4/15/2021 0 Comments 85 Yamaha Virago 1000
Main tank breathing is handled as in the 750s, but there is also a breather pipe between the main and lower tanks.The main points about fuel tanks are these: they need a way of letting fuel out to the carbs, and they need a way to let air in as the fuel goes out, to avoid a vacuum buildup in the tank.The concern is that we dont want fuel spilling out all over the place when this happens, so we need to block the fuel path and also the venting path, so that the fuel stays in the tank in a down situation.
Some models have one petcock and some two, but the method is the same. One petcock just requires a splitter and two pipes to feed the carbs. This means that they have a little diaphragm and valve set-up in them which responds to engine vacuum and allows fuel to flow only when the engine is running and supplying vacuum. This is how things work when the petcock lever is set to the on or run setting. So if the engine quits for any reason (as in a lay down situation), the petcock(s) will automatically close and prevent the fuel from flowing out. The purpose of this setting is to allow the carb float bowls to be filled when the engine is not running. Note that if you run your petcock(s) on the prime setting all the time, you lose the safety feature provided by the vacuum petcock. Note also that on this setting, fuel is prevented from flowing out through the carbs, into cylinders, etc.) only by the float valves in the carbs, which close when the carbs are full. Should one of these float valves stick open or leak, you then get flooding through that carb. These petcocks have inlet towers which stick up into the tank. In the on position, fuel flows down to the petcock through an opening high on the tower. So when you run out of fuel you actually have some more fuel sitting in the bottom of the tank which lies below this high inlet. When you turn the petcock lever to reserve, a lower inlet near the bottom of the tower opens, so you can gain access to this last bit of fuel in your tank. The sender (which resides in the fuel tank) is actually a switch designed to turn the warning light on when the supply of gas available through the high tower inlet is about to run out. California models were now vented through a hose running to a charcoal cannister which is designed to collect fumes from the tank (and the carbs) when the bike is at rest, and return them into the engine to be burned when the the engine runs. This breather hose also has a rollover valve in it, which peforms the same function as the ball bearing in the earlier gas caps. When the bike goes over, a floating ball rolls into a seat and prevents fuel from pouring out the vent. To my knowledge, Canadian and 49 state bikes retained this gas cap breathing method. The fuel now flows all the time from the main tank to an added lower tank through a big fuel pipe.
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