Dec. 09 - EVOLUTION FUEL DELIVERY - Part III: Petcocks - concludes

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Article concludes this thread.


The 1984-91 gas caps are not designed for 1992 and later models. They vent via a pressure/vacuum relief valve in the single gas cap, or the right gas cap if a dual gas cap setup. With built-in limitations, air can flow both ways through the earlier vented gas caps. The vacuum caused by gas flowing out of the tank will pull open the vent to allow air to flow in and replace the volume of liquid leaving. As gas fumes build up, they will push the valve the other way so the fumes can flow out. As pressure equalizes inside and out in both scenarios, the valve will close. The simple gas cap is not so simple.

How does the valve open and close? Certainly negative pressure will open it to allow air in, and positive pressure greater than atmospheric pressure will open it the opposite way to let expanding vapors out. As for how the valve closes from either direction as pressure equalizes, there are two springs, one on either side of the shutoff/open plate. A heavier, larger-diameter spring is on the outside, and a weaker, smaller-diameter spring fits inside. They don’t work against each other so much as engineers want the expanding vapor pressure to compress the heavier spring to open the valve, and less spring pressure to open the valve the opposite way in order to let air in. One reason for a heavier spring is a safety one. If the bike falls over, it will take more pressure to open the valve to let dangerous combustible gas to flow out of the vent. Each spring is strong enough to activate the closing of its valve once atmospheric pressure equalizes on both sides. Neither spring pushes its valve, but instead they provide seating resistance as both expanding gas and negative vacuum pressure causes their respective valves to open. The spring only pushes the valve back against its seat as the gas tank’s inside and outside
pressures equalize.

The valves for each spring are ingenious in design. The opposite ends of the springs fit into metal carriers for base support. The flat dual valves fit against and with each other. The smaller, flat plastic valve has an appendage that fits through a hole in the center of the larger-diameter, flat, rubber valve so they constantly align. As expanding internal tank pressure forces open its larger rubber valve for venting, the smaller, plastic, vacuum air valve seats against the center hole as if both pieces were one.

1992-2000 Cap Venting
In 1992, Harley-Davidson went from a dual-venting gas cap arrangement to a continuous venting system. The gas tank continuously vents while the gas cap now only vacuum-vents fresh air in to replace draining gas. Now I can get to the good stuff. The plastic screw-on section holds the two lower items. The white plastic piece holds the valve in against a seat located inside the black plastic screw-on bung. The plastic, rubber, and metal valve sits below and inside the center of the screw-on section. The small, round, gray plastic centerpiece has an extension that protrudes through the black rubber seal with a weak spring underneath it to hold the two together, thus providing a vacuum seal for the gas tank. As gas drains out of the tank, air must replace the decreasing volume.

The gas tanks vent through an internal metal tube called a standpipe that connects to an external hose to allow gas vapors to vent continuously. If removing the dash on the fuel tank, there may be two hoses or one hose, depending on model. One hose is an overflow that runs to the ground to allow gasoline to escape. To my mind, this is dangerous, since I’m always overfilling my tank and many times wind up with a puddle of evaporating gasoline underneath my hot motorcycle. Unfortunately, sometimes the gas will spill over my burning hot exhaust header pipe, below the gas tank. I have to discipline myself to pay attention, but my mind always seems to be thinking about a mechanical problem, analyzing something, or simply floating along in a daydream. It seems that if I’m on a customer bike, I always pay attention and give it the respect it deserves. On my own bikes, it’s a different story.

The other hose, which connects to the metal tube extending from the gas tank bung, is the evaporative tube fitting that the hose connects to for continuous venting. This hose also connects to the vapor valve on both 49-state and California bikes. After the vapor valve, another hose vents fumes into the atmosphere, away from hot components while the hose leading from the vapor valve on California bikes feeds the fumes into a charcoal canister for eventual purging back into the combustion chamber to safely burn as many harmful hydrocarbons as possible. The 1992 and later gas caps will not work on 1991 and earlier models because they are for a continuous venting system and do not act as the evaporative vents themselves.

Mechanics and riders servicing their own bikes must pay attention to the routing of hosing, especially rubber hosing that serves parts of the continuous venting system. With gas tank removal and reinstallation, it is extremely important to ensure that vent hosing doesn’t kink, preventing free movement of air and evaporative gasoline; that vent hosing does not clog up, again preventing free movement of gaseous air; and that the vapor valve is installed correctly in the up position or it will work opposite to the norm. The longer, thinner portion must face up. The vapor valve is firmly attached to an immobile object like the frame center downtube to ensure proper continual working of the internal venting piston and spring.

From a safety perspective, in order for a rider to enjoy the ride without impending danger it’s important the continuous venting hose and the overflow hose, if there is one, are properly routed out of harm’s way. Hosing must route and be tied off to prevent contact with moving components or the exhaust pipe. Moving components may frictionally rub a hole through the hose, allowing vapor to escape. Friction also causes heat that may ignite vapor, or the vapor may vent against an exhaust pipe or other danger spot. Contact with the exhaust pipe will burn a hole through the hose in a few seconds with possibly dire consequences.

Conclusion
Next month we’ll talk about types of carburetors and the principles involved.


Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com