'06 YZ450F Jetting Questions

I bought a used '06 YZ450F and the fueling was way off. It had a 165 main, 48 pilot, stock needle in the #2 position. I also discovered that the hot start did not have a reurn spring over the plunger. Given how high the idle screw was set, without a high rpm, I am guessing that this was the compensation for the plunger being engaged.

(Note: plunger, seal, and spring have been replaced)

I have the 165 main, 45 pilot, Fuel screw is just over one turn out, and the needle in the 4th position. Valve clearances are in spec. Power in all throttle positions is excellent now. The bike is a total SOB to start cold, and the hot start must be used all the time afterwards to ensure a first kick start when warm.

Cold start explanation (cold start plunger pulled): Will start on the first kick and run for about two seconds and die. Kick and kick and kick and it won't start. Hold the throttle at 1/8 to 1/4 turn and kick and it starts every time. Continue to hold for a couple seconds and it will idle on its own.

Hot start explanation is similar, but if I use the hot start every time it will usually start on the first kick when warm. Most of the time throttle is not needed while kicking.

As a six year owner of a YZ426, if you touched the throttle before the cold start, that bike would flood out. I would actually put my hand on the m/c reservoir to ensure no fuel was added. That bike was a first kick machine no matter how long it sat.

Two questions:

1. What would cause the necessity for the throttle to be held open while kicking during a cold start?

2. Should I step down to a 42 pilot in hopes of getting some adjustability for my fuel screw (say from 1 turn out to 2.5 turns out)? I ride between 1500 and 3000 feet...is this even necessary?

Any help would be greatly appreciated.

Hi! If You have a book store close go to buy the Motocross Action .MAY 2008.

You find your answer in it. if You cant buy I can Copy it for You.

I give it about three quick twists of the throttle and then give it a kick on top dead center with the choke on and it usually starts first or second kick. Holding it wide open does help if its having trouble starting. Ive talked to people with crf450s and they have to give it gas upon starting as well. The owners manual says not to, but it seems normal to do so.


I have the carb apart on the bench and decided I would check the TPS.

At 0% open, the resistance is 760 ohms (spec is 0-2k ohms) - PASS.

At 100% open the resistance is 3300 ohms (spec is 4k-6k ohms) - FAIL.

I had disassembled the carb once before to clean it. It ran fine beforehand.

Would a bad TPS cause the need to hold the throttle open?

Does the idle surge?

No surge. I was running/idling very smoothly after it started.

Hi Eddie. I found that article in the search and its the reason for question #2 above since the comments are closed. I'm just over one turn out with a 45 pilot.

Should I step down to a 42 pilot in hopes of getting some adjustability for my fuel screw (say from 1 turn out to 2.5 turns out)? I ride between 1500 and 3000 feet...is this even necessary?

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