Where is the logic in the Accelerator Pump?

Can someone help me understand the engineering behind the accelerator pump? I don't understand why you would want to jet to exacting specifications if all you are going to do is dump an un-managed amount of raw fuel into the intake with every twist of the throttle. Seems like it would just screw things up, or seriously hamper trying to achieve good jetting.

I know about the BK mod. I'm just wondering why it leaves the factory with up to a 3 second squirt. On some tracks you don't even hold the throttle open for over 3 seconds, so every time you open the throttle its always squirting. What effect does this have on jetting?

Assuming you have done the BK mod, does make your bike too lean? Does yamaha account for the long squirt in the Jetting specs?

Its just seems way to unscientific.

The accelerator pump corrects for a lean condition that results from rapidly opening the throttle when engine speed is low. The pump sprays more fuel depending on how rapidly the throttle is opened so it barely squirts if you just roll on the throttle rather than whacking it open.

well the small squirt is factored in in the overall fuel map,---but you are correct it is sloppy.

most of our race bikes use very little squirt and or no squirt from the ap at all,

the bike i ride doesn't have any squirt at all, i don't like the dam thing.

+ it stops your Buddy's from loading up your bike for you while your taking a piss ,, :naughty: witch used to happen to me all the time !!

i don't like the thing myself, but it works.

honda and all the factory's know that 4 strokes have got to be jetted crisp and way closer than the 2 stroke counterparts when leaving the factory, they know that people will use junk pump fuel in them and epa and emissions do factor in also , most people can't jet their 2 stroke and never even get them close, but they are way forgiving and it doesn't matter !!

but 4 strokes are way finickier and the factory's don't have the luxury of just making it fat and calling it good with these large carbs and lots of cam duration and the potential for a low velocity and poor signal to the carb when whacking the throttle from idle, so they have got to have an ace in the hole and that is the ap squirt,

but when jetted correctly and depending on fuel and how well you set up your bike and the type of riding you do you will start using less and less squirt

when you are really riding smooth and rolling on the throttle Early and hitting your lines good , your not hardly squirting any fuel from the squirter anyway,

the last two years of 4 strokes have slowed that circut down some


What is the Bk mod again?

That's some interesting info. Do you run a much fatter pilot to compensate? What other changes do you end up with in jetting. I hear what your saying about the throttle roll on but there always seems to be a couple places on any given track where I need to stab the throttle pretty quickly. This really interests me because I am looking to get my 05 YZ450 running as crisp as possible. I have to admit that a couple of the Hondas I've ridden make me a little envious with the real smooth low end jetting they seem to have. I like the way my bike accellerates but I want it to feel as smooth as possible comin off the low rpm's when I get a little tired and lazy in those tight corners late in a moto.

Great Link!

Pre-EPA cars had APs standard for the afore mentioned reason. I've driven cars where the diaphram failed and they are quite difficult to drive. You'd have to accelerate very slowly. If you opened the throttle a little too quick it would bog because it had gone super lean. Generally drivers behind you would celebrate your misfortune by giving you the classic salute.

the biggest thing for really getting these things to run good is the fuel your using, ( and being able to change ignition curves ),

if your running pump fuel you can get a bike to run very well with .2 .3 seconds of squirt when you hit it hard, witch is almost none.

some of the good race fuels you don't need any with careful needle selection and clip position,

the first early fcr carbs did not have any way to adj them hardly at all , and they were way to rich especially with race fuel .

that is what the bk mod was, just a way to alter the timing for the squirt,

racers all over the world have been doing this for 75 years,

i did not have our first yz 400 race bikes for 10 min at the track and that is the first thing i changed was the dam squirt time, i mean god it was terrible .

those first ones were like 4 seconds and they dripped and there was no way you were going to really jet the dam thing , or get it to start !!:naughty:

on all the emission carbs that is the first thing they do after it warms up is bypass the pump jet circut, and some close the idle circut until decell,

i mean non of this is new it been done for years,

when your bike is stink hot you would be supprized how little and or no squirt you need to keep the thing making better power :naughty:

all the top runners on 4 strokes have no squirt !!

Hey Kelstr,

What is the quick fix on my 05 crf450 to shorten the squirt time. It felt pretty good last weekend after I put that fatter slow jet in, but it is not as snappy as I think it could be and I'm pretty sure that the AP's dumping too much fuel. What exactly is the screw on the side of the carb doing? Does it adjust when the squirt starts, compared to slide movement? Does it adjust the duration of the squirt. It's just such a pain to pull the damn bike apart just to adjust, since you can't really tell while the bike's on the stand. I've read all of the threads on the BK mod and such, but there has got to be a simpler way of cutting down on AP duration, but make sure that it reacts instantaneously to throttle movement. Or do I just have high aspirations? :naughty:

If your squirt duration is too long, a larger leak jet will make it shorter. The leak jet is relatively easy to change out, because it is in the bottom of the bowl. I don't know if you have to disassemble the bike to get the bowl off on you Honda.

hi redbeard , barton is correct the leak jet can be increased in size to decrease flow .

what i like to do is with the subframe off i will play with the squirt quite afew times and time it and see where it is stock ,

most 05 450's are about .5 seconds on pump fule with a hard snap , so i note that and then i play with acoupple of different leak jets and time them and record what they are and now i know that one is .5 and one is .2 and so on and then just go test it and you can sure tell in a hurry what works better

i also like to make sure the streem is not hitting the slide or spraying on the needle or dribbling or so , you can correct it if it is ,

you can also adj the screw like what the bk mod was and also change it up slightly and change if it comes on early or slightiy later and alter the time sum.

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