Merkava_4
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I'll try to keep it as simple as I can. The internal combustion engine runs the best and puts out the most amount of power when the air and fuel entering the engine is at the correct ratio. A good starting point is 14 parts air to 1 part fuel. Stoichiometric Efficiency they call it. When the air to fuel ratio is outside of the ideal proportions, engine smoothness and power will decrease. The main variable is air density. Air density is not constant; it varies from day to day. A carburetor is a fuel metering device. It measures the correct amount of fuel for the air that's traveling through it. But carburetors need human intervention sometimes, especially when the air density changes; that's where the mixture needles come into play.
All a mixture needle does is restrict the amount of fuel flowing through a passageway. Too much fuel being allowed to travel through the passageway for a given amount of air traveling through the carburetor will cause what's called a rich condition. A rich condition means the ratio of fuel to air is outside the ideal ratio of fuel to air. Too much fuel and the engine will not sustain combustion at the most efficient level to make power. The same thing can be said for too little fuel. Too little fuel is a lean condition and the power output of the engine will drop off the same way it does with a rich condition. When the air density is less, like on a warm day, the engine requires less fuel. Conversely, cold air is more dense and requires more fuel. Altitude affects air density too. The higher up you go in altitude, the less dense the air is.
A carburetor will have two mixture needles for restricting the amount of fuel. One mixture needle is for idle speed and the other mixture needle is for high speed full throttle. Both needles will come with a bench setting. The service manual will specify the exact number of turns out for both the idle mixture needle and the high speed full throttle needle. Start with each needle fully seated. Then, turn the (L) needle out 1-1/2 turns. Turn the (H) needle out 3 turns. That's just to get the engine started. After the engine is started and running at idle speed, turn the (L) needle either to the left or the right to obtain the highest speed at idle. A turn to the left will richen the mixture. A turn to the right will lean out the mixture.
What you're trying to achieve is the smoothest operation and the highest speed at idle. Too lean and the engine speed will drop off. Too rich and the engine speed will drop off. Both too lean and too rich will affect engine speed and cause the speed to drop off. You're looking for the highest engine speed in between too lean and too rich. Adjusting the (H) needle is done the same exact way. The only difference is you will have the throttle trigger pulled all the way in when you're adjusting the (H) needle. The engine speed will be the highest and the sound will be the smoothest in between too lean and too rich. In other words, as you're going from a lean condition to a rich condition, the engine will speed up as you're turning the needle out which is to the left and then the speed will drop off at some point as you're tuning the needle to the left. It's at that drop off is when you went too rich. Turn the needle in to the right and you'll hear the engine speed up again. The highest speed in between too lean and too rich is the ideal ratio of fuel to air and when combustion is at its peak efficiency.
All a mixture needle does is restrict the amount of fuel flowing through a passageway. Too much fuel being allowed to travel through the passageway for a given amount of air traveling through the carburetor will cause what's called a rich condition. A rich condition means the ratio of fuel to air is outside the ideal ratio of fuel to air. Too much fuel and the engine will not sustain combustion at the most efficient level to make power. The same thing can be said for too little fuel. Too little fuel is a lean condition and the power output of the engine will drop off the same way it does with a rich condition. When the air density is less, like on a warm day, the engine requires less fuel. Conversely, cold air is more dense and requires more fuel. Altitude affects air density too. The higher up you go in altitude, the less dense the air is.
A carburetor will have two mixture needles for restricting the amount of fuel. One mixture needle is for idle speed and the other mixture needle is for high speed full throttle. Both needles will come with a bench setting. The service manual will specify the exact number of turns out for both the idle mixture needle and the high speed full throttle needle. Start with each needle fully seated. Then, turn the (L) needle out 1-1/2 turns. Turn the (H) needle out 3 turns. That's just to get the engine started. After the engine is started and running at idle speed, turn the (L) needle either to the left or the right to obtain the highest speed at idle. A turn to the left will richen the mixture. A turn to the right will lean out the mixture.
What you're trying to achieve is the smoothest operation and the highest speed at idle. Too lean and the engine speed will drop off. Too rich and the engine speed will drop off. Both too lean and too rich will affect engine speed and cause the speed to drop off. You're looking for the highest engine speed in between too lean and too rich. Adjusting the (H) needle is done the same exact way. The only difference is you will have the throttle trigger pulled all the way in when you're adjusting the (H) needle. The engine speed will be the highest and the sound will be the smoothest in between too lean and too rich. In other words, as you're going from a lean condition to a rich condition, the engine will speed up as you're turning the needle out which is to the left and then the speed will drop off at some point as you're tuning the needle to the left. It's at that drop off is when you went too rich. Turn the needle in to the right and you'll hear the engine speed up again. The highest speed in between too lean and too rich is the ideal ratio of fuel to air and when combustion is at its peak efficiency.
