Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn’t have enough gasoline can cause a backfire, too. A “lean” mixture is one that doesn’t have enough fuel, and too much air.
An air screw adjusts how much air is being delivered thru the pilot circuit: in is rich (less air) and out is lean (more air). A fuel screw adjust how much fuel (or air/fuel mixture) is being delivered from the pilot circuit. In is lean (less fuel) and out is rich (more fuel).
A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to have a surge or miss at idle and part throttle, stumble on acceleration, engine overheating, cause a lack of power, and create possible engine failure from the lean air/fuel mixture.
The engine should rev smoothly and quickly as soon as you apply throttle. If the vehicle is displaying any sort of sluggish performance or misfires when you apply the throttle, then more adjustments are required.
The idle mixture screw controls the air fuel mixture at idle speed, and often is located near the throttle plate.
On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.
Make adjustments 1/2 a turn in either direction to find the smoothest idle speed. Turn the screw counterclockwise and clockwise 1/2 a turn from the middle position and listen to the sound of the idle. Set the screw in the position where the engine’s idle sounds most even and smooth to balance the fuel mixture.
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Cleaning a carburetor without removing it is fine. However, it can and should never replace the wholesome cleaning exercises. This is because it does not impact the entire length and breadth of the engine as should be the case.
Q: How often should I clean my carburetor? Generally speaking, you should clean your carburetor every time you change your oil. For most vehicles, that’s about every 3,000 miles. However, you may want to clean it more often if your car sits for long periods of time.
The needle position has no affect on idle, it basically affects mixture around the 1/2 throttle range. If you need to change the WOT mixture you need to change the main jet, then change the needle to suit half throttle needs.
One of the signs that the carburetor float is sticking is when the engine will not idle. The float is not letting enough fuel into the reservoir, allowing for a constant idle of the engine. The carburetor float is stuck in the closed position, and only a small amount of fuel is seeping into the reservoir.
Within the needle jet the needle moves up and down to vary the orifice size to meter the fuel. The tapered shape of the needle controls the amount of fuel coming up through the main jet and through the needle jet. As it goes up, the opening gets larger, letting more fuel into the mix.
Fuel To Air Mixture Is Too Lean
Too much air and not enough fuel causes backfires to occur in the intake manifold. The exploding mixture then vents through the carburetor. Improper carburetor adjustments or vacuum leaks can cause this condition.
A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.
TLDR – running just a little lean could improve fuel economy and give extra power. However, run too lean and you risk engine failure because the engine runs too hot. Whereas running rich can waste fuel and increase pollution but will not damage the engine.
Vacuum leak: Vacuum leaks can cause a lean fuel mixture. Inspecting and replacing damaged vacuum lines with good hoses and clamps can solve a lean problem. Clogged fuel filter: clogged fuel filters will limit the amount of fuel requires by the engine.
Lean running conditions: If your engine is running too lean, the spark plug will be white. If the spark plug is black and oily, they are oil fouled.
Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns. If your carburettor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube.
Lack of secondary accelerator pump shot and a delayed secondary opening will increase fuel economy. The vacuum secondary carb’s fuel calibration is usually more efficient as well. In either case, make sure the idle and main air bleeds located on the top of the carburetor remain clean and unrestricted.
To balance carbs (or synchronise the carburettors) on your bike is simply the process of adjusting the throttle bodies so they pass the same amount of air and fuel mixture to each cylinder. It’s important because a well set-up fuel system will help the bike start easily, run smoothly, and be more efficient.
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