Glad you found your issue!
Trust me....I'm a tech and I'm also a customer (I own a couple boats/motors myself). You're not the only one who's done that
Demand valves....https://www.google.com/imgres?imgurl=https://www.attwoodmarine.com/UserFiles/Store/Product/Images/1413/full/9300FDV7-S_fu.jpg&imgrefurl=https://www.attwoodmarine.com/store/product/portable-fuel/fuel-demand-valve&h=519&w=600&sz=25&tbnid=uzFWz2774nZgcM:&tbnh=90&tbnw=104&prev=/search%3Fq%3Dfuel%2Bdemand%2Bvalve%2Boutboard%26tbm%3Disch%26tbo%3Du&zoom=1&q=fuel+demand+valve+outboard&usg=__5PeYI39qg-np6Qh1FPQY8XIMGo0=&docid=RZCFDC0N0sU-VM&hl=en&sa=X&ei=VFZzUeysL6nk2wXu4oFI&ved=0CE0Q9QEwBQ&dur=1593
It's supposed to keep fuel from flowing to the motor when the engine is not running. It's supposed to go in the fuel line, inline between the tank and motor. I've never seen one that actually works like it's supposed to.
The EPA mandated low-permeation fuel lines and fuel tanks in early 2011-which is why Yamaha changed their fuel line fitting style. The new style fitting stops fuel flow before it's disconnected (mandated by the good old EPA) so that no fuel will drip or spray out, which is supposed to help with evaporative emissions. They also changed the line itself to a low-permeation material. I like it-it's more flexible and seems like it doesn't get hard over the years of use like the old style lines did. And I like the connector as it's a simple push/turn operation rather than a squeeze the lever, fight with a stuck pin, pull out (and sometimes pull apart) ordeal with the old style. But, It's like $50 for a fuel line. And about $100 for their fuel tank which has the correct fitting for the "round" fuel line connectors. Most guys cut the round connector off of the Yamaha original fuel line, then clamp a typical "old style" yamaha/mercury/mariner fitting to the new style low-perm line. The problem lies in the fact that the aftermarket has not caught up yet. Noone offers the EPA 2011 round-style connector with a 1/4" fitting to screw into the normal aftermarket tank. So what most guys do is to cut the round connector off and clamp an old style mariner/mercury/yamaha fitting onto the fuel line, then screw in a matching merc/mariner/yamaha 1/4" NPT brass fitting into the tank. The problem lies in that when that fuel line is disconnected, since the low-perm tanks DO SWELL UP (and build pressure) fuel can spray out. The idea behind the demand valve was to prevent it from spraying all over when the line is disconnected at the motor. But it adds another avenue for failure, in my opinion. I've seen a couple have problems. We aren't supposed to change the fittings on the lines per EPA guidelines. We at the dealer sell Yamaha fuel tanks, and give customers the option for the attwood....but they have to buy/install all the fittings. The aftermarket really needs to catch up.....that or yamaha can make a 1/4" NPT screw-in fitting for the "round" style connectors. Thus far, neither has taken place.
BTW-lawn mowers also are mandated to the new EPA low-perm fuel tanks. Price has gone up because the tanks cost more to manufacture. And they can't be vented like they used to. So now we have tanks that swell. And if the belt(s) are anywhere near a fuel tank, and the tank swells, there is contact...and the belt(s) can wear holes into the tank itself. So some manufacturers have added a valve which leads to a carbon canister-identical to what cars have had to use for the last 40 some years. If you ask me, it's ridiculous.