Question: What makes fuel come through the carby jet?
Hi friends,
I've just got my bike ripped apart and I've never been good with mechanical things and engines but I'm learning now through pocket bikes.
I've just wondered, what causes fuel to come through the carby jet and how does using the throttle (moving that little pin up and down) increase and decrease the amount of fuel that comes through? Is it just gravity and air pressure feeding the fuel through?
edit: Oh and I just had a flashback to my childhood watching some pommy .edu show on the ABC, I remember something about air flow running over the top of a straw and water getting sucked up the straw and sprayed out the top. I'm guessing this has something to do with it.
Hi there all I can help you with is that it starts as being gravity fed on these bikes once the bike is started I guess the piston moving up and down is causing a vacuum and sucking the fuel through the carby the needle and seat I will start with the seat the size of the hole will determine how much fuel your Carby will recieve the needle is tapered works like a cork to put it simply the more you twist the throttle the higher the needle raises thus opening the hole in your seat then letting the fuel through then there is the issue if the carby jets depending on size again this will allow the fuel to be sucked into your engine the bigger the hole the more fuel the basics of any engine working whether it is a 3hp pocket bike of a 800hp big block is to get the fuel in and out as fast as you can so just rejetting your carby wont give you an extreme hp improvment when you do one mod there is always something else that needs to be done to compensate
hope this has shed some light on your question but I am sure there are way more qualified people in this forum that will probobly correct me if I have stuffed anything up and will lead you to your desired answers
I myself am very new to 2 stroke engines but I have spent 20 yrs building chev small blocks for my cars these lil 2 stokes are a great challenge for me and a lot easier to throw around my shed if something isnt working the way i want it too :lol:
What is the "seat" you talk about? Where is it in the carby?
I didn't know the throttle pin in the carby was tapered. I was wondering how fuel got past it if it wasn't all the way out. This would explain that. I see now how that would affect the flow of fuel into the carby.
I also think that air rushing over the top of the carby jet (I think it's the carby jet anyway, the thing the pin slides into) must affect the amount of fuel that gets drawn from the carby bowl, so the more air you can allow to rush over the jet, the richer you can run the bike/bigger the piston you can have?
if you have a stock carby the needle screws to the top of the carby the needle itself has about five slots I think that allows you to adjust it the seat is where the needle point goes into to and blocks the hole as for running it richer you mean more fuel the bike actually goes quicker if you give it less fuel up top and more air but be very carefull on this point because as I have been told (mind you after I melted 2 pistons) is that the leaner it runs the hotter it runs in 2 strokes apparently it is the fuel that actually cools your piston down and if it isnt getting enough fuel it runs hotter and faster but does do damage so by moving the position of your needle up you will get more fuel through the carb if you drop your needle you will get less fuel because it will still be partially blocking the hole
Think of a lawn mover when you go to shut it down I used to turn the fuel tap off so that the carby would not get fouled up with oil while it just sat around ok you have turned your fuel off and the mower is still running with whatever fuel is in the carb but as it starts to run out it also starts to rev more this is happening because it is sucking more air than fuel but if you kept it that lean for too long the mower wouldnt last that long
hope this helps you somehow like I said before I TOO AM NEW TO 2 STROKES
Well, starting at the top, the fuel flows in from the tap to the top of the carby and in through the needle and seat. The needle I'm talking about has a black neoprene tip and sits on that little metal Y shaped pivot and the seat is the brass bit built into the body of the carby. The bottom of the needle is spring loaded to stop it leaking through vibration. As the fuel fills the bowl of the carby the white plastic doughnut shaped thing (called, funnily enough a float) floats up on the fuel and presses on the Y shaped metal pivot which pushes the needle up into the seat and shuts off the fuel. This is why fuel shouldn't piss out everywhere, this system allows the correct amount of fuel in the carby bowl at all times, as you use it up, the float goes down, needle comes away from the seat and more fuel comes in etc etc etc.
Now for how it gets into the motor. At the bottom of the carby submerged in the fuel is a brass tube with a jet (brass screw with a precisely sized hole in it) in the end of it. The tube is called many things, emulsion tube, needle jet and so on, but I'll just call it the emulsion tube. The jet needle (the one in the middle of your slide) goes up and down inside this tube as you open and close the throttle, but I'm getting ahead of myself here. Back to the mainjet. The hole in the mainjet controls the rate at which fuel can flow can flow up the emulsion tube to mix with air before entering the motor. This is how you can adjust the richness or leaness of the motor by increasing or decreasing the size of the hole, kind of like drinking through a straw with a restriction in the bottom.
Ok, but what makes the fuel go up the emulsion tube? It's called venturi effect. As the air is drawn into your motor by the suction of the piston, it has to squish down to get through the hole in the carby. As it squishes down, it gets faster and faster (this is why I put such a big taper in the mouth of my carby, it helps this effect). as the air goes zooming over the top of the emulsion tube it creates a vacuum (like when you blow across the top of a soft drink bottle) which draws the fuel up from the carby bowl. The air and fuel then mix together and head off to the motor to burn together.
What about the jet needle, the needle in the middle of the slide that goes up and down when you twist the throttle? Take it out and have a good look at it, see how it's tapered? There's a very good reason for that. When the throttle is closed, the fat part of the taper is at the top of the emulsion tube, closing off most of the fuel flow because at idle you dont need much fuel. As you open the throttle and lift the jet needle, the taper means that the amount of jet needle blocking off the top of the emulsion tube is smaller so more fuel flows out to mix with the proportionaly greater amount of air flowing through the carby. This is also why there are five groves in the top of the jet needle. If you have the clip in the middle position but find that your motor is running too rich, you can move the clip one grove higher which will lower the needle putting more of it into the emulsion tube and therefore reducing the amount of fuel avalialble. Since the needle is tapered, the effect is felt throughout the entire throttle range. It's more of a fine tuning method, often, as in my case, when you make changes to the carburettor or engine which affect the amount of air flow you need to compensate those changes with more fuel, but in my case one groove is a bit on the lean side, while the next groove down is a bit on the rich side. This is when it's time to change mainjets.
Thats a very basic explanation of how the simple carby on a pocketbike works. Other more complex carbys have idle bypass circuts, low speed jets as well as mainjets, power jets, accelerator pumps (4 stroke only) etc etc, but we don't have to worry about any of that stuff. Pulse rate carbys (like Walbro) work completely differently (which is also why they're so much better), but I don't know enough about them (apart fromt the fact that they use vacuum pulses from the motor to squirt fuel in via a diaphram) to explain it, and besides, that wasnt the question :lol:
Anyway, I hope that made some kind of sense to you, I explained it as best as I can from behind a keyboard :wink:
Thanks very much :oops: :wink: Now shall we go into reeds, 4 petal vs two petal, reed tension, dual stage reeds, carbon fibre vs fibreglass reeds, boost bottles, powervalves, exhaust porting...
Cool, I remembered that venturi effect (though I didn't know the name) after I posted this. I figured this would probably how fuel gets drawn up from the carby bowl. Didn't know about the grooves in the carby needle or that it was tapered until you both explained that now. I have to muck around with the throttle cable more and rip it apart I think. The sticky spring gives me the shiats and needs fixing.
On air intake, I've seen those "Hi-Clone" things for cars that makes the air intake swirl like a tornado. Does this do anything? If yes, what? I'll take a guess and say that it would make the air move faster or perhaps mix the fuel and air better? I know the fastest way to empty a soft drink bottle is to turn it upside down and swirl it to make the water 'tornado' out. Does air move in a similar way?
No, for the volumes of air and flow rates that we're dealing with a Hi-Clone type device won't achieve much. May well end up being a flow restriction actually.
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Really wasn't looking forward to getting typers-elbow. :roll:
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