See also Choosing Fuel & Oil | Does Avgas Run Cooler?
The fuel system provides fuel to the carburetor. It usually consists of a tank with vent line, primer bulb (optional), engine driven fuel pump (pump is integral to most membrane carburetors) and shutoff valve (optional). Make sure any valves are opened, there’s gas in the tank, disconnect the fuel line from the carburetor and blow into (or pressurize somehow) the vent line. Fuel should push up and out of the carburetor end. If it does not, the filter (if installed) may be plugged or the fuel line may kinked or pinched or a valve has failed closed.
Fuel tanks can usually be purchased from your dealer or from cart shops like Comet Kart Sales.
Quality fuel line is Tygon, available from many PPG vendors and GoKartParts.com. The usual size is 1/4″ inside diameter and 3/8″ outside diameter to fit on Walbro and most other carbs. Number is VTC-30.
Air In Line
Air bubbles mean air is leaking into the fuel line somewhere. Try to see where the air bubbles are first visible—that’s your culprit.
- Primer bulbs are common problems. Consider removing the bulb from the system and joining the fuel line with a simple nipple to see if that cures it. Even if you don’t see air bubbles, if the bulb’s check valve fails, you may not get sufficient fuel flow.
- Fuel line cracks. These can be impossible to see. Usually they show up where the fuel line goes into a nipple or around a bend or gets chafed (rubbed enough to cut into the material).
One method to check for an air leak is to remove the fuel line all the way from its gas tank pickup to the carburetor. Plug one end while putting all but the carburetor end under water. Blow into the carburetor end to to see if you get air bubbles anywhere. If you do, that’s where your leak is. (suggested by Bruce Brown).
Tygon is the gold standard for fuel line and lasts up to three years.
Don’t use wire ties on fuel line since they don’t hold the fuel line evenly around its radius. In fact, they frequently cause air bubbles to form.
Fuel leaking down the fuel line is a sure giveaway. Pump fuel from the primer or blow in the vent. If fuel comes dripping out, bingo, you’ve got a leak.
If fuel has been in the tank for a long time then it may have gone bad. On average fuel will last about a month even under the worst conditions. How long you can store fuel depends on several factors:
- Type: Avgas has fewer additives and is made to not deteriorate. It should remain usable for several times longer than pump gas under the same conditions. It has higher lead content, though, so using it may allow lead deposits on the piston and cylinder. These deposits are easy to remove — about like carbon.
- Sealed: Fuel in an airtight container will last several times longer than if stored otherwise. If your tank hisses when you open the cap then it’s airtight.
- Temperature: Fuel stored in high temperature will deteriorate the quickest.
- Quality & Delivery Age: Not all fuels are created equally. It’s true that fuel is made in a very few refineries and differences usually come down to minimally significant additives by the different brands. Quality is usually consistent but not always. Plus, if the fuel was old to begin with, your shelf life will be reduced. Avgas is rarely a problem although it may not be as fresh when pumped since airports sometimes get deliveries less often.
Combining factors will make matters worse. For example, storing an open container of car gas in a hot garage for over a month is asking for problems. Sealed avgas may last for several years.
Bad fuel may prevent the motor from starting at all and will cause it to run poorly if it does start.
If you suspect that the fuel flow is insufficient there are two possibilities. A blockage in the fuel path or vent path.
- Vent: The gas tank must vented to allow air in as fuel is burned. Otherwise a vacuum forms that eventually prevents free fuel flow. Unscrew the cap and blow into the vent line. If you cannot blow into then either a one-way valve has malfunctioned (if installed) or there is something blocking it. Replace the vent line and/or one-way valve.
- Fuel Line: Make sure there’s gas in the tank, disconnect the fuel line from the carb and blow into (or pressurize somehow) the vent line. Fuel should push up and out of the fuel line’s end. If it does not, the filter (if installed) may be plugged or the fuel line may kinked or pinched.
- Plugged Carburetor Filter: Most carbs have some sort of filter. Insure that it’s clear. Hair or lint is the most common cause of these to foul.
Fuel can leak from anywhere along its feed path and it’s usually pretty obvious where. A fuel leak may also mean that air can get in your feed system (See Air In Line above). Here are some common places to leak fuel.
- Fuel Line: Usually where it enters or exits or has sharp bends. A leak at the carb nipple can be hard to notice since fuel may run down and away but this is one common location.
- Vent: You lean forward and gas runs up the vent and overboard. You may be able to buy a one-way valve that lets air pass but not fuel. It must allow air out or else building pressure in the gas tank can force fuel up through the carburetor and possibly overboard or into the motor.
- Tank: A hole in the tank can be repaired with variable success using zzz T.
- Carburetor: If its leaking around where it attaches, replace the gasket or make your own using a gasket material that is fuel tolerant.