Andy_S wrote:This is from the Polini CP Carb instruction page, which I presume will also apply to the PWK type.
The atomizer is a tube that connects the main jet to the venturi and it is called “two-stroke type” when it doesn’t have any hole. Otherwise if it has a series of holes along its entire length it is called “four-stroke type”.(Attention: to call the atomizers “2 or 4 stroke” is popular but technically incorrect because you can safely use atomizers with holes for the 2 stroke too. I.e the Minibikes have atomizers with “4 stroke” type holes). Its working is very easy: due to the depression in the venturi then, from the atomizer tube the liquid fuel is drawn, metered by the main jet and by the conical needle, while a certain airflow comes from the channel. Air and fuel mix together and the result is a pulverized spray sucked by the engine. To intervene in this process, in addition to the atomizer hole, there are the air channel diameter, the height of the part of the atomizer protruding into the chamber and the height of the step located in the nozzle. In addition to the atomizer’s hole diameter, the variables are therefore the diameter of the air channel, the height of the atomizer’s side that protrudes in the chamber and the “step” of the delivery nozzle. The “four stroke” type produces leaner mixture and more controlled; in fact since it is equipped with a series of holes the emulsion is done inside the tube, before the mixture reaches the nozzle in the venture. Obviously the arrangement of the holes and their diameter influences the delivery. Holes machined in the lower part of the atomizer are bathed in the fuel of the bowl so, by increasing the numbers of holes the flow of fuel increases and goes to emulsify itself with the air. The holes in the upper part of the atomizer are exposed to air and by increasing them you obtain weaker carburation at low rpm.
The POLINI CP carburetor can be adjusted so that it works with both the types, 2 or 4 stroke, changing the brake air jet.
Close air brake jet: 2 stroke atomizer type working as an atomizer without holes
Open air brake jet: the atomizer holes work and make the engine weaker.
URL for the page this was taken from - https://www.polini.com/en/technical-are ... tructions/
I have read similar information produced by other manufacturers, regardless of the type of carburettor.
The manufacturers have the knowledge & resources to make virtually any of their range of products work on a two or four stroke. In fact, Dell''Orto refer to the 'step' (in Polini speak) as being the norm for two strokes (a fact I'm certain we can all agree with) yet acknowledging that some racing two strokes may not have them fitted. Interestingly, as far as I am aware, the PWK's that we discuss here have no needle shielding in the venturi, yet the larger PWK's fitted as OEM to 'crossers do. I am not about to even try to question that, but I would vouch for the clean carburation in any case of the PWK's we discuss here to suggest that they probably do not need it. That conclusion is drawn by the fact that I have a converted VHB Dell''Orto that has a two stroke conversion via the PH needles, atomisers & venturi needle shield. IMHO, no doubt due to the flat guillotine slide, it works better than the round slide product that donated the bits (I hope that one day there will be a market for converted VHB carbs! Ha!)
The point I'm trying to make, is that given unlimited resources & a product with two stroke 'friendly' slides, then a good combination of atomiser, needle, slide cutaway & body insert could be found. Let's not forget the effect of atomisation upon the fuel air mix in a two stroke, which gains larger surface areas as the revs rise, resulting in the need for less ignition advance. What will occur if that efficiency were to be increased via 'extra' atomisation of the mix down the scale? Further retardation @ low revs?
Personally, I can't see much point in going down that route with the PWK's that I rate highly enough to work with what I have. The only augmentation likely is the adjustable power jet that I have grafted on. Currently it adorns a reed valve engine so can wait until it is back on a piston port engine.