Alright I'm new to this and I have a question I have a 04 Crf450r that back fires like crazy when you let of the throttle. I have played with the jetting a lot and can't get it to stop. I even had the air pilot screw out so far it about fell out.  I am looking for some ideas I will list what I have tried and my mods.

I have checked the following:
New plug
Timing checked out
Shop just did valves
Cleaned carb
No cracks in header
Cleaned stater
New plug cap
Clean filter

My mods are:
A high compression piston
+9oz flywheel
Full Dr.d exhaust
180 main jet
45 pilot jet
R&D flex jet air pilot screw (also have mouse mixture screw)

Anything?

Race bikes like yours, running perfectly, will usually pop ocassionally on decel.

If turning the fuel screw past 2 turns has no effect, you have an air leak somewhere.

check the hot start plunger.

It pops like crazy tho. I will check the hot start tonight. Were could air be entering?

fletcher2597, on 11 April 2012 - 06:06 AM, said:

With the engine running, fuel screw at 1.5 turns out, hot motor, try adding hot start. It should raise the idle. If it doesn't, you are too lean somewhere. Take spray brake cleaner and spray the carb and pipe junctions, as well as the hot start nut. The hot start plunger will corrode inside the carb body, and cause a leak too.

fletcher2597, on 10 April 2012 - 12:09 PM, said:

I had to log in and stop your suffering.  It sounds like the plate on the slide in your carb is in upside down, it goes in opposite of what it looks like it should, I walked around he pits once before practice frantically looking for a size 60 or whatever PJ and people thought I was nuts I jetted around it barely and figured it out that night..  How do you like that 9 oz FW ?

I will check that tonight. The 9oz fw works like a dream for the woods

fletcher2597, on 13 April 2012 - 04:15 AM, said:

Hope thats it, I'm curious to know.  Also, DO NOT clean the slide plate with carb/contact cleaner, it will ruin the rubber gasket thing attached to it.