If you are using a plastic tank (Dubro,Sullivan, etc), there are aftermarket clunks that have been shown to reduce the bubbles in the supply line. 

OS ENGINES Bubbleless Clunk  (https://gravesrc.com/osm71531000-os-engines-bubbleless-clunk/)

Dubro Tank Filter  (https://gravesrc.com/dub161-dubro-tank-filter/)

MIRACLE RC IN TANK FILTER CLUNK  (https://gravesrc.com/mirh-013-miracle-rc-in-tank-filter-clunk/)

Take note that I'm new to CL Stunt but I've been playing with model engines (Mainly RC Stuff) since the early 80s.  I don't know if any of these suggestions will transfer to use in a stunt model, but I've seen them make a difference in RC models (mainly helicopters).  They may be worth a try.

In my experience, bubbles in your supply line usually indicate fuel foaming that is caused by vibration.  Reducing the amount of vibration that the tank experiences would be the best fix and I'd exhaust all possibilities here before trying a fancy clunk.  All the obvious stuff applies....use a well balanced prop...ensure that the nose of the model isn't overly flexible...isolate the tank from the air frame with foam rubber...etc.

Finally, I have heard tell of people adding substances to their fuel to reduce foaming.  Ive read that adding Armor All (not sure of the amount used) to the fuel can reduce foaming/bubbles in the line.

Make sure to report back if you try any of this.  I'd like to know if any of it helps.

   You will not eliminate 100% of the bubbles, but can get rid of enough of them to make needle settings easier. First thing is to make sure your props are as balanced as good as you can make them. You won't eliminate all vibration, but every little bit helps.  Fuel filters are discussed for their obvious benefit of stopping junk from getting to the needle but they can strain out air bubbles also. If you use one of the larger units, it gives a place for the air to accumulate and nothing but fuel gets to the needle. I have used Armoral in the past with no problems. You just add a tiny little squirt to a jug of fuel, and I'm talking about just a few drops, then shake it up and you should not see any foaming action. This doesn't last forever but will get you through a flying session. I have demonstrated this to several R/C guys while flying at Buder Park. One guy had a neat 1/2A jet with a Cox TD-051 mounted in a pusher configuration. It would just not peak out the ROM he needed to get it airborne from a hand launch. I talked him into trying a dab of Armorall and promised to buy him a new jug of fuel if it didn't work. He fired it up again, and it was night and day different. he had enough power to get it in the air and fly it, and he could visibly see no air bubbles in the fuel line to the engine. It's easier to just eliminate the source of the foaming, which is vibration from an out of balance prop or loose engine mounts, and get the best fuel feed that you can. Brett has discussed doing what you have to do to get larger fuel lines in your tank and fuel feed to the engine on larger models and this can help out a lot I think. Go to 5/32" tubing for all the tank fittings and large fuel line stepped down to the medium you need at the NVA nipple. I intend to play with that concept when I get back to flying my larger models.
  Type at you later,
   Dan McEntee

Please make sure the uniflow exit in the tank is not too close to the fuel pickup. Some tank designs are very risky wrt to bubbles in the pickup. Also RC tanks with 2 clunks can give bubble issues...

Hi Steve

I've used the Dubro sintered bronze clunk filter (middle link above) since the early 90's. Your fears about the engine starving out, with fuel still in the tank, just have not happened to me in all that time. My tanks are empty of any fuel at the end of a flight. I also use the same style of filter in my fuel jug pickup line. I like the idea of prefiltering the fuel before it is put in the tank, just because I'm a little obsessed with having clean fuel. It seems to help keep the engines running as they should.

Most of the tanks I use are RC plastic tanks converted for CL use and run with a fixed, non-moving, not connected to the clunk, uniflow venting arrangement. I have had the best success with that style of venting.

There's another benefit that comes from this arrangement. I make 2 small bends in the uniflow line. The first one will go inside the tank to position the end about the middle of the sidewall. The second one will be made after the tank is assembled. Experience has taught me that this bend does not need to be more than about 30-45 degrees, it's only there to help you move it, up or down.

Moving it up or down, effectively changes the relative height of the tank, without having to physically move it.

I believe that when the uniflow vent end, inside the tank, and the clunk pick-up are connected, your relative apparent tank heights could change every time the clunk moves up or down. Perhaps this may be part of the reason some people have a bad taste in their mouths when trying to use a plastic uniflow tank.                                         

There are several other advantages inherent to using plastic tanks.

RC plastic tanks are usually less expensive and more readily available.

No soldering is involved, with the problems of bad joints, and residue inside the tank. Having to desolder to see, or correct problems inside.

Depending on the brand and type, some are easy to see inside. In any case, they are easily opened up should the need arise.

You can simply swap out the annealed, brass tubing for copper tubing to avoid the brass tubing cracking, or splitting from reaction to our fuels. This can be accomplished as you assemble the tank the first time.

Easy tank height adjustment as detailed above.

One of the possible cons could be trying to use a style of plastic tank that may not work for your setup. Unless you have no need to adjust your tank height, I suggest you not use the slant front style for stunt. It is difficult to impossible to properly raise or lower your apparent tank height with that style of tank. I prefer the rectangular style with the tubes coming straight out the front. Other styles can work as well, as long as the stopper has the tubing, outside the tank, aligned parallel to the top and bottom.

I apologize for the thread drift and the length of my reply. In a way, it does have relevance to the subject matter. The sintered bronze-style clunk pickup has been shown to have helped some with minimizing bubbles in the fuel line. Split or cracked brass tubing in the metal tank will, at the least cause bubbling. There have been more than a few posts relating to this on the forum.

If desired, I can dig out the CAD drawing I did that shows how I set my tanks up and repost it. It was Allen Brickhaus, who passed several years ago, who showed me how to set up a plastic RC tank for stunt work. I made a few modifications to make it easier to use and since then, with 1 or 2 exceptions, that's all I use.

John Miller

 

The double clunk tank works for me. Don’t know why it works, but it does. Indeed, I’ve tried every other tank arrangement with little success. I only fly profile models, maybe the double clunk is a good solution for profiles?

Another thing. I use a nice solid piece of angled aluminium as a shelf to mount the tank. the tank height can be accurately shimmed up ur down with thin ply strips. Accuracy to within 1/32” is needed for final adjustment.

Steve