Gas Helicopter Fuel Tank Plumbing - My Way

[carey shurley]

I’ve read quite a few posts about whats the proper way to plumb the fuel system for a gasoline helicopter. Like most model helicopter things, there’s no one way to do anything There are always many and often all will work.

So I’m going to offer my technique as an example, its not the only way but I virtually guarantee that it will work correctly every time.

Generally the fuel tank is provided with the model so I’m not going to spend much focus on them. If for some reason you have to provide a tank, use a Dubro or Sullivan HDPE tank that is thick and hard. I’ll explain why later.

Unless they are already molded into a custom designed tank, the first thing you’ll have to decide is where to put the fittings. Depending on the exact setup you’ll need three or four:

• The fuel pickup – feeds the carburetor
• The fuel return – from the primer circuit
• A tank vent – the carburetor has a negative pressure fuel pump, it will pull fuel from the tank so it must be vented or it will collapse
• A fuel fill – this is optional depending on how the tank is vented – more when I discuss venting

The exact position for these will depend on how the tank mounts, frame layout, tank location etc. You’ll want the fuel pickup fitting to be as near the center/centerline of the tank so that the fuel clunk can move to the most extremes of the tank. Unlike glow models its position won’t really affect the fuel flow because of the fuel pump involved.
The other fittings should all be on the top of the tank since the model is usually sitting on its skids when you put fuel in it.

Parts Used List

• Fuel Tank - Any Dubro or Sullivan RST style
• Tygon fuel tubing
• Viton fuel tubing - Zenoah Part # 1799-85400
• 90 degree inlet - Miniature Aircraft part #MA0410-90
• Fill/Vent/Return - Miniature Aircraft part #MA0405
• Fuel clunk - Walbro/Zenoah #125-527-1
• One way valve - Stens/Echo #610-079
• 3/4" hole plug - Jasco - Ace Hardware
• Aluminum tank stopper - Sullivan #478
• T fittings - Sullivan #486
• Fuel plugs - Thunder Tiger #PV0251

Here’s an couple of examples of my tanks. You can see the pickup fitting on the side and the other 3 on the top.

The fittings I use are from Miniature Aircraft and the part numbers are:
Inlet - 90 degree #0410-90
Fill/Vent/Return #0405

FYI with two piece fittings like the ones shown, I use neoprene washers (available at Ace hardware) to help seal them to prevent any leaks. These aren't required but as shown they don't leak.


Typical fuel tank with fittings


Another view of tank fitting positions with one way vent installed

Some older designs use tanks with removeable screw top caps. I’ve seen many people fuel their model by turning it on its side, removing the cap and just pouring in fuel. If your model uses one of these tanks and its accessible, that’s another option for filling the tank (and you wouldn’t need a fill fitting)

Whats inside?


Clunks - Typical (Left) - Stens (Center) - Walbro -preferred (Right)

So typically glow models use a rather simple clunk like the one on the left. This will actually sort of work in a gas helicopter but I don’t recommend it. Because there is always more vibration, this type of clunk tends to dance around on the bottom of the tank. As you get lower on fuel, its much easier for it to pick up air and cause the motor to get leaner. I prefer to use a filtered clunk which is very common with most small gasoline motors. I’ve shown two types, the one in the center is a Stens/Poulan #610-381 which uses a plastic sleeve and comes stock with the Spectra-G helicopter. This will work fine for a while but over time the porous plastic gets dirty and will slow the flow of fuel. Its not really cleanable. I don’t recommend it. The one on the right is a Walbro/Zenoah #125-527-1 which uses a felt filter. Its cleanable and is slightly heavier than the stens. I recommend that you only use this filter.

You’ll need to plumb the inside with a gasoline tolerant fuel tubing, I recommend the tubing that comes with Zenoah motors, its Zenoah Part # 1799-85400. Due to the size of the clunk fitting, you’ll need to use 1/8” ID tubing. Tygon in this size is fairly stiff and doesn’t let the clunk move around well enough and it can wind up out of the fuel supply in certain model positons. The Zenoah tubing is very flexible and comes like this: (the stopper and clip are easily removeable)


Zenoah Part #1799-85400

Usually in a glow setup you run a very short line such that the tip of the clunk can just touch the tank in any position. Because these filtered clunks are much heavier use enough fuel line on the feed that the clunk can literally move anywhere in the tank. A typical length would be 6-8”. Heres an example of how that should look.


Plumbed clunk with loop

What if the clunk won’t fit through any hole on the tank?

Now suppose the tank that came with the kit doesn’t have a hole large enough to use one of these felt clunks? Don’t let that stop you, its very important for consistent operation to use one of these clunks. The felt soaks up the fuel even when there is very little left. You’ll have fewer unexpected lean runs. So it won’t fit what do you?



Clunk won't fit? Cut a hole and plug it


On the top of the tank, in a flat area, cut a ¾” hole. Then run the fuel line through the normal tank hole and then through this large hole. Connect the clunk, pull it into the tank and then insert the normal fuel pickup plug in the hole where it goes. Go to the Ace Hardware store and get a ¾” hole plug. You’ll find it in the yellow Jasco hardware bins that most Ace stores have. Get some silicon RTV, apply it around the edge of the plug and seal it into the hole. That’s it, done.

Plumbing the tank/carburetor

So how to plumb the tank to the motor. Here’s an example that I put together. The lengths of the tubes are only for example, any part can be pretty much any length but I’d keep the fuel pickup to less than 12”

For the fuel inlet, you want to run a straight line, no filters or T’s from the fuel pickup on the tank to the fuel inlet on the carburetor. If you use this as a fuel line especially with a T fitting, you’ll get air bubbles in the lines that even priming the fuel system won’t remove. However when you get it in the air the vibration will pull it into the carb. I’ve seen that cause a hiccup. With the straight line this can’t happen

When you pump the fuel primer, it pulls fuel through the carburetor and then out through the primer outlet. I have seen these simply left bare or have a tube that simply lets the excess dump on the ground. I prefer to let it return to the tank and it also makes a good line to put fuel in or to use a filling vent. (explain more in a minute). So plumb the fuel return on the carb to a fuel return fitting on the tank. Put a T fitting in this tube, run the T line somewhere on the model and insert a fuel plug of some sort in the end of it.

Look a little further down and I have some obvious examples. These are the only connections you need to the carburetor

Vent/Fill plumbing

Now onto the vent. No matter what you have to vent the tank because all of these carburetors use a negative pressure fuel pump, they are pulling fuel from the tank. The fuel input and primer circuits are sealed so if you don’t provide an air vent for the tank when you get the motor running hard it will suck the tank into itself, go very lean and then quit running probably when you least expect it.

So you need to use a vent. There are two popular ways

1. Coil some fuel tube as shown and connect to a vent fitting
2. Use a one way valve Stens/Echo #610-079 – the XCell Spectra includes one on the kit

Vents - Loop (Left) - Stens - preferred (Right)

There are some dependencies here based on what you do so be careful with this part.

To summarize the choice here:

1. If the tank is hard plastic I recommend the one way valve and an additional fill fitting on the tank
2. If the tank is soft (flexible), use the looped tube and you won’t need the additional tank fitting.

Plumbing with loop vent - no fill needed

The simplest way to vent the tank is with the coiled tube. Its simple and easy. In my opinion its not very neat but it is simple. It will also work perfectly regardless of whether the tank is flexible or hard plastic.

In case you’re wondering why its coiled, if you just run a straight line gas will simply run out when you invert the model not only making a mess but it could easily ignite on the hot parts. So using 2-3 coil loops pretty much keep any from ever making it all the way out. The coil can be any size just make sure you loop it at least twice

Another advantage of this vent is that since its always venting, you can use the tube on the T fitting from the fuel return line to fuel the tank instead of making an additional fuel fitting on the tank.


Plumbing with one way vent - fill needed

The neatest way (in my opinion) is to use the one way valve. Virtually EVERY lawn implement you have will have one of these little vents on them. They are cheap and easy to get.

It does have two prerequisites:

• It won’t work with a soft plastic/flexible tank. It takes a certain amount of vacuum pressure to open the valve. Over time the valve gets sticky and it takes more pressure to open. With a flexible tank, the tank will literally distort instead of venting. This will cause a slight lean condition that’s very hard to diagnose. The Dubro and Sullivan RST tanks are hard enough. The Nalgene bottles that some mfgrs use normally are not.
• The other is that it’s a one way valve, air can’t get out. So when you fill the tank you have to have another open line. The T fitting on the return line provides one of the two you need. But you’ll either have to remove one of the other lines to fill into or my suggestion is to add a fueling fitting on the tank connect a tube and then plug the end. Now you’ve got a vent and a fill both with removeable plugs. Locate them somewhere accessible on the model and fueling becomes simple.

The bottom line:

• There are many ways to plumb the fuel tank
• This way is not the only one but it is virtually guaranteed to be easy to use and problem free

[Azalin]

Thanks for the info. Where can i find those one-way valves?

[st99]

Wow, what a set up. Looks so clean. Also need a link to the valves.

[carey shurley]

well, these can be had from many places: Here is just one:

One Way Valve
Fuel Filter

[kenh]

What silicone are you using with the 3/4" plug?

In my experience (power sports mechanic) I've found silicone turns to "snot" when exposed to gasoline. Even the "good" automotive stuff.

[carey shurley]

last used DAP Auto/Marine Sealant - 100% silicone

hasn't jello'd yet although that doesn't mean it won't eventually.

since the plug, if necessary is always on the top of the tank it doesn't sit in petrol for very long.

[klamp]

Where do you get the fittings and the rubber grommet looking things?

[carey shurley]

The fittings I use are from Miniature Aircraft and the part numbers are:

Inlet - 90 degree #0410-90
Fill/Vent/Return #0405

I use 3/16" ID neoprene washers from Ace Hardware

[Azalin]

Originally Posted by carey shurley

well, these can be had from many places: Here is just one:

One Way Valve
Fuel Filter

I missed this reply. Thanks Carey.

[sundust]

Very usefull info Carey thanks.

For the first set up (Plumbing with loop vent - no fill needed) :
You fill the tank from overflow line thats ok but
where do you empty the tank from?

[carey shurley]

I usually run the fuel down to the point where I don't need to remove it however if for some reason I need to pull it out, I break my rule about removing the fuel inlet from the carburetor fuel feed. As long as I'm careful and don't get anything dirty its never caused a problem.

[sundust]

We are planing with Azalyn to make a hole under the tank (for trex 700 G) and put this inlet for refill and emtying the tank... How does it sound?

[carey shurley]

sure you can do that. Just make sure the hole fits it tightly, seal it with a rubber washer and make sure you secure the fuel line or plug to it.

[Azalin]

OK this is what sundust mentioned. The idea of placing a fitting to the bottom of the tank is to being able filling/emptying the tank. That's all. The rest are similar to the common.

Regards,
Suat

[Doug Darby]

I have not had great luck with fittings in the tank. Your setup looks great. Somewhere on a thread I saw a plumbing setup that all goes into the tank through the original plug. I believe it was a 3 hose setup.
What are your thoughts on that type of a fuel system?
I am about to start my HWC Trex 700 conversion on Friday. I was hoping you might have yours posted prior to my build. Any progress on that front?

[Azalin]

Hey Doug,

Are you asking me or Carey? If it's me my Helibug 600 plumbing is exactly the same as you have mentioned above. All the lines goes to the main stopper. No other fittings needed. Works as well.

I have a thread in "Gasser Helicopters" section about my HB 700 but unfortunately i haven't put detailed build data in it. I believe the best HB7-RC build thread is Rototerrier's.

[carey shurley]

I don't yet have the conversion kit so I haven't started the build thread yet.

however if the tank is fairly stiff I will again use my proven method of 4 connections using fittings, one to feed the carb, one using a one-way vent valve, one as a primer return and a 4th as a fill tube.

The main issue I have with putting the "fill tube" on the bottom is that it effectively becomes only a "defuel" tube because when you remove the plug, fuel is going to pour out of the tube unless you clamp it. So you still wind up using one of the other tubes to fill with. Its one more issue to deal with.

I tend to run all the fuel out OR pull the feed tube off the carb and pump it out as thats the easiest thing to do. So I run the fill tube on the top for simplicity. So far I've never needed to defuel the model with it inverted!

Of course this can be done any number of ways, I can virtually guarantee you what I posted will work every time.

[Doug Darby]

Carey,

I am certain what you posted will work great but don't care for the look of multiple holes and fittings in the tank, each with some potential to leak. With all the lines running through the plug it seems much cleaner with less points to have leak. But I may end up with your method anyway.
Will see what mood I'm in when I get to that point.

Thanks too all for your thoughts.

[ben_beyer]

How often do you have to replace the black fuel tubing that you're using in the tank?

[fastflyer20]

I had bad luck with the Zenoah fuel line, it split after a few weeks of use. I now find the thinest Tygon I can. I wish it was more flexible. but it holds up much better.