Question about Harley Davidson FLSTF - FLSTFI Fat Boy Motorcycles

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I am getting fuel leaking from the bottom of the carb and I have adjusted the screws on the bottom of the carb but it is still leaking...it is a 1997 flstf fatboy

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Carburetors have a Float Bowl on the bottom. This float bowl has a small amount of fuel, that the carburetor uses. (The float bowl's amount of fuel, is replenished from the gas tank. The fuel is gravity fed from the gas tank, through the Fuel Line)

The float bowl has a Float Needle inside. The float needle is attached to the Float. The Float resembles a thick doughnut. On one end of this doughnut, is a flat piece of metal shaped like a U on the end. Looking at this -> U shape, there are two holes at the top. There is a metal pin that goes through these holes, and the pin is attached on it's end's, by the main body of the carburetor.

The flat metal U shape, and the metal pin, works together to form a type of hinge, allowing the float to pivot on it.
Where the U shaped hinge is attached, the float needle, clips to it.

The float needle has a tapered neoprene tip, and sits in a Float Needle Seat. This seat is a barrel shaped piece of brass, and has a tapered hole at the bottom. Fuel comes up through this seat, and the float needle regulates how much fuel can come in.

As the float drops down, it draws the float needle out of the float needle seat, and fuel comes in. As the float bowl fills with fuel, the float rises, and pushes the float back up, also pushing the float needle back into the float needle seat, cutting off the flow of fuel.

There is a specified height of the float, in relation to where the float sits in the float bowl. If the float sits too high, the float bowl fills up with fuel, and overflows. This overflow goes through an Overflow Tube, on the carburetor.
(If the float sits too low, the engine starves for gas)

A LOT of times, crud builds up in the gas tank, makes it's way through the fuel line, and gets into the float bowl. It can get in-between the float needle, and the float seat, thereby flooding the carburetor out, and fuel comes out of the overflow tube.

The float bowl is also attached to the carburetor with a brass main jet nut. At the bottom of the float bowl, this will look like a brass head of a bolt. Under the brass head is a small O-ring. This can leak. First step is too insure this is tight. (Do Not Over tighten! This is part of the main jet, and is expensive! It's Brass)

Where the outer part of the float bowl itself, attaches to the carburetor, there is another O-ring.
A large one. This O-ring, like the main jet O-ring, can deteriorate over time. Additives that gasoline has in it, heat from the engine, and heat from the sun does this. Plus just age. Neoprene breaks down after time.

It could also be that the neoprene tip of the float needle has broken down.

You indicated you adjusted screws on the bottom.
These are the Main Jet, and Idle Jet -> Air Mixture screws. All they adjust is how much air goes through these jets.
The amount of fuel going through, is predetermined by specific passageways, that are drilled through the main body of the carburetor.

This only comes into play when the engine is running. With the engine off, and fuel leaking out, you have crud under the float needle, or one or more of those O-rings has deteriorated.

Posted on May 12, 2009

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1997 Kawasaki VN 1500 Vulcan Classic how to adjust the air-fuel mixture screw


Hi, Anonymous it should be noted that the "AIR FUEL" mixture screw adjustment "ONLY" manages your idle and has no effect on any other circuit also any intake leaks must be repaired before the A/F adjustment procedure can be performed otherwise you will never obtain a proper idle and you will waste a lot of time chasing the impossible. The A/F mixture screw's purpose is to fine tune the fuel charge entering the combustion chamber. The following applies to both 2 and 4 stroke engines:
1. The mixture screw may be sealed at the factory with a Welch Plug please review the following video for removal.
https://www.youtube.com/watch?v=wAXcksgvDkM
2. The mixture screw manages a range of 3 complete 360-degree counterclockwise turns from the bottom/closed position.
3. The mixture screw should have a spring and o-ring for tension and sealing integrity.
4. Turn the mixture screw clockwise until it gently bottoms out, this makes the fuel charge very lean and the engine should not idle if it does then the pilot/idle jet is too big and needs to be replaced with the next size smaller.
5. Turn the mixture screw 1 and 1/2 turns counterclockwise to establish a baseline for starting the engine.
6. To fine-tune the idle circuit, adjust the mixture screw 1/4 turn in or out to achieve maximum idle RPM, wait 15 seconds between each adjustment for the idle to settle.
7. Never go past 3 full turns out this will make the fuel charge rich, foul plugs, and produce black smoke out of the exhaust, if the engine RPM keeps increasing past 3 turns the pilot/idle jet is too small and needs to be replaced with the next size larger.
8. After achieving maximum idle back out the mixture screw another 1/8 of a turn then adjust the throttle cable idle stop screw to 950-1050 RPM.
9. This procedure works great on 99% of all engines, for the 1% that demand a more robust throttle response on aftermarket monster fuel delivery systems additional tweaking outside the box may be necessary.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
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1. The mixture screw may be sealed at the factory with a Welch Plug please review the following video for removal.
https://www.youtube.com/watch?v=wAXcksgvDkM
2. The mixture screw manages a range of 3 complete 360-degree counterclockwise turns from the bottom/closed position.
3. The mixture screw should have a spring and o-ring for tension and sealing integrity.
4. Turn the mixture screw clockwise until it gently bottoms out, this makes the fuel charge very lean and the engine should not idle if it does then the pilot/idle jet is too big and needs to be replaced with the next size smaller.
5. Turn the mixture screw 1 and 1/2 turns counterclockwise to establish a baseline for starting the engine.
6. To fine-tune the idle circuit, adjust the mixture screw 1/4 turn in or out to achieve maximum idle RPM, wait 15 seconds between each adjustment for the idle to settle.
7. Never go past 3 full turns out this will make the fuel charge rich, foul plugs, and produce black smoke out of the exhaust, if the engine RPM keeps increasing past 3 turns the pilot/idle jet is too small and needs to be replaced with the next size larger.
8. After achieving maximum idle back out the mixture screw another 1/8 of a turn then adjust the throttle cable idle stop screw to 950-1050 RPM.
9. This procedure works great on 99% of all engines, for the 1% that demand a more robust throttle response on aftermarket monster fuel delivery systems additional tweaking outside the box may be necessary.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
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