Transworld Motocross: Tuning the Accelerator Pump

Now that four-stroke motocross bikes have made a full swing back and are obviously here to stay, two-stroke mechanics need to learn a few more tricks if they plan on owning a happy thumper. Valves, cams, oil pumps, and counterbalance shafts are all in the mix, but one subject nobody seems to talk about is carburetor differences. It is common knowledge that four-strokes feature push-pull throttles and a fuel screw instead of an airscrew, but in general the words "accelerator pump" aren't heard too often at the local track.

Todd Brown came down from the Yamaha of Troy headquarters to give us an idea of how he sets up Brock Sellard's YZ250F for competition. Yamaha introduced the new-age four-stroke motocross bikes first, so it seems proper to talk to a Yamaha tuner to get the lowdown on how to keep the bikes running crisp.

One of the biggest battles when jetting a thumper is getting rid of the bogs off the bottom. The basic two-stroke jetting theories apply to four strokes, but there are a few different components that play a big role. Four-stroke bikes have an accelerator pump built into the carburetor, and usually this is the most important jetting tool in fixing carburetion problems.

Every time the throttle is twisted, the accelerator pump shoots a stream of gas into the carb. The throttle has to be closed so the pump can reload and wait for the next time the throttle is twisted. This is why your bike gets flooded when your buddy twists its throttle on the stand, and probably why you are considering how good of a buddy he really is when you try to start it.

 THE CONFUSING PART

When riding a Supercross track the throttle opens and closes frequently. This can result in too much fuel loading up the carburetor before it has a chance of cleaning out. Even though you aren't on the gas for long, it creates a rich condition. The opposite situation is a fast, loamy motocross track with long straights. The throttle is always wide open and burns the accelerator pump's fuel quickly while pulling fuel from the main jet. When the throttle is shut off for a corner and stabbed back on for the next straight, chances are good that there won't be enough fuel for the motor, thus a lean bog is created.

Now that your head is spinning and you are completely confused, it is best to go try it out for yourself. Practice makes perfect, so buy a cheap thermometer/humidity gauge and a logbook. Keep track of your settings for different tracks and conditions. This will make life easy in the long run, since it is really hard to jet an accelerator pump with the bike on the stand. (Even the factory wrenches don't get it right all the time.)

 YZ25OF CHEAT NOTES

All four-stroke carburetors on modern motocross bikes work on the same philosophy, but here are some cheater tips for '03/'04 YZ250F owners. Todd told us that the stock jetting is really close on the 250F, but there are a few things that have to be changed. The fuel screw should be between 1.5 and 2 turns out. On the YoT bikes, the motors are obviously works, so they opted to go up one size on the pilot jet. (They bump it up to a #45 pilot jet). On a stock bike this shouldn't have to be changed, but Todd noted that if your bike is hard to start this can often make it easier.

The other jet that is changed is the bleed jet, which we will discuss later. The stock bleed jet is a #90, but YoT replaces it with a #75 to coincide with their accelerator pump setting.

When working on a YZ250F, these tools will be needed: three and four millimeter Allen keys, a 14mm wrench, Philips #2 screwdriver and a small flat blade screwdriver.

 JET AWAY

STEP 1) On the right side of the bike, where the throttle cables connect to the carburetor, there is a cover protecting the throttle linkage. This cover must be removed.

STEP 2) Using your fingers, grab the cam that actuates the accelerator pump. Try to feel if there is any free play before the cam actually moves the accelerator pump. There may not be any, but don't worry; we'll adjust that next.

STEP 3) Factory mechanics actually measure the engagement point of the accelerator pump a little differently than how the owner's manual normally says. On the YZ250F, turn the Philips screw out until there is no play in the linkage, or if there currently isn't any play, turn the screw in until there is some play. The start point that has to be achieved is the exact point where there is no play. Take your time and make sure it is right, remembering that it is easiest to start with play in the cam and turn the screw out slowly until there is no play.

Now that the starting point is found, it can be adjusted. The more play there is, the longer the delay on the starting point of the accelerator pump, which makes the bike lean. If there is no play at all, the accelerator pump will start to pump fuel into the carb as soon as the throttle is twisted, resulting in a more rich condition.

On the YZ250F, Todd sets his accelerator pump delay screw 0.5 to 1 turn in from the starting position previously mentioned. This allows for some free play or delay on the accelerator pump. Now that the timing of the accelerator pump is set, it is time to check the amount of fuel that is being pumped into the carburetor.

STEP 4) The bleed or leak jet controls the amount of fuel that is pumped back into the float bowl, offsetting the amount that flows into the carburetor. This is something that will have to be played with to achieve the correct jet. Once the proper jet is installed, the jetting will usually be adjustable with the delay screw. To change the bleed jet, the carburetor must be twisted and the float bowl must be removed. On a 03-04 YZ250F it is a good idea to remove the hot start housing before rotating the carb. It can easily be broken if not removed. (The 14mm nut must be unscrewed before it pulls out.) Now that the hot start is removed, loosen the float bowl screws and remove the float bowl.

STEP 5) Using a small flat blade screwdriver, loosen the bleed jet located in the float bowl. (Bleed jets are typically the same as two-stroke power jets.) Some testing will have to be done to determine the best jet. YoT changes to a smaller jet (#90 to a 75), in turn making the accelerator pump shoot more fuel into the carb. The smaller the jet size the smaller the hole, resulting in less leakage into the float bowl and more into the carburetor venturi.

The YoT setup obviously shoots more fuel into the carb at a delayed time. They have found a bleed jet that will work well when the delay screw is from a .5 to 1 turn from the "no free play" starting point. This makes changes at the track easy; all they have to change is the amount of delay on the accelerator pump.

STEP 6) Before replacing the float bowl, check the accelerator pump diaphragm by removing the three screws and pulling off the cover. YoT changes their diaphragms every five hours, which Todd admits is overkill. For the average guy it wouldn't be a bad idea to change it a couple times per year or any time the accelerator pump adjustments don't seem to be working properly. When installing a new diaphragm, make sure the part numbers that are cast into the rubber are always facing you. It is possible to put it in upside down, and it obviously won't work if you do.

 FINAL NOTES

If this article has scared you more than it has helped you, it might be a good idea to leave it up to the pros. Companies like Pro Circuit, Bill's Pipes and FMF spend days finding the best jetting specs for every production bike. Call any of these companies and they can help you with a good base setting. Their advice might not be perfect for your local track, but by using Todd Brown's methods it will be easy to get it spot-on from their base settings. Good luck!

Reprinted from Transworld Motocross February 2004

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