What's the real difference between Primary Type needle jets and Bleed Type jets

[Jrasm]

I've seen this question asked in various ways across the interwebs but none of them seem to really get to the heart of what's going on with these. Usually the discussion devolves into specifics of who uses what on which model with no real specifics as to how they arrived at their conclusion.
But from what i can gather on another thread,
http://www.thumpertalk.com/topic/888059-needle-jets-bleed-type-vs-primary-type/

So what's the real difference between a Primary-Type (p-type) and a Bleed Type (b-type) needle jet?

They say typically you use P-type with 2stroke motors and B-type with 4stroke. Which means...nothing really.. Typically? Says whom? Has It just been that way since the dinosaurs?
From what i can gather the rationale behind this is that on a 2stroke you spend more time in the larger throttle opening ranges, where on a 4 stroke you spend more time in the <50% throttle ranges (traffic and such). and that because of this, B-type jets are typically leaner up top then P-Type which may be richer at >50%.
Also, atomization of the fuel. Better, more appropriate atomization for 4stroke on a B-Type


If this is true, then isn't it really a simple matter of picking a B-Type when you need a bit leaner but don't want to go a full size down with the P-Type?

And furthermore, if this is really true, couldn't you just pick a wider needle instead of moving to a new jet?

So there has to be more too this than simply "leaner vs richer".

Either way, this is creating some dissonance in my understanding of how these actually work and when to use one vs the other or if they are simply not interchangeable.

For a case in point, my Vm30s came with P-Types. Why? Isn't the application typically(my assumption) for 4stroke motors? Seems silly to think that the good peeps at Sudco wouldn't know the difference.


anyway thanks for the discussion

jeff

 

[Sonreir]

The real difference is the fuel slopes.

Primary needle jets are designed for two-strokes which require less fueling at lower throttle ranges and higher fueling at upper throttle ranges (as compared to four stroke engines).

The reason for these fueling needs doesn't have to do with rider habits, but in the way the engines work. Two strokes suffer more from reversion than do four strokes and so need to be leaner at low RPMs because of the double-carburation. At low RPMs, two stroke engines push yet-to-be-burned fuel/air mix back out of the intake port and that mixture richens the income fuel/air mix even more. The solution is to have a carburetor that provide a leaner mix at that range of operation. At higher RPMs, when a two stroke gets "on the pipe", the volumetric efficiency of the two stroke increases very rapidly and requires fueling that matches that rapid increase in breathing.

Basically, it is about "rich vs lean", but it's not simply adding more fuel or taking fuel away. It's about adding it or taking it away at the right time.

You can mimic this curve by changing the needle taper, but needles are tapered in a linear fashion and fuel delivery doesn't always desire this model of delivery.

 

[Sonreir]

Also... VMs are designed for both two strokes and four strokes (which is why the needle jet can be replaced). They come from the factory jetted for two strokes because, historically, that's where they were used more often.

 

[teazer]

It's also about fuel atomization and fuel droplet size. two strokes darg teh fuel and air into the crankcase and beat them up and squeeze them through transfer passages and squirt them out into teh barrel and all that motion tends to break up fuel molecules quite nicely into small droplets. 4 strokes are a lot more civilized and need the fuel to be emulsified to get smaller droplet sizes. Small droplets increase the surface area to volume ratio and since fuel burns on the outside of the droplet, more area for a given mass of gas means faster more consistent combustion.

Sonreir touched on another key issue of fuel slope. A P type nozzle fuel slope is very different to a B type so it makes a huge difference to low or high speed fueling. So do air jet sizes and height of a spray bar. Fuel slope is the rate that fuel is delivered at constant throttle over a range of engine speed. We change different jets for different throttle position, but at a CONSTANT throttle, fuel delivery is not linear with revs. Changing needle tapers changes fueling over different throttle positions but it's constant throttle that's the second order effect we are discussing here.

With jetting it's important to think in terms of 3D maps. We draw fuel charts as if it was throttle against fuel flow regardless of the biggest varaiable - engine speed. Think three variables Speed, throttle position and airflow. They all effect fuel flow and fuel and air flow musty match engine needs at all engine speeds and throttle positions to work well.