denisj Posted September 30, 2019 Hi all I am sorting out a new to me ‘02 YZ250 that ran terrible. After a tear down and inspecting everything with no repairs..the bike managed to fix itself. I never found the problem with poor acceleration..now it’s fine. I am looking for symptoms of the following so I can put them in my mental notebook Reading on here I found a number of issues that may contribute to a rich running issue over 1/3 throttle. What does it feel like when: 1. The reeds are worn out 2. The muffler packing is all gone 3. The muffler (chamber is new)is coated inside from years of buildup 4. The power valve is incorrectly set up/assembled 5. The Keihin jet block seal is bad 6. The electric power valve is failing 7. The throttle position sensor is failing or out of adjustment I’m trying to get a grasp on the possibilities because the bike checks out OK. I’m not a huge fan of a bike fixing itself and then the issue returning mid trail a million miles from the truck Share this post Link to post Share on other sites
mrjata Posted September 30, 2019 1 hour ago, denisj said: Hi all I am sorting out a new to me ‘02 YZ250 that ran terrible. After a tear down and inspecting everything with no repairs..the bike managed to fix itself. I never found the problem with poor acceleration..now it’s fine. I am looking for symptoms of the following so I can put them in my mental notebook Reading on here I found a number of issues that may contribute to a rich running issue over 1/3 throttle. What does it feel like when: 1. The reeds are worn out 2. The muffler packing is all gone 3. The muffler (chamber is new)is coated inside from years of buildup 4. The power valve is incorrectly set up/assembled 5. The Keihin jet block seal is bad 6. The electric power valve is failing 7. The throttle position sensor is failing or out of adjustment I’m trying to get a grasp on the possibilities because the bike checks out OK. I’m not a huge fan of a bike fixing itself and then the issue returning mid trail a million miles from the truck Woah there kemosoby. one at a time. 1-reeds are valves, it will fall on its face if they are trashed or not working right 2 if packing is gone, might loose some power, some. 3 if chamber has a bunchof carbon build up, it is like breathing our through a straw while running a matathon. Need to get that out. I ran a propane flame inside and burned out the carbon, just knocks right out, no biggy 4 If the power valvve is wrong, you won't have any top end to speak of. 5 jet block seal, humm, ya lost me there 6 I have no clue about an electric power vlave 7 got me there too Hope this helps a little. Get the service manual!! I do on any bike build, huge help and is great for resale. Share this post Link to post Share on other sites
Smoking 2's Posted October 1, 2019 (edited) I'll start where mrjata left off 5 - jetting will always fluctuate 6 - no electric power valve, operates off governor in right side clutch case with an linkage going up right side cylinder under a cover 7 - Book has adjustment specs for it and I've never losened mine to find out but my believe is just a lose of power When you said your bike fixed itself, probably have dirt in carb and jets partially clogged and the jet cleared itself but may come back if more dirt in there. Pull carb and clean everything really well Edited October 1, 2019 by Smoking 2's Share this post Link to post Share on other sites
denisj Posted October 2, 2019 On 9/30/2019 at 11:56 AM, mrjata said: Woah there kemosoby. one at a time. 1-reeds are valves, it will fall on its face if they are trashed or not working right 2 if packing is gone, might loose some power, some. 3 if chamber has a bunchof carbon build up, it is like breathing our through a straw while running a matathon. Need to get that out. I ran a propane flame inside and burned out the carbon, just knocks right out, no biggy 4 If the power valvve is wrong, you won't have any top end to speak of. 5 jet block seal, humm, ya lost me there 6 I have no clue about an electric power vlave 7 got me there too Hope this helps a little. Get the service manual!! I do on any bike build, huge help and is great for resale. It helps thanks! The jet block is in the Keihin PWK carb. It is the small tower block type thing that has 2 screws holding it to the carb body in the float chamber. It carries the needle jet, main jet, pilot jet and emulsion tube Electric power valve - This is screwed into the side of the carburetor. It is responsible for enrichment at low rpm and apparently it closes off the enrichment port at a cpu determined rpm where it leans the overall mixture to allow the engine to run fast. A modern version of the old internal power valve and just like the external hose from the body to the carb inlet you will see on the universal replacement PWK carbs. TPS - I can not verify that this is only for ignition. I have also failed to verify if this is a potentiometer or a switch. Share this post Link to post Share on other sites
denisj Posted October 2, 2019 I am concerned it may have been ignition - The symptom was similar to bad spark plug wires when putting an engine under load. The only difference is under light loads it would still start to run badly like it was missing the main jet. Gonna take it out again Sunday and see how she goes. Hopefully it behaves Share this post Link to post Share on other sites
