By Mark Cantrell - Member # 6179 on http://www.thumpertalk.com/
This is just the basics of what the AP is, how it works, and how it can be tuned.
As the piston goes through the intake stroke, the intake valve is open and air is being sucked into the cylinder. This 'sucking' creates a vacuum. How much depends on the RPM of the engine and the throttle position. Vacuum is higher at high RPMs and higher at closed throttle (slide covers almost all of throat, blocking the air from the airbox). Vacuum is highest on high RPM and closed throttle (when closing the throttle at high RPM, i.e., when decellerating). Vacuum is lowest when the RPMs are low and the throttle is open (when giving it full throttle from low RPM, i.e., when accelerating). Vacuum is what pulls the fuel up through the pilot, needle, and main jets.
(There are two vacuums, one due to the intake stroke sucking against the throttle slide and through the air filter, and a second one due to the flow through the narrowest part of the intake. Where the intake is narrowest, the velocity will be highest, and according to Bernoulli's law the pressure will be lowest (vacuum highest). I'm going to pretend that all of this is just vacuum pulled against the throttle but will be glad to expand if anyone is interested).
Four stroke carbs usually have accelerator pumps. When the throttle opens rapidly, especially at low RPMs, the vacuum goes away reducing the 'signal' (or indication or method) to the jets to pull gas up from the bowl. As the revs build, the vacuum comes back. This means that between a rapid throttle opening and the revs rebuilding, the amount of fuel will be too little.
The accelerator pump, unlike any of the other fuel circuits on the bike, doesn't rely on vacuum to pull fuel. Instead, an actuator connected to the throttle pulley goes into a plunger (diaphram) on the bottom of the carb, squirting fuel as the throttle is twisted. This produces a squirt only while the throttle is being added, not at a constant throttle or trailing throttle. Again, the purpose it to make up the fuel lost to low vaccum until the revs build recreating that vacuum.
Like any other circuit (e.g., pilot circuit, needle circuit, or main jet circuit), there may be more fuel added or less fuel added than is needed by the AP. In addition, unlike any other circuit, the AP squirt may be the right amount but not last long enough (to build revs) or too long (being rich after revs build).
You can 'see' the squirt clearly. Take the carb off the bike with the bowl on and full of fuel. Twist the throttle pulley (with a rag, it has sharp corners where the throttle cable comes in). Do not be looking into the intake side. A stream of gas should shoot out 4 to 8 feet. It will last between about a 1/2 second to 4 seconds. If you don't want to take the carb off (and who does?), you can loosen the single phillips screw on the hose clamp that holds the air boot on the rear joint of the carb. Push the rubber boot out of the way and wedge it behind the subframe. You will see the back of the throttle plate blocking the throat of the carb. There will be a small brass head just behind the throttle plate on the bottom of the throat and slightly to the right of the middle. Have a buddy twist the throttle. You may need a drop light or flashlight. You will see the throttle go up and a thin squirt of gas shooting forward out of the brass thing. Notice there is no other gas, because there is no vacuum to pull it through the pilot, needle, or main jet. Anyway, that is the AP squirt. Blip the throttle, get a little. Whack the throttle, get a lot.
You can 'feel' a correctly functioning AP, if you like to abuse your bike, by selecting a high gear and riding on level ground just above idle. Whack the throttle. It will lug (at least on a 250F, a 426 might just leap away) because at that low RPM, it can't make enough torque to accelerate much. For a second or two after you whack it, it will be OK because the AP is making up for the low fuel condition caused by low vacuum. After the AP squirt is over (it lasts longer in practice than you measure in the garage, I don't know why), it will be too lean. It may pop, stutter, vibrate, or even die entirely. This doesn't mean it needs adjusted, no AP is tuned for this condition where it can't build revs. The rider is not supposed to let that happen.
Newer bikes (either 2001 and up or just 250Fs) have a leak jet that 'leaks' some of the squirt back into the bowl. The AP is purposely built too strong so a smaller leak jet would send most of the fuel into the throat and a larger leak jet would send less into the throat (that is, more would leak back into the bowl). This allows adjustment from too much to too little.
Similar to the way leak jets reduce the volume of the AP squirt, the BK mod reduces the duration of the squirt. The BK mod consists of drilling and tapping a blind flange on the carb, putting in a small screw with a spring to keep it in place, that limits the travel of the cam connected to the throttle pulley that actuates the AP. It will not make the squirt longer, but by limiting the travel, you can adjust the squirt shorter than whatever it naturally is.
Most of these bikes come with enough or too much and too long AP squirt. Larger leak jets and the BK mod control each. DOC has found bikes that have too little duration and/or volume. He plugs the leak jet entirely and then uses the BK mod to adjust the duration back down.
If your bike is idling on the stand, it can build revs faster than under any riding condition (except downhill or decellerating). In this case, the AP should not have to squirt very long for the vacuum to rebuild. If you are lugging (remember our 5th gear lug show and tell above?) (too high a gear, going up a steep hill, etc.) it will need the longest squirt. With the BK mod, you should adjust your squirt for your riding conditions. I'm not sure that the .3 seconds (Brian Kinney's original suggestion and the gospel for most on this forum) is ideal. I'm sure Brian knows what is ideal for Tim Ferry on a MX track. I'm not sure that would be enough for me in a tight gnarly hare scramble (partially because I suck in tight gnarly and am often in too tall a gear).
OK, now how to tune. First, find out if you have a leak jet. Look in the manual under Specifications where all the jets are listed. Is there a leak jet? If so, you're in luck for this part. Drop the bowl on the carb (Jetting 101 describes this in painful detail but to reduce the suspense, you don't have to pull the carb). In the bottom of the bowl, about 1/2 way from the center to the back, brake side is a tiny brass jet with a flat head screwdriver slot in it. Unscrew it and see what number it has on it. Bigger jets give less squirt (more is wasted back into bowl). Smaller jets give more squirt (less is wasted back into bowl).
If you have a leak jet and want to learn about it, remove the leak jet and put the bowl back on the carb without it. Now all of the squirt will just go back into bowl, completly diabling the squirt. Now start the bike. Hmmm, started just fine. Why, because according to the drill we never twisted the throttle. Without twist, there is no AP squirt. Therefore the AP has nothing to do with starting (except to flood the bike if you use it before running). Now roll on throttle slowly. The revs should build fine because there is never a drastic dip in vacuum. If you ride it and just roll the throttle slowly, it will probably run better than ever. This is because it won't be getting the too rich condition of the AP when it isn't needed (just slowly rolling on throttle). Now, back on the stand, blip it. It will cut out, run rough, cough, or just die. If it doesn't die, it will eventually catch and run fine. If you whack it, it will just die, even on the stand. Now get out and ride it. Since the revs build slower (since it has to haul your ass around now), the coughs and stumbles last longer on small blips. If you ride it around a while, you will learn that in lower gears, you can roll it on faster from low RPMs than you can in higher gears. Again, all this makes sense. This is a pretty cool learning method. It's exactly what I did first.
So the goal in adjusting the AP is to select enough squirt to get it past the low RPMs but not enough to outlast the low RPMs or create a too rich condition during the low RPMs. It will burble like a two stroke with the choke on if too much and just have low acceleration. It will cough, cut out, or die without bucking at all if too little. I bought a 40, 60, 80, 105, and 125 leak jet and tried them all. The stock for my bike as a 60 and even though I have YZ'ed it (cam timing, uncorked, throttle stop, grey wire (now Vortex), etc.) it is almost perfect. 40 is way too much squirt. 80 coughs too much, 105 and 125 just die with almost any throttle unless on stand.
Now for the part I haven't done. I have not done the BK mod. I have a squirt of just under 1 second and it works well for me. So take the following as untested advice. The BK mod came with a suggestion of .3 seconds or a certain gap between the screw and the cam (I don't remember what the gap was, but it was a feeler guage kind of thing). This was suggested by Brian Kinney who wrenches for Tim Ferry. You may have noticed that a top 5 kind of MX/SX rider like Ferry doesn't lug his bike around the track. He keeps the RPMs up, always picks an appropriate gear, and doesn't ride in the woods where the speed changes are more drastic. That tells me that the .3 seconds will be about right for me when H*ll freezes over (unless you count while I'm day dreaming of passing Ricky and Bubba in the SX finals). Early 400's came with close to 4 seconds squirt, which everyone agrees is too much. Maybe for a retiree on a 1200 pound street bike complete with Grandma and luggage for two weeks in Florida, its just right.
My guess is somewhere between .5 and 1 seconds is about right, depending on the caliber or rider, the type of track/woods, and if you keep it up off the bottom. At over maybe 6K RPMs, the AP is just wasting fuel.
Anyway, I'm sure there are mistakes in what I've said, both editorial and content. Please correct me with replies. I'm also sure there will be controversy, no post pleases everyone here. Most will think I'm lazy or slime for not having done the BK mod. I probably am.