Question about Yamaha YZ 250 Motorcycles
98 yz 250
Posted by Anonymous on
Hi, Anonymous it should be noted that your performance issues are not electrical ie. faulty charging system, battery condition, and connections etc. If your fuel system (gas tank, filters, fuel valve and carburetor) is contaminated with ethanol sludge, varnish, rust, dirt, water etc. or your bike has been sitting for months or years without running these components must be "PROPERLY" cleaned and reassembled "CORRECTLY" before any adjustments can be made. Tuning your carburetor is fairly simple once you understand the basic principals. You engine is a simple airbox sucking air in and blowing it out, it is finely tuned at the factory for maximum performance once you upset that delicate balance by changing air filters, camshafts or exhaust systems your performance may go down the and the engine may run poorly, you need to compensate the air-fuel mixture in the carburetor in order for the engine to run smoothly and at peak performance. If you are running multi carburetors you need to sync them first and make sure your air cleaner element is clean and dry for paper elements or lightly oiled for foam and meshed elements and properly installed. Here is how and where you compensate trouble: "TIP" if your engine "BOGS" your not getting enough fuel.
1. Closed to 1/8 throttle is managed by the air screw and pilot/slow jet.
2. 1/8 to 1/4 throttle is managed by the air-screw, pilot/slow jet, and throttle slide.
3. 1/4 to 1/2 throttle is managed by the throttle slide and jet needle.
4. 1/2 to 3/4 throttle is managed by the jet needle, needle jet, main jet, and air jet.
5. 3/4 to wide open throttle is managed by the main jet and air jet.
6. Wide open throttle is managed by the main jet.
If you are running lean, spark plug electrode color is white, engine runs hot and feels like it is starving for fuel you need to go up on the jet size or move the c-clip down one notch. If you are running rich, spark plug color is black or dark gray, engine runs cool, and bogs down when accelerating you need to go down on jet size or move the c-clip up one notch. When your carburetor is properly tuned for maximum performance your spark plug electrode will be a light tan color like coffee with cream. If you prefer fuel economy over performance you can go down on main jet sizes until a satisfactory level of lower performance is acceptable versus MPH, your spark plug color will be whiter and your engine will run warmer. These tuning adjustments will only make improvements if your intake and exhaust system have no air leaks or sealing issues and the entire electrical system is in proper working order and you have no mechanical issues. For more information about your issue and valuable "FREE" downloads that you will need please click on the websites below. Good luck and have nice a day.
Some help with my yz250 carb settings please Yamaha 2 Stroke
Two stroke Carburetor jetting How to performance tune your two stroke carb...
YAMAHA YZ250 LC Owner Service Manual
OEM parts for Yamaha
Posted on Jul 25, 2016
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Posted on Jan 02, 2017
SOURCE: 91 kx 250 stock carb setup.
Low speed screw 1.5 turns out from bottomed. Middle position for clip #3 position.
I never rely on the factory settings for the low speed screww, set it to factory and keep adjusting it 1/8 turn at a time until idle sounds best and quick throttle blips dont sound "stumbly" the later being the most important.
The needle position is for low altitude.
Posted on Jan 26, 2009
Higher octane fuel is needed from what you describe. Without knowing where you are located and how hard you ride the bike, it's impossible for anyone to offer any jetting advice. Higher octane fuel will likely help, but if you (or a previous owner perhaps) have shaved the head for higher compression, you might have to run higher octane racing fuel.
Also - check to make sure the pipe is sealed well at the head pipe connection - if not, it will **** air at high RPM and can cook your piston.
Check also for leaking crank seal on the flywheel side (left) which can also **** air but will normally be accompanied by a very irradic idle.
Posted on Apr 12, 2010
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