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I am trying to get the jetting on my 07 yz450f right. I am not sure where i need to start though. The bike is a little hard to start sometimes, backfires a lot, and it doesn't stall when I screw the mixture screw all the way in. Also it bogs if I snap the throttle. I have practically redone this whole bike now I just want it to run right, riding at about 500ft 60-80 degrees. Not quite sure which jets are in it most likely stock seeming the bike was completely stock when I got it, I am gonna pull the carb soon just haven't yet cause it's so much of a pain to get out.

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Have you checked valve clearance lately? If you did, was the timing done correctly? Is your gas fresh? Is your fuel line 100 percent clear? Is your tank clean? Spark plug new? This time of year people are having that old gas problem. You may look and find a shop manual or find the thread for the stock jetting sizes for your bike on here. Keep track of your maintenence and do it on schedule and you can maximize your engine life.

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Read here: https://www.thumpertalk.com/forum/showthread.php?t=694917

1 hour ago, XAaronJohnstonX said:

Also it bogs if I snap the throttle

Snapping the throttle open on a four-stroke is a non-starter.  You can actually eliminate almost all of the stumble that results from it, but you will end up with an engine that runs too rich in real time operation during off idle maneuvers.  Tuning an engine for better off idle response than is practically necessary when you're underway is counterproductive to overall performance, and can lead to stalling by flameout on sudden closing of the throttle from moderate loads at low speeds. One important adjustment you need to make is to learn to "roll", rather than snap, the throttle open, beginning a little more slowly and increasing opening rate as you progress from closed to open. 

Also, most folks miss the fact that the pilot jet is not primarily responsible for covering such maneuvers.  That's the job of tuning the accelerator pump and of selecting a needle with a suitably sized upper straight (non-tapered) section, identified by the 4th letter character of the needle ID.  The tapered section has no effect until around 1/4 throttle because until that point, it hasn't started to lift clear of the main nozzle yet. Stock needle is an NFLR, clipped in the 4th groove down.  Using an NFLP will make a noticeable difference in off idle sharpness, and could even be too rich at 1/8 to 1/4.  In that case, go with an NFLQ instead.  Part numbers are in the manual tuning section, and the manuals are free from Yamaha.

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Have you checked valve clearance lately? If you did, was the timing done correctly? Is your gas fresh? Is your fuel line 100 percent clear? Is your tank clean? Spark plug new? This time of year people are having that old gas problem. You may look and find a shop manual or find the thread for the stock jetting sizes for your bike on here. Keep track of your maintenence and do it on schedule and you can maximize your engine life.

When I got the bike I checked and changed everything from spark plugs, to valve clearances and timing, everything was in spec then but it still back fired a lot and seems to not respond as fast as it should. The old gas isn't a problem for me, my bikes don't sit and if for some reason they are I drain all the gas from the tank and carb, and clean em when it's time to ride. When my buddy got the bike it was still hot out, and seeming we haven't really had a cold winter since I have still been riding it a lot.

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