Aloha, I am new here and looking for some help. I live here in Honolulu, Hawaii. I have 2008 YFZ 450 and just put a slip on FMF 4.1 Titanium Silencer on it. On the main jet I installed 162 and on the pilot I installed a 48. I also installed a moose racing idle mixture screw. I set the idle mixture screw 2 1/2 turns out to start. The bike starts fine. When I go to wrap the throttle it bogs big time and then it pops out the exhaust. when I try to hold the throttle just between idle and 1/4 throttle the bike dies out. I tried to mess with the idle mixture screw and it does nothing. I checked for leaks and my air filter is clean and spark plug is new. I also checked the fuel bowl and it is clean. It was doing this same problem prior to putting on the mods. Could it be my TPS??? Wrong jetting????I would really appreciate some help here. Mahalo(Thank you)

 

I think your problem is in your pilot system, which is from idle to 1/4 throttle. Especially if adjusting your air mixture screw did nothing. Try going back to the stock pilot jet. I have never had to increase my pilot jet even after significant changes to my intake and exhaust. Also, try changing only one thing at a time as a rule of thumb. This will help you find your problems much quicker and easier. I would go back to stock on everything, and start by changing the main jet. I don't know what other mods you have done like to the intake or if you have a jet kit so you have many jets to choose from, but I would start with 2 jet sizes up from stock and make sure you are running slightly rich. When you get that right, work backwards. Good luck! I'll keep checking back...oh, and send me some of that warm weather! I'm tired of the cold here!

 

Aloha, Thanks for the help. The only thing I have on the bike is the FMF titanium 4.1 silencer. Just the slip on. I kept the stock air box set because I ride in trails here in hawaii and it gets pretty muddy and I ride through some water puddles. I run also run race gas in the bike 103 octane unleaded. Right now I have a 162 on the main and a 48 on the pilot. I am still having problems tuning the bike. Its been three days now. But thanks for the help. Mahalo.

 

k first off dont run then high octane your piston is only 11.4 to 1 compression run regular 91 pump gas. put all your jets back to stock like firehog said and put a 165 main jet in and turn your fuel screw one full turn. this should make your bike run perfect. you run high octane gas when you up your compression on your piston, i got a 13.1 to 1 compression piston and im running 95 octane. hope you figure it out this should work, and maybe check your spark plug now it should be chocolate brown you might have burned it up running that high octane.