Question about 2003 Yamaha YZ 250

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Very low end power

I just bought a yamaha blaster atv 2-stoke with many aftermarket parts and it has little take off power but once it gets mid and high rpm range it runs great. what can i do to get it back to it's prime!!!

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  • joemforeman Feb 09, 2009

    well i replaced the reeds with boyseen power reeds and tried to adj carb jets and very little change. Do i need to rejet the carb or im i missing something.

  • neo 8a Feb 13, 2009

    my 200 blaster cuts out at full throttle in all gears whats causeing this?

  • joemforeman Feb 22, 2009

    hey thanks for the good information i did some looking and trying differant things after reading your response and here is what i come up with. there was a spacer inbetween the carb and the reed valve when i removed it a 100% better,but i now have a slight stumble from idle to low rpm. The carb is jetted at 340 and a stage three boyseen reed valve should i go bigger or smaller on the jets and stock on the reeds. I did try to move the poistion on the needle but no it made it worse.



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How do i adjust my carb on my yamaha blaster with 30 over bore kit

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Replace the reeds, Clean the carb jets and adjust mixture screws.

Please rate this a fixya

Posted on Feb 06, 2009

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  • Randy Hough
    Randy Hough Feb 09, 2009

    Here is everything you need to know to get the power you are looking for, other than porting and major engine modification.


    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 to 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. 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.


    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 to 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 to 1, is ideally suited for high rpm power.



    There are two popular mods hop-up companies are doing to crankshafts; stroking and turbo-vaning. Stroking means to increase the distance from the crank center to the big end pin center. There are two techniques for stroking crankshafts; weld old hole and re-drill a new big end pin hole, or by installing an off-set big end pin. The method of weld and re-drilling is labor intensive. The off-set pin system is cheap, non-permanent, and can be changed quickly. In general, increasing the stroke of a crankshaft boosts the mid range power but decreases the engine's rpm peak.


    In general a small diameter carburetor will have high velocity and a good flow characteristic for a low to mid rpm power band. A large diameter carburetor works better for high rpm power bands. For 125 cc engines a 34mm carburetor works well for supercross and enduro and a 36 or 338 mm carburetor works best for fast mx tracks. For 250 cc engines a 36 mm carburetor works best for low to mid power bands and a 39.5 mm carburetor works best for top end power bands. Recently there has been a trend in the use of air-foils and rifle-boring for carbs. These innovations are designed to improve air flow at low throttle openings. Some companies sell carb inserts, to change the diameter of a carb. Typically a set of inserts is sold with a service of over boring the carb. For example; a carb for a 250cc bike (38mm) will be bored to 39.5mm and two inserts will be supplied. The carb can then be restricted to a diameter of 36 or 38mm.


    Think of a reed valve like a carburetor, bigger valves with large flow-areas work best for high rpm power bands. In general, reed valves with six or more petals are used for high rpm engines. Reed valves with four petals are used for dirt bikes that need strong low end and mid range power. There are three other factors to consider when choosing a reed valve. The angle of the reed valve, the type of reed material, and the petal thickness. The two common reed valve angles are 30 and 45 degrees. A 30-degree valve is designed for low to mid rpm and a 45 degree valve is designed for high rpm. There are two types of reed petal materials commonly used, carbon fiber and fiberglass. Carbon fiber reeds are lightweight but relatively stiff (spring tension) and designed to resist fluttering at high rpm. Fiberglass reeds have relatively low spring tension so they instantly respond to pressure that changes in the crankcase, however the low spring tension makes them flutter at high rpm thereby limiting the amount of power. Fiberglass reed petals are good for low to mid power bands and carbon fiber reeds are better for high rpm engines.

    Boyesen Dual Stage reeds have a large thick base reed with a smaller thinner reed mounted on top. This setup widens the rpm range where the reed valve flows best. The thin reeds respond to low rpm and low frequency pressure pulses. The thick reeds respond to higher-pressure pulses and resist fluttering at high rpm. A Boyesen RAD valve is different than a traditional reed valve. Bikes with single rear shocks have off-set carbs. The RAD valve is designed to redistribute the gas flow to the crankcases evenly. A RAD valve will give an overall improvement to the power band. Polini of Italy makes a reed valve called the Supervalve. It features several mini sets of reeds positioned vertically instead of horizontally like conventional reed valves. These valves are excellent for enduro riding because of improved throttle response. In tests on an inertia chassis dyno show the Supervalve to be superior when power shifting. However these valves don't generate greater peak power than conventional reed valves. Supervalves are imported to America and sold by Moto Italia in Maine.


    The exhaust pipe of a two-stroke engine attempts to harness the energy of the pressure waves from combustion. The diameter and length of the five main sections of a pipe, are critical to producing the desired power band. The five sections of the pipe are the head pipe, diffuser cone, dwell, baffle cone, and the stinger. In general, after market exhaust pipes shift the power band up the rpm scale. Most pipes are designed for original cylinders not tuned cylinders.


    Silencers come in all sorts of shapes and sizes. A long silencer with a small diameter enhance the low to mid power because it increases the bleed-down pressure in the pipe. A silencer with a short length and a large core diameter provides the best bleed-down pressure for a high rpm engine. Too much pressure in the pipe at high rpm will radically increase the temperature of the piston crown and could cause the piston to seize in the cylinder.


    Here is how changes in the static ignition timing affects the power band.

    Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of ****** at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.


    Rotating the stator plate relative to the crankcases changes the timing.The center mark is the standard timing. If you loosen the plate mounting bolts and rotate the stator plate clockwise to the flywheel's rotation, that will advance the ignition timing. If you rotate the stator plate counterclockwise to the flywheel's rotation, that will ****** the ignition timing. Never rotate the stator plate more than .028in/.7mm past the original standard timing mark. Yamaha stator plates are marked.


    Msd makes a good one for your blaster

    Carb - The piston diameter and carb bore diameter are closely related. The larger the ratio between the piston size and the carb size, the higher the intake velocity. That makes the jetting richer. Figure on leaning the jetting after an engine is over bored.

    Ignition Timing - The timing can be retarded to improve the over rev. Normally over bored engines tend to run flat on top end.

    Pipe and Silencer - Because only the bore size is changed, you won't need a longer pipe only one with a larger center section. FMF's line of Fatty pipes work great on engines with larger displacement.

  • Randy Hough
    Randy Hough Feb 14, 2009

    Have you checked the exhaust?(It may need to be repacked if it is clogged.)

    Rejetting is always good.

    Check the timing as well.

    Hope this is a fixya

  • jay_m_theria
    jay_m_theria Mar 18, 2012

    Had the EXACT SAME problem with my 03 YZ250 - I removed the spacer that was installed before I bought the bike and BANG!!!! Runs like a completely different bike. Low end is definitely there now (almost came out from under me on the first test!)

    I know this is an old thread but I had to comment and say that after tinkering, jetting, adjusting mix screws, and everything else anyone could recommend, this is what fixed the problem. Took me 6 months of listening to everyone else who thought they knew the solution before I finally tried this.




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