Question about setting up extended run gas tank

Hey guys,
Pretty new here, but have been lurking for a while. I searched for info on this topic but had no luck, so figured I’d record a video and ask.

I’ll type the question out here as well.
I’m working on modifying my pickup truck power wash setup. My buddy gave me a 6 gallon marine gas tank and I’m curious if I can rig it to my pressure washer’s Honda engine to achieve extended run gasoline supply. I wanted to ask here before I did any damage to my machine. I’m thinking of drilling a hole in my gas tank cap and inserting a brass barb with a gasket. This would achieve an air tight gas tank with an inlet. I’d attach the inlet to the supply line for the marine gas tank, which includes a primer bulb. I’ve seen people run these types of gas tanks to Honda generators, but haven’t seen it done on a Honda pressure washer. Can’t think there’d be a difference. I’d assume that if both the gasoline chambers are air tight, this would allow for the gasoline to be drawn through the siphon up into the engine. Thus, the marine gas tank wouldn’t need to be gravity fed, and could be positioned below the pressure washer rather than above.

Would this work? Worst case scenario, I ruin my gas tank cap and buy another one.
This might make more sense by watching the youtube video I linked to above.
I’m ready to try it, but figured I’d ask beforehand. I’ll post results.


Aren’t those caps vented? I mean, air has to replace the gas somehow as it’s used up.

I’ve read a couple people mention adapting a pulse(?) fuel pump to hook up the 390’s to an auxiliary tank. You have to tap into the motor or carb somehow to get a pulse. It was way over my head mechanically and sounded way more trouble than it was worth.

There are also kits to convert the gx motors to fuel injection. That was even more way over my head.

But I have toyed with the idea of getting one of those 5 gallon round aluminum fuel cells on ebay, mounting it above the motor somehow, and just gravity feeding it directly into the fuel line. :thinking:


@infinity is right. Long story short, you won’t get enough vacuum to pull the gas out of that extra tank. If you wanted to do what you’re thinking, you’ll need to bypass the metal tank altogether and tie into the fuel line going to the carb. Then you can either install a pulse pump or a 12v fuel pump (battery required) to pull from the 6 gallon tank. You might be able to raise the 6 gallon tank to get a gravity siphon but it could starve sometimes. Cutting out that primer bulb is a necessity either way

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Yeah, that’s what I liked about the aluminum fuel cells. They typically have the fuel port at the bottom of the tanks, so it could gravity feed just the same as the stock tank.

See image 6:


Right on. That’s very helpful.

Yeah that fuel cell you linked to is a brilliant idea. I might sell the marine tank and put the money towards something like that.
Yes, the cap is vented. Didn’t think this through. You are right that air would need to enter into the chamber.
Thank you for the ideas. I’ll see what’s most cost effective. It’s fun to learn new mechanical tricks.

This is gold. I’ve never though to look for these. Thanks for sharing. I think I may use mine more like a fuel as you go type deal with a fitting attached to the tank itself. I can check it and just turn a valve and let it fill the stock tank.

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If you go that route, make sure to get one with 1/2” or larger plumbing. 1/4” would be painfully slow to watch refuel.

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You can also get a pulse from the valve cover. Just drill a hole, screw in a hose barb, and it’ll pull enough vacuum to run a fuel pump. I’ve also done the carb spacer with hose barb. It’s a quick and simple solution to being able to use a larger tank.

Let’s not reinvent the fuel system fellas. These larger tanks have been on gx390’s for many years now you just have to look. Thers plenty of generators that have 25 litre fuel tanks on them designed specifically for the gx390, you need one of them and a custom mini skid to hold it above the engine. Very common setup on outdoor compressors where long run time is necessary. 25 litres on a gx390 is a tonne!

It’t not reinventing the fuel system. It’s nothing new. A pulse pump is their design they just chose not to put it on all engines. The valve cover barb or carb spacer barb will work on any engine so you can use a fuel pump. They even sell the carb spacer with a hose barb already in it. I had to figure out a way to get a pulse when I put an 18 hp clone in my golf cart. I didn’t have room to gravity feed. The valve cover only takes 5 minutes and you’re fuel pump ready. That’s the route I went so I didn’t have to order the carb spacer. I have used both methods though and both work fine.

Here’s the carb spacer for a Honda GX390.


Super simple, 9 hours run time


I just purchased this pretty stoked! No more fear of my muffler igniting the fuel as I refill.


Even more simple, 8 year run time. :grin:

carb spacer hose barb

Diesel tank


Awesome, let us know how it goes.

On a side note, I always thought it would be cool to tie directly into the truck’s fuel tank.

The only downsides I could see, are not being able to claim an off-road-use fuel credit on your taxes, and not being able to track gas mileage accurately.


@dperez does this. Would be nice only having to refuel the truck.

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By the time you boys and girls find your pulse to feed a 390cc engine I would have gone through 6 of those lol :sunglasses:

We use 12v fuel pumps wired into the ignition key and use 36 gal tank, 12 gal, and two 8 gals. Got tired of the pulse pumps…went in the trash along with the 12v fatboys :confused:

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.[quote=“marinegrunt, post:17, topic:26558, full:true”]

@dperez does this. Would be nice only having to refuel the truck.

I’ve sold all my F550’s and C5500’s they were gasoline powered machines so yes the machines were tapped into the trucks fuel tank. The diesel burners used 30 gallon under belly mounted fuel tanks. I run diesel powered machines now so they run off a 50 gallon under belly mounted tank that also feed the burners