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:applause: my friend has an 03 450 and it always pops on decel and is hard to start when cold and when warm. it also idles erratic, sometimes high sometimes low, sometimes snapping the throttle will bring a high idle down and sometimes not. so &%$#@!??? we decided to check the valves and found the intake side to be a little tight so we reshimmed and now they are right on. we also went through the carb and cleaned everything and found a 162 main and the needle on the 3rd from the top. we live in utah at 4000 ft.

we then decided to repack the silencer just cause. we used stainless steel wool around after the stock steel then fiberglass. put everything back together and the carb setting back the same. now the thing really pops a lot after blipping the idle and the pipe got so hot after a few minutes it started melting the air box. it runs worse then before. I am thinking we might be a tooth off on the timing but am not sure is there anything else that could be the cause of this?? could it be the silencer with the new packing? what if the inner part of the silencer is broke and some of the inside isn't lined up. I herd something that sounded wierd when we put the silencer back together. I don't want to redrill the rivets out again unless I have to.

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Model: 2003 YZ450f

Edit Revision:12/12/05


Main Jet: 158

Pilot Jet:38 (edited)

Starter Jet: stock?

Leak: n/a

Fuel screw: Zyp Ty 1 1/2 turns out

Needle: #3 clip ( stock)

Air box: Twin Air and Unis

Pipe/muffler: Stock header, FMF Q (older version)

Altitude: 5700 feet and up to 11k

Temp:current 40-80F

Humidity: 30%

After Edit Satisfaction: Bike runs perfectly now. Lesson learned is that a very rich pilot and a very rich main jet seems to mimick a lean condition. High hanging idle, surging and sometimes hard to start. Except it could foul a plug.

Pre-edit, I'm on the verge of dropping the needle back down, ie placing the clip at the #4 position, back in the middle position to richen it up slightly.

#4 position at 8000 feet and 80'f was horrible, creating a rich bog in the middle and stumbling from 1/4 to 3/4 throttle. Probably because needle richness in conjunction with AP injection. #3 needle clip is best setting

I love the Q by the way. Its sufficiently quiet that you can now hear a lot of engine noise. Man these motors rattle and hum

Degree of statifaction prior to the 38 pj change: bike is g00d,no bog, good throttle response and runs like a striped ape WFO BUT it has a real annoying midrange surge that I cannot cure either with richer or leaner jetting, needle height etc. Sometimes hard to start.

Note: Temp has ranged from 70'f to 95'f recently so everyday its something different. Altitude has ranged from 5700 to 9000 feet.

As of 11-1-05 the current jetting is perfect for my conditions. Italics denote notes prior to finding a good combination of lean needle and pj range.

I think your bike is horribly rich. It sounds like the way mine used to run. You need to give us the full specs on your jetting.

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4 strokes don't use air screws for their carbuerators....

It's called a Fuel Screw on a 4 stroke carb.

Turning it out will enrich the mixture, turning it in will lean it out.

I think Motogoalie is right about this one......

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you knoe I pulled the spark plug to find it running really lean. the plug is brand new and was almost white. does reshimming the intake valves normally require a jetting change?? I did go to a 45 from a 42 on the pilot jet and raise the needle to the 4th clip with a 162 main the fuel screw was @ 3/4 turn but I went to 2.5 cause of the plug. it still popped but not as bad and the pipe was not as hot either. I think if I go up one more size on the pilot and see what happens. it's just wierd that I am running into this big jetting problem after the reshim. could I be a tooth off on one of the cams?? thanks guys!!

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