[crawllercrane]

installed power core 4 and rejetted main to 148 and pilot to48 but still will not stop backfiring try a adj screw for air mixture in out still will not stop HELP

[aoyue2702]

It's probably your smog pump causing the noise.


You can either

Take the pump off, JB weld inside of it to block it off, let it cure and reinstall it.

OR

Buy a smog block off kit for around $30-40

[rdalley]

If you havent opened the airbox, you wont get the results with the exhaust. If you have, then the next step is taking off the smog pump.

A 148 main is WAY too lean after these mods. I run a 158-160 depending on temperature at 5500' elevation. I dont know your elevation, but if you havfe done all the mods and you are closer to sea level I would think a 45 pilot and a 165-170 main would work much better. You probably need to see what other guys in your elevation are doing with their needle.

[Kiwi]

While removing the factory smog stuff may help, I don't think that it is the real problem.

I have a 2007 CRF450X with modified airbox, Uni filter w/no backfire screen, NCVS needle on 4th, 172 main, CRFs fuel screw at 2, Boyesen QS3, 450R cam, FMF Megabomb header, and Pro Circuit T-4 muffler. I still have the factory smog stuff on and the 450X runs awesome with no backfire pops, throttle tip-in hesitation or such.

I think the real problem is:

1. Your 450X is way too lean. The 148 main jet needs to be upped to a 160 or 165 (depending on engine mods, altitude, etc.).

2. What needle are you running (stock??) and at what clip height? For about $15.00 you can get a NCVS needle (16233-MEN-671) from the Honda dealer that will perform much better than the stock needle. Set the clip height to the 4th and you should be ready to rip. [Be sure to adjust the fuel screw after jetting. Approximately 1.5 - 2.0 turns out will be required]