MCRIPPPer
Active Member
- Messages
- 31
- Location
- carmel valley ca
ok guys, i have read some posts, where people are misunderstanding how a walbro membrane carb works.
i would like to help explain the inner workings of these little devices.
their are 3 main parts to a carb: 1. the fuel pump (a membrane, a pulse, some passages, and a gasket) 2. the metering chamber (a big diaphragm, a needle valve, lever, and spring. 3. the bore (jets, throttle shaft [choke in most cases] and needles.)
lets start from square one: the fuel is pumped from the tank, by a membrane system. the common belief is, that the metering diaphragm pumps fuel. this is a myth.
on one side of the membrane, their is the pulse port. it is a small passage coming directly from the crank case. it delivers a steady train of positive and negative pressure pulses, which drive the membrane up and down. on the other side of the membrane, their is fuel. when it goes up, it pushes fuel threw a one way valve, and when it goes down, it sucks fuel threw another one way valve... and the process continues again, and agin.. this simple system pumps fuel threw a passage, threw a filtering screen, and into the metering/float chamber.
the float chamber has a needle, spring, and lever. when fuel is sucked out of the chamber, the atmospheric pressure pushes the diaphragm down, which moves the lever, opens the the needle valve, and lets in fuel, to refill the chamber. this maintains a steady amount of fuel in the chamber note: lowering the pop off pressure (
), will let the fuel in the chamber get replenished sooner, effectively giving the engine more available fuel.
now you have a set amount of fuel, ready to be used. it travels threw some passages into some small chambers, where the adjustment needles are. when you turn the needle in, it closes up a small hole, allowing less fuel into the bore. the fuel gets to the bore, by some jet holes.
the bore is small at the front, and larger at the back. this causes low pressure in the bore, which sucks fuel in to the engine.
it is easier, at least for me, to fix, tune and mod something, when i know how it works. i hope this can help some guys get to know how their carb works, and get some more performance, and enjoyment out of your large scale rc.
now on to popoff pressure. i have an old 668 that i set the pressure with, than i put the spring in my 603. (i didnt feel like taking the carb off the baja, so i set the spring in a diferent carb. im not sure the exact pressure.) all i know, is that the modded spring is WAYYYYYYY lighter than the stock spring. i set it to about 16psi in the 668. i also have a gold spring i set to 10 psi that is way to soft, and floods the motor.
the 16 psi spring works great.
throttle response improved drastically. the dreaded lowend bog is gone :w00t: , and it idles much smoother.
every baja/largescale owner should reduce popoff pressure. it is one of the best things to do for your engine. :cup-1: :cup-1: :cup-1:
i hope this can help some people out with modding and fixing their carbs.
thanks for reading
i would like to help explain the inner workings of these little devices.
their are 3 main parts to a carb: 1. the fuel pump (a membrane, a pulse, some passages, and a gasket) 2. the metering chamber (a big diaphragm, a needle valve, lever, and spring. 3. the bore (jets, throttle shaft [choke in most cases] and needles.)
lets start from square one: the fuel is pumped from the tank, by a membrane system. the common belief is, that the metering diaphragm pumps fuel. this is a myth.
on one side of the membrane, their is the pulse port. it is a small passage coming directly from the crank case. it delivers a steady train of positive and negative pressure pulses, which drive the membrane up and down. on the other side of the membrane, their is fuel. when it goes up, it pushes fuel threw a one way valve, and when it goes down, it sucks fuel threw another one way valve... and the process continues again, and agin.. this simple system pumps fuel threw a passage, threw a filtering screen, and into the metering/float chamber.the float chamber has a needle, spring, and lever. when fuel is sucked out of the chamber, the atmospheric pressure pushes the diaphragm down, which moves the lever, opens the the needle valve, and lets in fuel, to refill the chamber. this maintains a steady amount of fuel in the chamber note: lowering the pop off pressure (
now you have a set amount of fuel, ready to be used. it travels threw some passages into some small chambers, where the adjustment needles are. when you turn the needle in, it closes up a small hole, allowing less fuel into the bore. the fuel gets to the bore, by some jet holes.
the bore is small at the front, and larger at the back. this causes low pressure in the bore, which sucks fuel in to the engine.
it is easier, at least for me, to fix, tune and mod something, when i know how it works.
i hope this can help some guys get to know how their carb works, and get some more performance, and enjoyment out of your large scale rc.
now on to popoff pressure. i have an old 668 that i set the pressure with, than i put the spring in my 603. (i didnt feel like taking the carb off the baja, so i set the spring in a diferent carb. im not sure the exact pressure.) all i know, is that the modded spring is WAYYYYYYY lighter than the stock spring. i set it to about 16psi in the 668. i also have a gold spring i set to 10 psi that is way to soft, and floods the motor.
the 16 psi spring works great.
throttle response improved drastically. the dreaded lowend bog
is gone :w00t: , and it idles much smoother. every baja/largescale owner should reduce popoff pressure. it is one of the best things to do for your engine. :cup-1: :cup-1: :cup-1:
i hope this can help some people out with modding and fixing their carbs.
thanks for reading
