Monday, March 31, 2008

Something technical this time perhaps Mr. Bond??

Recently, I have been having too many thought and discussions about bikes, the reason being the passion and determination of two maniacs and also part of my own drive, which was increased out of logic and also out of my cancelled trip to the western coast. The two maniacs I am talking about are my good friends and along with me, share a long standing passion with bikes, two-strokers to be precise. Those two now have the bike of their dreams and although those two machines now put together produce the same horsepower as one of them used to produce when it was brand new, they still ride it as it is an engineering masterpiece. I think they're right, they are engineering masterpieces according to me, the only problem I've seen with those bikes is running out of petrol! Where as, I've seen some strange things happening to the current generation mass produced Indian motorcycle, strange noises, unexplainable lack of performance, extreme temperature sensitivity and most of all “sudden death” which has left a few unfortunate souls in the middle of the highway scratching their heads thinking why the bike won't start! Although I too have been the victim, in recent times, to very mild glitches and gremlins, I've spent enough time and effort to see that the bikes keeps its head together always, literally!

It is in connection to the last sentence of the previous paragraph that I am writing today, about a very special part of the bike, something which is rapidly being outdated and replaced by newer technologies, but still remains to be the personal favorite to true bikers. The Carburetor.

Think of the carburetor as a burette (I make this comparison cause it's easier to attach to the word 'burette' which rhymes a little to the word 'carburetor') in a titration. The aim of the burette is to see that you have a measure of the amount of liquid (in this case petrol) you're giving in order to complete the reaction in the flask (in this case the engine) to the maximum. Yes, I know, I suck at comparison sometimes so lets get down to business.

The carburetor or the 'carb' for short is a device which gives the charge to the engine..... the charge here being a mixture of petrol and air. The aim is to provide the right amount of fuel to the amount of air being demanded by the engine or to put it shortly, the 'quality' of mixture. But the engine is never being run at the same speed, so as speed increases, the amount of charge demanded also increases, so the 'quantity' of charge needed should also be managed by a carb.... hmmm... tough job you may say. But there are ways in which it can be achieved! The carburetor has varying demands, from a minimal mixture at idling speed to a lean mixture at slow speeds, normal at regular, rich during acceleration and a slightly lean again during cruising. There are ways of doing that, beautiful ways.

The carb is designed with a converging-diverging section also called a 'venturi'. The narrowest point of the venturi is the metering point of the engine's demand. The quantity of fuel is controlled by a throttle at the engine-side of the carb.

Now, for the quality, there are multiple circuits of fuel which are cast into the main body of the carb during the time of it's manufacturing, these circuits are in turn metered by jets which are always immersed into a pool of petrol in a 'float chamber'. The petrol is then drawn up from one or many of these jets based of the engine speed and the pressure at the throat of the venturi and many other factors. But, what goes through where and when?? that's the big question, right?

When the bike is idling, there need not be much of a rich mixture, to control this there is an orifice which is partially closed by the pointed tip of a screw called the idle mixture screw. This takes fuel from the slow jet and controls the quality of the charge right from idle to nearly about half throttle under hard acceleration, then the size of the opening and the slow jet and also the drop in pressure occurring at the throat makes the idle jet partially useless at moderate throttle and completely useless at moderately-high throttle. So there was a second jet introduced, this is called the emulsion jet in some places or the needle jet at some places, there are reasons for both names, cause both hold true. The emulsion jet cause the fuel is partially converted into an emulsion by bypassing the air into the emulsion jet before the fuel is completely atomised in the main venturi. The emulsion jet sits right under the throat of the venturi and feeds fuel based on the.................. aha, this is where I have to confuse you a little further cause there are two main variations in the design of the carb. One is the simple slide carburetor which has a tapered needle (called jet-needle) sitting right inside the emulsion jet (called the needle-jet). The needle is connected to a barrel which keeps blocking the venturi thus limiting air flow into the engine and therefore the pressure at the throat. This barrel also acts as the throttle, the minute the throttle is twisted, the barrel moves out of the way allowing more air into the engine and reducing the pressure at the throat and also simultaneously removing the needle out of the emulsion tube and uncovering the Small holes drilled all along the sides of the tube. Thus the emulsion tube will be producing more emulsion and spewing out more mixture to be atomised, thereby feeding the engine with more fuel. The second type is called the constant depression or variable venturi or constant vaccum type, in this the barrel and the needle is placed under a spring loaded vacuum piston governed by the amount of air which is being drawn into the entrance of the carburetor. The difference in pressure makes the vacuum piston move up thus removing the needle out of the needle-jet. The throttle is a different part in this type of carb, and is placed at the engine end of the carb.

This mechanism gives enough fuel from nearly quarter to three-fourths throttle. The final part of the punch is provided with some thing called the main jet, which is screwed on to the emulsion jet and is completely immersed in fuel in the float chamber. When the demand of fuel is more, even the emulsion jet can't meet the quality standards, at that time, the pressure of air rushing directly draws fuel into the emulsion tube from the bottom of it through the main jet and feeds it into the venturi. At this point, all other circuits inside the carburetor would've stopped providing fuel to the engine and the main jet will be the sole supplier of fuel.

There are however a few other mentionable circuits in a carburetor, but these are of special design and almost never available in the commercially produced, factory fitted carbs of today with a few exceptions of the past. One of them is called the top end enrichment jet, or so I call it. It's job is to enrich the fuel mixture far beyond what the main jet can. It sits in the air filter end of the venturi and at full throttle, provides a spray of fuel, again controlled by pressure difference, to cool the engine and also to give it that extra punch. Till today, I have only read about this type of carb. The second modification is called the coasting enrichment circuit, this circuit kicks in and starts leaking fuel into the engine as soon as the throttle is closed. Activated by the manifold vacuum, the circuit starts giving out small quantities of fuel into the engine and therefore keeps the valves from burning out and also help the engine and the bike coast under closed throttle. But the huge quantity of air being taken in during coasting coupled with the small amount of fuel being delivered makes it a non-ideal mixture and therefore it does not ignite in the engine but passes into the exhaust manifold thus cooling the engine. But occasionally, the mixture does ignite, and when it does the heat of the exhaust gases passes into the exhaust manifold and ignites the unburnt charge which is present there , thereby causing the characteristic 'pop' of one famous and extraordinarily brilliant bike of the late 90's and the early 2000's.

This was what I learnt during the last one month, when I had set about to tune my carb for my trip. Now looking for something else to learn about. I wonder what is it going to be next!! Till then, cheers!

1 comment:

  1. Well written, but at throttle position above 75%, other circuits do not 'stop' providing fuel. Fuel enters through all ways and the main jet too.. Means to say, the pilot jet, needle jet and the main jet would be spewing out fuel at full throttle!