Question about Yamaha YZ 125 Motorcycles
Hi, i have a 1991 yamaha yz 125. it starts and runs fine. when u pull off it bogs down when throttled
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.
UPDATED 2005 YZ125 bog issue with videos
MototipsTV Replacing Float Cleaning Carb
YAMAHA YZ125 Owner Service Manual
OEM parts for Yamaha
Posted on Jul 16, 2016
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Posted on Jan 02, 2017
SOURCE: 1994 yz 250 bogs down
Have you cleaned the carburator ? It seems the main jet has some kind of obstruction.
Try cleaning the carburator and well a tune up would be fine too after the clean , check the plug/s & air filter too.
Posted on May 10, 2009
SOURCE: Carburetor issue
make sure the oil level is correct also on carb is a screw fiddle with this mixture screw and get back to me if it made no difference i will send a pic in a while
Posted on Oct 06, 2009
SOURCE: yz 125 bog
Sounds too lean to me or lacking fuel.
Partially blocked jet in carb. May have to remove / clean.
Check also that carb butterfly / piston is travelling to full open position.
Posted on Dec 21, 2009
A compression test will tell you your condition. (Throttle wide open while doing that) If that's good, your jetting is too rich, could be the issue.
Posted on Jan 09, 2010
You have to adjust the jets on the carb, theres a high and low jet, Turn them both all the way in ( dont force them) just turn until they stop, now losen each one in 1/4 intervals and try running it,
Posted on Apr 04, 2010
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