How-To: Jet a 4 Stroke Dirt Bike

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Year, Make, Model

The dirt bike featured in this blog post and How-To video is a 2006 Yamaha YZ250F. All the 4 stroke dirt bikes are equipped with Keihin FCR carburetors, so this post should guide you in the right direction no matter what brand you own.

 

how to 4 stroke dirt bike jetting

 

Circuits We Are Going To Jet

  • Pilot circuit – This is effective from 0 to 1/8th throttle positions.
  • Accelerator pump – This circuit is responsible when the throttle is quickly opened, and squirts fuel in engine when slide opens.
  • Needle & clip position – This circuit effects ¼ to ¾ throttle positions.
  • Main jet – Main jet is ¾ to wide open throttle (WOT)

 

Pilot Jet & Fuel Screw

I like to start jetting a carburetor by getting the pilot jet sized correctly and the fuel screw set properly. Reason being, the pilot jet contributes to all other circuits by a small amount, and can cause the following symptoms if not set correctly. 

Some of these symptoms include

  • Hard starting HOT or COLD. Using the hot start lever often.
  • Hanging idle which indicates lean mixture.
  • Decel pop. Do not try and richen the pilot circuit to eliminate decel pop. Check the silencer packing, which may help. 

I cover pilot jet sizing here in detail. You want the fuel screw to fall between 1 to 2.5 turn out max. Any further out, and you run the risk of the fuel screw falling out. I personally like to have the fuel screw fall at 2 turns out so there is some adjustability left either way. This adjustability may be needed for temperature changes, etc. If the fuel screw ends up at 1 turn or less, decrease pilot jet size. If it ends up at 2.5 or more, increase pilot jet size. 

 

Also, do yourself a favor and install an adjustable fuel screw. The stock screw is impossible to get at and you will usually burn yourself.

 

adjustable fuel screw fcr mx

 

 

 

Below is a video showing you how to properly tune your fuel screw. I would run through this video and set up your fuel screw before removing carb, so you can make the decision on whether to change the size of the pilot jet.

 

Accelerator Pump Circuit

The accelerator pump (AP) squirts fuel into engine only when when you whack the throttle really fast. This is needed to compensate for the lean condition of the slide opening. If you roll into the throttle really slow, the AP does not squirt any fuel at all. I’ll explain this later on. There are 2 adjustments that can be made to the system, fuel volume and timing of squirt. 

First let’s talk about the AP timing. The AP timing screw (see image below) is located near the throttle wheel can advance or retard the squirt timing. I usually do not adjust this from the stock setting. If you need to modify timing, turning the screw out will advance the squirt. The opposite is true for turning the screw in. It seems most desirable for the timing to be as soon as possible and it must miss the slide as it opens. In the past, I have adjusted the timing to just miss the slide. You can test this with air box removed and engine NOT running. Whack the throttle and pay attention to the squirt. 

 

o ring mod ap timing screw

 

Next is the leak jet. Changing the leak jet is the ticket to eliminating the bog. Here is how the leak jet works: 

Fuel bowl > leak jet  > AP squirt nozzle 

The above represents the path the fuel takes (left to right) from the fuel bowl to AP squirt nozzle and eventually to the engine. Notice the leak jet is in the middle. Imagine a garden hose hooked up to your spigot at one end and a sprinkler at the other end. These represent the fuel bowl and AP squirt nozzle. Now go poke a hole somewhere in that hose between the two. What happens? Less water will squirt out the sprinkler. The poked hole represents the leak jet, except it will bleed fuel back into the fuel bowl. So a numerically larger leak jet will have a larger orifice and bleed more fuel back into bowl and less fuel to squirter. The opposite is true for a numerically smaller leak jet. Also, if you open the throttle slowly, all fuel will be bleed to bowl via the leak jet. There is no need for a squirt of fuel if throttle is opened slowly.

 

leak jet location float bowl 4 stroke mx fcr

 

Now that you know how a leak jet works, they usually come from the factory with a 70-100+ size. Since these bikes are meant to run WOT, and they are probably trying to conform to some environmental standards, they are usually too lean from the factory and it that is why they all BOG when you whack the throttle from a roll. 

I typically install a 45-55 leak jet in most of the 250F’s. You will want to decrease leak jet number size until the BOG is gone.

Now, there are several companies that sell adjustable leak jets. The nice thing about these aftermarket kits is the ability to adjust the the leak rate by turning a screw with a screwdriver that is super easy to access. Given how much of a PITA it is to remove a carb on a 4 stroke, this may be a better option for you if you only want to tear the carb apart ONCE.

 

Another AP improvement that is super easy and inexpensive is the “O-ring mod”. The o-ring mod simply couples the AP pump arm to the adjacent shaft (not sure what it is called). This will move the pump arm quicker and result in a much stronger pump shot. You can go to any hardware store and ask for a #78 o-ring, it fits perfect. You can also use safety wire, however I prefer an o-ring since there is a little compliance if needed, reducing any binding.

 

o ring mod ap timing screw

 

Needle & Clip Position

To be honest, I don’t have any sound advice on how to select needles. I usually surf the web and see what all the talk is about, and then usually try it for myself. Forums and magazine articles is usually where I start. 

As far as the needle clip position is concerned, I use my butt dyno to see what change feels the best for ¼ to ¾ throttle positions. I typically move the needle clip up and down to see what runs best. Remember, the needle clip position is always referenced from the top. Moving the clip up lowers the needle causing a leaner mix. Moving the clip down raises the needle causing a richer mix. 

This 06’ YZ250F came stock with a OBELQ needle with clip in 3rd position. In 2008, they changed the needle to a NFPR with clip in 5th position. The 08’ needle is a dual taper that claims more bottom to mid performance. If interested in this needle, just click the ad below and navigate to the OEM parts finder, 2008 YZ250F, carburetor diagram, and order the needle.

 

Main Jet

I have a post here on tuning a main jet. The main jet is ¾ to WOT positions. You’re going to need some open space to perform this test! As you can see in my video, I was not able to do so.

 

Comments

Has this blog post & How-To video helped you? Please comment below.

 

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