'06 KX450F Backfire

[biketech]

i just bought a 06 kx450 and im experiencing some backfire on deceleration. i have heard different sides on this subject. is this normal with these bikes or is it the jetting? what needs to be done? i have only owned two strokes so this bike is going to be a learning curve for me.

 

[oldguy]

Normally if your jetting is on then when the temp drops you will be lean and get the deceleration backfire. Try backing the fuel screw out a 1/4 or 1/2 turn.
We just had a case of severe backfiring in the YZ450F after a topend and it turned out to be 2 causes- first and formost the timing was off by one tooth (an oversite by my son ) and second he forgot to tighten the bolts holding the header on. We went over to Eric Gorrs shop for a quick evaluation of the cause and he caught the timing immediately and once it was taken care of Spider fired the bike up and ran it across the parking lot only to get the backfiring. Eric was starting to scratch his head when Spider noticed the loose header.

Today he went out riding and the temp had stopped rising at about 36 and he was getting the decel pop and 1/2 turn out on the fuel screw cured it

 

[biketech]

the large thumb knob on the left side of the carb is the fuel mixture screw, right?

 

[BigRedAF]

No! The large thumb screw is the idle adjustment.

The fuel screw is under the carb in the front near the rubber intake manifold. It is very difficult to get to with normal hand tools. There are several after market curved screw drivers that can be used or an after market replacement screw that can be adjusted by hand.

 

[biketech]

richened the fuel screw about 3/4 turn and no more backfire.i got to the srew with a little hobby screwdriver for three bucks. no need for a fancy tool to get to it. although the aftermarket replacement fuel screw with the thumb knob would be nice. thanks for the help.

 

[IndyMX]

It is.. I have one on my 450.. Thinking about getting one for my brothers 250, so I don't have to mess around with tiny screwdrivers in tight spots. God forbid he tune his own bike!!

 

[whenfoxforks-ruled]

Try doing it on a hot bike! That screw is worth its weight in gold. Oh yeah, it is the price of gold.

 

[oldguy]

The fuel screw is always the first add on for every new 4 stroke we get- just put one in my son's new YZ250F last week.
I couldn't imagine trying to adjust in a crowded staging or on the start line without it