We would like to thank Josh- AKA-RedBeard450 for taking the time and sharing this information we will be providing. Under no circumstances will anyone take responsibility or liability for anything covered in this How-To Article for modifying your Accelerator Pump, attempt at your own risk. After recent review and input, it was brought to our attention that this mod has been performed in the past, but never formalized. If this is the case, we are not trying to claim sole credit for this mod. We did not have any prior knowledge when starting this about any previous explored AP Modifications other than the Yamaha BK Mod. We share this information with the full intent of helping others gain our experience and performance.
This modification consists of altering your Accelerator Pump timing and altering the stock parts associated with the accelerator pump. By performing this mod, there have been noticeable gains in throttle responsiveness, making the bike less prone to low end bog and making the bike perform better than stock. While we understand that each bike is different, we have successfully test this mod on a 2004 CRF450R, 2005 CRF450R, and a 2006 CRF450R, all with noticeable performance gains.
Tools and Parts Required:
Leak Jets: Various
Spare AP Diaphragm(Optional)
Boyesen Quick Shot(Optional)
Feeler Gauge or Small Measuring tool
Dremel or Grinder or Sander
Bike Tear down
Seat, Tank, and Shroud Removal
We will be providing the information based upon a 2006 CRF450R. There might be some slight differences or variations depending on your Model and Year, so keep that in mind when reviewing this document.
The first thing we need to do, is start off with a clean bike, it will make things much easier and cleaner to work on. We will need to remove the Seat Bolts, Tank Shroud Bolts, Tank Bolt, and Right Side Number Plate and Bolt:
Once you have these bolts removed, you should be able to lift the seat out of place exposing the Air Box and the Tank Strap
that connects to the Frame/Sub frame. Remove the Tank Strap
, and then remove the Fuel Petcock Mount Bolt
Once these bolts are removed, make sure your fuel petcock is turned to the OFF position; disconnect the Fuel Line from the carburetor (some fuel might leak out). The tank and shroud assembly should lift out of the frame. Be careful when lifting the tank out, you might have to guide the petcock and fuel line through the frame spar and over the top of the camshaft cover. Once out, set aside.
Partial Sub Frame Removal
The next step consists of loosening the Intake Boot (1) on the carburetor side and Air Box Boot (2) where the air box meets the carburetor.
Once these items have been done, we can move on to the partial sub frame removal. In order to provide enough room, and for the ease of accessing the carburetor, we will be only removing part of the sub frame.
In order to accomplish this, we need to remove the TOP Sub frame mounting Bolt(s) (1). We will also need to loosen the LOWER sub frame mounting bolts (2) located on both sides of the bike.
In order to facilitate moving the sub frame out of the way, you will also need to remove the muffler assembly. The muffler assembly is bolted to the right rear part of the sub frame (1) and the lower right side of the sub frame(1) or frame, depending on your year.
You will also need to loosen the muffler clamp where it meets the head pipe. Once you have the muffler assembly removed from the bike, you should be able to slightly angle the sub frame down, but be careful, you will also need to disconnect the Crankcase Breather Tube (3) from the air box to allow full movement downward.
Now that we have pretty easy access to the carburetor, you will want to disconnect the Throttle Position Sensor and remove the Breather Tube from the cylinder head cover. Route the breather tube under the throttle cables and under the frame spar on the right side. Also, follow your throttle cables up from the carburetor and see if there are any OEM ties that are holding them to the shroud mount or on the top of the radiator, if so, undo them so the cables are free.
Now that we have everything out of the way, you can gently pull back on your carburetor and lift it out of position. It should be able to rest over the right frame spar.
The next thing you need to do is to remove the Throttle Drum Cover Bolt.
Once you have the Throttle Drum Cover off, we can then expose part of our meat and potatoes to this mod.
Below is a picture depicting what the STOCK AP Link Lever, AP Set Screw, and AP Timing Gap resemble. The stock gap can very depending on the bike. For our purposes, I set mine to .100” which is recommended by Boyesen for their Quick Shot.
Without the AP Mod, when you hold the throttle wide open, there is a gap between the AP Link Lever and the AP Set Screw. This is caused by the AP Diaphragm Rivet length which we will discuss later.
As you can see, the AP Link Lever does not follow the AP Set Screw throughout the throttle range. The gap between the AP Link Lever and AP Set screw has measured up to 1/8”.
AP Mod 1
This is the Easy-Way-Out version of this mod. If you are not completely comfortable with everything we will discuss, you can try this version, and if you are happy with the results, and would like to do the complete mod, you can refer to Mod 2 discussed later in the article. While some may contest that this completely throws the AP Timing out of whack, I personally did this mod first on my 2006 with positive results. While I did notice some On-The-Stand bog, I didn’t notice any on the track.
For this part of the mod, you will need to hold the Throttle Wide Open, and adjust the AP Set Screw until it meets the AP Link Lever, about 1/8”.
Once you have the AP Gap set, then take some think safety wire, and make a loop and start your twists so it will be easy to tighten.
After you have made your loop, slide the loop over the AP Set Screw, and around the AP Link Lever, and twist until all of the slack is taken up and the wire is snug around the two. Here are some various pictures of the AP Link Lever and AP Set Screw wired up.
Another variation that was brought up by Jimbo45 on Thumper Talk, was to use a Rubber O-Ring (5/16” I.D. x 7/16” O.D.)
This concludes the How-To for AP Mod 1. If you are interested in performing the complete AP Mod, then read on. Otherwise, follow the steps back through for putting your bike back together just as we described taking it apart.
AP Mod 2
Removing the Fuel Bowl – Leak Jet
First, remove the 4 corner screws that hold the fuel bowl to the carburetor. You will also need to remove the Idle Adjustment Bracket screw in order to take the fuel bowl off.
Once you have the fuel bowl off, flip it over, remove the Leak Jet and check what size leak jet you have.
In order to fine tune the AP Squirt Aperture, we will need to install a larger Leak Jet. Since we will be forcing fuel through the AP Circuit at the same rate as the throttle is twisted, and the AP Circuit will be slightly longer, because of the amount we shaved off the AP Diaphragm Rivet. Because of this, we will need to thin out the AP Squirt by installing a larger Leak Jet. By installing a larger Leak Jet, we are allowing more fuel to flow back into the fuel bowl during activation of the AP. RedBeard450 has tried a few different Leak Jets, and is still testing different sizes for the most absolute throttle response. Currently he has installed a 58 and 62 Leak Jet, and will be trying slightly larger sizes to test the results.
Removing the AP Cover
In order to complete this version of the mod, we will be removing the AP Cover, AP Diaphragm, and shortening the AP Diaphragm Rivet.
Flip the fuel bowl over, and remove the 3 AP Cover screws:
Once you have the AP Cover Screws removed, be careful when removing the bowl. There is a Diaphragm Spring that will be inside the AP Cover when you remove it. If it happens to fall out, make sure you clean it before reinstalling.
With the AP Cover removed, note the AP Diaphragm, it has a rather long, in our case, rivet that protrudes out the bottom. This rivet is what bottoms out on the inside of the AP Cover when twisting the throttle to Wide Open, thus limiting the amount of travel the AP Link Lever can move. ( The picture show the Fuel Bowl on the carburetor, pay no attention, we are simply showing the AP Diaphragm Rivet)
And here’s another picture of the AP Diaphragm Rivet:
(Pay no attention to the Diaphragm sitting on the AP Bowl, the diaphragm is actually flipped over for viewing the side of the rivet only!)
I actually tried to measure the rivet length, and although the picture doesn’t show it clearly, it is about 4.5mm.
Shaving the AP Diaphragm Rivet (Option 1)
We want to take about 2.5mm off of that, or about 1/8”. RedBeard450 has expressed that measuring exactly how much you shave off is Not Necessary, so go ahead and shave it down a fair amount. If you happen not to take enough off the rivet, it will cause binding with the AP Link Lever and AP Set Screw when wiring them together, so make sure you shave it down a fair amount. For shaving the Rivet various tools can be used, for instance sand paper, Dremel tool, grinding blade, or sander. For shortening my rivet, I used a metal cutting blade on my Skill Saw and gently shaved the rivet down.
After you have shaved your rivet down, measure if needed (optional), or if you just shaved it down a good amount, then the hard part is over.
Now that we have our rivet shaved down. We can proceed to putting the AP Cover back on.
Purchasing a Different AP Diaphragm (Option 2)
It has been brought to our attention that there are AP Diaphragm’s that have shorter rivets. If you do not feel comfortable shaving the AP Diaphragm Rivet from your bike, you can purchase a different AP Diaphragm. We have not tested or purchased any additional diaphragms, for our purposes. Some additional diaphragms that might have a shorter rivet would be a CRF250F or KTM which was noted by James Dean of Thumper Talk. Here is a picture he posted representing the differences in rivet length:
Take the AP Diaphragm and place back into the bottom of the Fuel Bowl with the shaved part of the rivet facing away from the carburetor.
After you have the AP Diaphragm in place, then take the AP Diaphragm Spring, and place it in the AP Bowl, and place the AP Bowl into position on the bottom of the carburetor. Secure the AP bowl with the 3 screws.
Fuel Bowl Assembly
After you have gotten the AP Cover back on, it’s time to put the Fuel Bowl back on the carburetor. Carefully position the Fuel Bowl back into place, and tighten the 4 fuel bowl screws, and don’t forget about the Idle Adjusting bracket. Once you get the fuel bowl and idle adjustment bracket back on, we can then move on to verifying the AP Mod we did.
AP Mod Validation
After you have tightened the AP Bowl to the carburetor, turn the carburetor right-side up so that you are looking at the Throttle Drum. Go ahead and hold the throttle wide open, that gap that was previously seen between the AP Link Lever and the AP Set Screw should be gone. The AP Link Lever should now stay mated all the way through the throttle range. Make sure you verify there is NO GAP at WOT, if there is, then you didn’t shave the AP Diaphragm Rivet enough, and you need to go back and shave the rivet down some more.
Wiring the AP Link Lever
Now that we have eliminated the gap between the AP Link Lever and the AP Set Screw, we are now ready to wire them together. As noted earlier, Jimbo45 was successfully able to utilize an O-Ring (5/16” I.D. x 7/16” O.D.) to secure the AP Link Lever to the AP Set Screw. This is also an option. By utilizing a rubber o-ring, you could possibly eliminate some of the added stress that might be induced to the AP Diaphragm. We have not encountered any Diaphragm failures due to wiring, but the choice is up to you.
Start of by grabbing some thin safety wire. Cut a small piece off, and make a loop, go ahead and start a few twists. This will make it easier to attach and fasten.
After you have your safety wire loop made, place it over and behind the AP Set Screw and around the AP Link Lever. Gently tighten until all of the slack is out and you have firmly fastened the AP Link Lever to the AP Set Screw.
Not Recommended Wiring Method
Make sure you place the safety wire behind the AP Set screw, otherwise you risk the chance of it coming off and binding with any of the AP Mechanisms. Here is an example of how NOT to wire it:
After you have completed the wiring, you can them place the Throttle Drum Cover back on, and secure it with the Throttle Drum Cover Allen bolt.
Once the Throttle Drum Cover is in place, you can follow back through the steps on connecting everything back together, i.e. Carburetor, Breather Tube, Throttle Position Sensor, OEM Ties, Sub Frame, Air Box Boot, Gas Tank, Shrouds, and Seat.
This concludes the How-To AP Mod by Professor RedBeard450. Please join us at
to discuss anything that might come up. Please feel free to provide feedback for us, along with Ride Reports so that we can continue exploring the possibilities to make our bikes better. Special Thanks to: