My boys 05 YFZ 450 have this lil problem when he starts to rev high rpm's or drag race his quad it chokes then dies out. When he tries to start it in 1st gear then it just dies. Then if he tries to rev while letting go the clutch slowly it back fires. I'm thinking its the clutch or tranny acting up. It won't let him startup normally until the engine cools down for about 20 mins. Again if he cruises at low rpms and drives normal, there is no problem but when he starts to gun it the problem persist. He brought it to the Yamaha dealer twice and they can't seem to find out whats the problem. Its pretty hard to explain to a Yamaha mechanic when your at the dealer and your quad starts up fine and runs normally. Until aggressive riding takes place, where he cannot do around the area when the problem begins. ANyone out there with a 05 yfz 450 with this problem and had fixed it please help out my boy.

 

Sounds like he needs to richen up the air/fuel screw. It's located on the bottom front of the carb, just turn it out until it runs correctly. Out to richen, in to lean it out. Try a half a turn first, if that does'nt work try another half turn. Good luck!

* I just noticed you said it dies and/or cuts out on to top end too, that probably means he needs to richen up the main jet too. It's located on to bottom of the carb also, just take off the 17 mm nut on the bottom and you'll see it.

 

try running it with the lights on

 

My buddy has a 05 yfz. He has the same problem and it crept up again today. Yamaha techs told him that the stator cannot fully recharge the battery and the yfz's throttle is electronic controled. Check the battery! I think that the battery has to have 15-20 volts in order to run well. It has been a while, the numbers might be wrong but get a trickle charger for the battery and it should solve the problem.

Brian

 

Did you take the spark arrestor and/or air box lid off with out a re-jet?
YFZ's come w/ a 158 main jet to pass emmissions, try a much needed 168 mj in their.

 

Yea we always run with the lights on, the salesman recommended it. For the spark arrestor causing the problem is not likely because I run my yfz with and without a spark arrestor without messing with the jets and its fine. Its most likely a jet problem or the throttle position sensor, I have posted on other threads and guys with the same problem said it was it. His quad is only 3 months old and its been in the shop longer than at his home.......... maybe he just got a lemon.