back firing?

[badburn]

my new yz450f back fires all the time when i let off the throttle is this normal?

[RMSpeed6]

Not normal, get it jetted properly

[YFZ450_725]

too rich...head over to the jetting section.

[JB MCS]

too rich...head over to the jetting section.

too lean!?

i thought they backfired only when lean. popping off decel...?

clean the carb, and get some jets

[RMSpeed6]

^^^ Agreed, it's a lean issue.

[YFZ450_725]

Oh, guess I over looked the "decel" part....good catch guys!

[grayracer513]

OK, now that that's over:

Read: https://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

[rdrata]

Yeah, but there was some learning going on there...

[grayracer513]

Yeah, but there was some learning going on there...

...a fact that seems to have escaped my notice.

[rekros33]

his bike is an 09.. would it have anything do to with their "new and improved" restricted exhaust system thats on the stock 09 yz450F SE?

why would a brand new bike, with 0 mods done to it, already have jetting issues? the bike has around 10 hours on it.

we ride at 3000-5000 feet, usually in about 5-25 degrees Celsius weather.

would a JD jet kit solve this issue?

or just one size up (from the stock 170 (I think) to a 175?) in the main jet if its running too lean or what?

thanks

[grayracer513]

It's like that because:

1. Yamaha tends to jet the pilot circuit on the lean side
   
2. A properly jetted YZ450 should backfire on decel a little, some of the time
   

The main jet has little or nothing to do with it, and at that altitude, the stock pilot may even work. Start by trying to adjust the pilot fuel mixture screw:

http://www.thumperfaq.com/jetting.htm#PJ

https://www.thumpertalk.com/forum/showthread.php?t=696672

If you believe the pilot jet is still to small, confirm that with this procedure:

https://www.thumpertalk.com/forum/showthread.php?t=327405

[WILLIAMS37]

I just installed the jd kit on my 08 yz450f. I ran a 160 main jet all last season but it was way off once I upgraded my exhaust. The JD kit suggested a 170 for my bike, so thats what I put in it and it does exactly as gray says. Once in a while on decel i'll notice a little backfire. But its not every time I let off the gas. It was also a little cold out today, i expect it to run even better once it gets nicer out. But I will definitely recommend the JD kit to anyone, makes extremely simple for anyone who hasn't done it before (me).

[rekros33]

09 yz450f motor is the exact same as 08s motor