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Love my new 52 Pilot Jet, NO more hard start issue.

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Hello Guys,


I just wanted to share some recent interesting jetting experience from my 2007 WR250F.


Just some back ground of the bike, the bike was bought from a dealer it was a demo bike.


Right after acquiring the bike I went ahead and used the JD jetting kit per instructions


Red Needle

#3 clip

185 MJ

Stock Pilot Jet (#45) (revise to #52)

Stock Leak jet (#70)

Stock starter Jet (?)

Air Screw 2 ½ out (revise to 1 1/2, read below)

Grey Wire

Moded Air Box

Exhaust baffle removed

AIS delete



For most part the bike ran good around 500 to 3500 ft Pacific Northwest condition.


Having the Air Screw out 2 ½ bother me I keep thinking the pilot jet is lean, when I use the electric start it still require to open the throttle by some to get it started, not sure if it was requiring more air or fuel. The air jet lead to believe it was lean because every time I adjust it to rich condition the bike react sligthly better starting and better fast throttle response. With that observation it lead me to believe the pilot jet is lean. I  did a research here and over the internet and consensus is nothing bigger than 45 pilot jet. I’ve only seen one guy posted a 50 pilot jet. Majority are somewhere between 42 to 48 pilot jet.


So I check the stash of jets I have and found I have 52 pilot jet was hoping for 50, so I proceed to replace it. Wow the bike started first try using electric start without a choke, in slow idle setting and in 45 deg temperature. Not to mention I did not prime the carb using the pump and with throttle fully close. All I did was turn ON the ignition and press the start button and viola. After a good warn up, I was excited to see if I have introduce a rich bog condition during quick throttle race launch and boy the bike just took OFF with authority, NO bog or lagging throttle response. It was a linear response all the way through. The hot start was also improved just first try all the time and without needing to use the hot start, wow!


The air screw is not set to 1 1/2  setting.



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Wow. That is pretty rich by most people's standards. However, I always go by the saying "that there's no right or wrong jetting numbers, it either works well or not" .

It sounds like you found something that works well. Go with it.

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I cannot beleive that a 52 pilot works properly. When you have a hot engine, it is going to be very hard to start. Does the bike run if the fuel screw is closed? What size is the slow air jet?

Pilot jet and fuel screw are for idle only and not for acceleration, that is the AP.

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Your carb doesn't have an air screw. That is a fuel screw and it controls fuel flow at closed to about 1/16th throttle. Turn it in = less fuel.

Cold starting without requiring the choke is NOT a sign of a wonderful bike. Rather, that is a direct signal from above that your jetting is much too rich. Properly jetted cold engine requires use of the choke and should run 15-30 seconds or so before beginning to sputter. Properly jetted cold engine should probably never start at all without use of the choke or accel pump action.

The 52 jet is not responsible for your starting. Unless you are running straight alcohol, go back to a jet something in the range of 40 to 45.

The 52 PJ could improve your throttle response because your leak jet is too lean.

Edited by BazookaJoe

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