Pressurized Milemaster 6gal tanks and similar

[JBrit12]

Hey guys, very new to the forum but undertaking a rebuild of a tank (old style 1956 and prior) for my father's QDL-18 10HP Johnson. My buddy is also doing a rebuild of a 54 5.5HP Johnson and has the 4 gallon tank while I have the 6 gallon. He's a little ahead of me and actually had his running last weekend but was very concerned that his tank was building way too much pressure and is certain there should be a check valve or something to release pressure at a certain level. Is this true? I've been looking around and haven't found anything definite either way. I was thinking that the 2nd spring located on the opposite side of the primer button spring may have something to do with it but I don't really know either. The parts for that area are called pressure release valve spring, pressure release valve, and pressure release valve washer so I'm thinking duh, that is it. If it is, at what pressure should it open to release some pressure when fully functioning? He said his hissed for a long time when he unscrewed the cap after about 5 minutes of running the engine. I read that they can build between 5 to 7psi normally. Just wondering if anyone can give a good explanation of how that works. Thanks. I should be working on the project a lot this weekend minus time for yard work as most of my parts for the motor and tank reassembly should be here. Much appreciated and I'm really getting into this and my goal is to some time maybe having every 1957 ever made! My next project will be the boat and a 1.5hp Evinrude from 1968 which looks in amazing shape with very few hours on it.

 

[RCO]

I think they are designed to vent at around 7 psi from the rubber stopper with the spring under it. I have personally never seen one vent, and I have put a lot of hours on them. The crankcase pressure shouldn't ever get that high.

 

[F_R]

The motor pumps about 4-5 psi of air into the tank, which pushes fuel up the second hose to the carburetor. Leaving it out in the hot sun certainly can cause the pressure to rise above that. But it builds too high, the pressure relief valve and spring will open and allow that excess pressure to escape. Don't worry about it not working. Much more common is failing to seat, preventing normal pressure build-up from the motor.

People with those motors love the satisfying hiss when the cap is loosened at the end of a good day on the water.

 

[JBrit12]

I think they are designed to vent at around 7 psi from the rubber stopper with the spring under it. I have personally never seen one vent, and I have put a lot of hours on them. The crankcase pressure shouldn't ever get that high.

Thanks, sort of what I was figuring and my father said they never had any issues way back when. The tank is completely cleaned and all parts look pretty close to new and totally operational so all I'm replacing is gaskets. I'll post back when I get it up and running or have any issues to troubleshoot. Thanks again.

 

[JBrit12]

The motor pumps about 4-5 psi of air into the tank, which pushes fuel up the second hose to the carburetor. Leaving it out in the hot sun certainly can cause the pressure to rise above that. But it builds too high, the pressure relief valve and spring will open and allow that excess pressure to escape. Don't worry about it not working. Much more common is failing to seat, preventing normal pressure build-up from the motor.

People with those motors love the satisfying hiss when the cap is loosened at the end of a good day on the water.

Thanks. You reiterated my thoughts and findings. I'm way less worried about it than my buddy as I wasn't there when he tried his but he did mention when he took his apart his spring was collapsed so maybe he has a bad spring or put it back together incorrectly. I'll pass it on and again, thanks.

 

[Chinewalker]

If he's worried about it, he can burp the air out any time. Always a good idea to either leave the tank with the cap loosened a tad, and/or to disconnect the fuel fitting when you're done for the day anyway. If pressure were to be left built up, it's possible a stuck carb float, or even tilting the motor up, could cause the pressurized fuel to seep through the float needle and either flood out the motor, or drip out the carb and leak all over the boat, motor well, and water.