two common valve angles are 30 and 45 degrees. A valve is designed for low to mid rpm and a 45 degree is designed for high rpm. are two types of reed petal commonly used, carbon and fiberglass. Carbon fiber are lightweight but relatively stiff tension) and designed to resist at high rpm.

Fiberglass have relatively low spring so they instantly respond to that changes in the crankcase, the low spring tension makes flutter at high rpm thereby the amount of power. Fiberglass petals are good for low to mid power and carbon fiber reeds are for high rpm engines.

Boyesen Stage reeds have a thick base reed a smaller thinner reed on top. This setup the rpm range where the reed flows best. The thin respond to low rpm and low frequency pressure The thick reeds respond to pulses and resist fluttering at rpm.

A Boyesen RAD valve is than a traditional reed Bikes with single shocks have off-set The RAD valve is designed to redistribute the gas to the crankcases evenly.

A RAD valve give an overall improvement to the band. Polini of Italy a reed valve called the It features several mini of reeds positioned vertically of horizontally like conventional valves.

These valves are for enduro riding because of throttle response. In tests on an chassis dyno show the to be superior when power However these valves generate greater peak than conventional reed

Supervalves are imported to America and by Moto Italia in Maine.


The exhaust of a two-… engine attempts to the energy of the pressure waves combustion. The diameter and length of the main sections of a pipe, are to producing the desired power The five sections of the pipe are the pipe, diffuser cone, baffle cone, and the stinger. In after market exhaust shift the power band up the rpm

Most pipes are designed for cylinders not tuned cylinders. like MOTOWERKS custom design and fabricate pipes on the cylinder specifications and the type of band targeted.


come in all sorts of shapes and A long silencer with a diameter enhance the low to mid power it increases the bleed-down pressure in the A silencer with a short and a large core diameter the best bleed-down pressure for a rpm engine.

Too much pressure in the at high rpm will radically the temperature of the piston crown and cause the piston to seize in the


The flywheel is to improve the engine’s tractability at low to mid There are two different types of weights, weld-on and thread-on. performs the weld-on flywheel service. Steahly makes flywheel weights. This threads onto the fine threads that are on the center hub of Japanese magneto rotors. the threads are used for the flywheel tool.

Thread-on flywheel can only be used if the threads on the are in perfect condition. The advantage to weights is they can’t come off.

Yamaha RD 350 F
Yamaha RD 350 F

External flywheels have a larger than internal rotor so they have greater inertia. Internal rotor give quicker throttle


is how changes in the static ignition affects the power band of a dirt bike. Advancing the will make the power hit harder in the mid range but fall on top end. Advancing the timing the flame front in the combustion adequate time to travel the chamber to form a great rise.

The rapid pressure contributes to a power band’s In some cases the pressure can be so great that it causes an pinging noise from the As the engine rpm increases, the pressure in the becomes so great that losses occur to the piston.

is why engines with too much advance or too high of a compression run flat at high rpm.

the timing will make the band smoother in the mid-range and more top end over rev. the spark fires closer to the pressure rise in the cylinder as great. The emphasis is on gaining degrees of retard at high This causes a shift of the from the cylinder to the pipe. can prevent the piston from at high rpm, but the biggest is how the heat affects the tuning in the

When the temperature rises, the of the waves in the pipe increases. At rpm this can cause a closer between the returning compression and the piston speed. This extends the rpm peak of the pipe.


Rotating the stator relative to the crankcases changes the Most manufacturers stamp the plate with three near the plate’s mounting The center mark is the standard If you loosen the plate mounting and rotate the stator plate to the flywheel’s rotation, that advance the ignition timing. If you the stator plate counterclockwise to the rotation, that will the ignition timing.

Never the stator plate more .028in/.7mm past the original timing mark. Kawasaki and stator plates are marked. stators have a sheet plate riveted to one of the mount

This plate insures the stator can only be installed in one If you want to adjust the ignition on a Honda CR, you’ll have to the sheet metal plate, a 1/4in rat-tail file.


The latest innovation in systems is an internal rotor bolt-on discs that as flywheel weights. PVL of Germany these ignitions for modern dirt bikes. Another to the PVL ignition is that they a variety of disc weights so you can the flywheel inertia to suit conditions.

MSD is an aftermarket ignition manufacturer. They are making systems for CR and RM 125 and 250. MSD’s system features the ability to the number of degrees of advance and

These aftermarket ignition sell for less than the OEM


In the mid European electro-plating companies service centers in America. made it possible to over cylinders and electro-plate them to tolerances. This process is by tuners to push an engine’s to the limit of the racing class or make the engine legal for a class.

When you change the of the cylinder, there are so many to consider. Factors like; compression ratio, exhaust carb jetting, silencer, and timing. Here is an explanation of you need to do when planning to bore a cylinder.

Port-Time-Area This is the size and opening of the exhaust and intake ports, the size of the cylinder and the rpm. increasing the displacement of the cylinder, the has to be bored to a larger diameter. The enter the cylinder at angles of 15 degrees.

When the cylinder is bore is larger, the transfer ports in height and retard the timing and of those ports. The exhaust gets narrower. If you just bored and plated a cylinder, it have much more low end than stock.

Normally have to adjust the ports to the demands of the larger engine Those exact dimension can be determined with TSR’s computer program.

Cylinder — The head’s dimensions be changed to suit the larger The bore must be enlarged to the bore size. Then the band deck height be set to the proper installed squish The larger bore size increase the squish turbulence so the squish band may have to be The volume of the head must be to suit the change in cylinder

Otherwise the engine will run at high rpm or ping in the mid range detonation.

Exhaust Valves When the bore size is the exhaust valve to piston must be checked and adjusted. pertains to the types of exhaust that operate within proximity of the piston. If the exhaust aren’t modified, the piston strike the valves and cause engine damage.

Carb The piston diameter and carb diameter are closely related. The the ratio between the piston and the carb size, the higher the velocity. That makes the richer.

Figure on leaning the after an engine is over

Ignition Timing — The can be retarded to improve the over Normally over bored tend to run flat on top end.

and Silencer — Because the bore size is changed, you need a longer pipe one with a larger center FMF’s line of Fatty work great on engines larger displacement. Some use silencers that are shorter larger outlets to adjust the in the pipe for the larger engine

Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F
Yamaha RD 350 F

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