RC_Fanatic

My Feedback: (1)

I set up the fuel tank in one of my OS46AX powered T-34 racers as a uniflow tank. I cannot get the engine to run smoothly at WOT, and it is down in revs from what I got with a standard tank setup (probably because it keeps hunting about +- 200 rpm at around 15,500). It runs fine at mid throttle and idle. Anyone else seen this with the uniflow tank?

Sport_Pilot

I have had the engine running a bit lean right after switching to uniflow, but ran fine after opening the needle up a bit. The submerged vent will cause a slight reduction in pressure.

jaka

Hi!
All my models are supplied with Uniflow tanks, both racers and sportplanes as well as scale, where I don''t use Tettra "Bubbleless "tanks.
You must check your tank set-up! Uniflow tanks are slighly better than the usuall two-line tank set-up! I use it in all my Q-500 racers (No Tettra "Bubbleless" tanks allowed over here in Q-500 racing).

-Are the two clunks side by side, one slightly shorter than the other? Should be.
The longer fuel line with clunk should go to the carb, the shorter with clunk is pressure.

I recommend using a Tettra tank without the inner silicon bladder or an ordinary Tettra tank as those are superior to Du-Bro or Sullivan or any other tank, as they have a better system for sealing the front of the tank (No rubber expander that very often leak).

pe reivers

+1
needle tuning is needed for lack of exhaust pressure in the uniflow system. Also, tank and pickup location becomes much more important. The pickup should be at about spraybar height. Any lower than that will cause leaner mixtures.

jaka

Hi!
I mount all my tanks just as with any two line tank; according to the "tank mounting rule": Center of tank in line with fuel intake orifice in the carb when plane sits horizontal.
Works just perfect!

1QwkSport2.5r

I am using a uniflow setup using a Sullivan tank on one of my airboats powered by a SuperTigre S90K. I have the tank centered with the spraybar ("the tank rule") and the feed clunk is sintered and at the back of the tank. The pressure line sits in the middle of the tank. The system definitely delivers a more consistent fuel flow, but I find the mixture tends to be richer, especially coming back to idle from a WOT run. Otherwise, I find it working fine. I have a APC 13x6 on it and run 5/20 fuel.

Don't forget to pull the plug from the third line before filling the tank. I forgot one time and made a mess.

RC_Fanatic

My Feedback: (1)

Well, that explains the problem. With the engine upright, as required by the rules, even the top of the tank is below the spray bar. I thought that the uniflow would be less sensitive to tank position than the normal setup. That seems not to be the case. I thought it was worth a try as I had to replace the tank anyways. I'll go back to the normal setup which seemed to work reasonably well.

Thanks for your help.

1QwkSport2.5r

If you use a demand regulator like a cline or iron bay, then tank position isn't as critical. The uni flow is more sensitive because the exhaust pressure has to overcome the fuel above the clunk. My SuperTigre 90 tends to have fuel in the pressure line until I get passed 1/4 throttle with a full tank. Otherwise it's working good on a picky engine.

freakingfast

My Feedback: (3)

I haven't found the Uniflow setup more sensitive to tank position per say. However, what the carb sees is a lower yet more constant pressure similar to a normal tank that's always near empty. That's why you must open the needle more for the Uniflow tank. The normal "tank rule" is out the door with Uiniflow, optimal is now spray bar even with near bottom of tank, but it isn't necessary to move the tank in most cases if you have proper muffler pressure. (for instance, if you raise the tank, then the needle setting would be as before).


If you had a low mounted tank, marginal muffler pressure or poor fuel draw before, it will it wont help (lean). If you had excessive muffler pressure or fuel draw before, there's a good chance the engine could die from over rich if you go suddenly from full throttle to idle.

Charlie P.

When I set up a two klunk tank I always use the longer one to the carb (or smoke pump)and the shorter one for fill/drain. Not sure it matters but that's myway.

flybyjohn

My Feedback: (2)

Here is my take on the uniflow setup. If the tank is in the wrong position it won't fix the problem. What the uniflow does is maintain constant pressure to the carb from the top of the tank to the bottom of the tank. This is what happens when you have a regular two line setup: when the tank is full you have all the fuel weight (head) in the tank pushing down on the outlet tube to the carb and you also have full muffler pressure pressurizing the tank. So you have a rich setup. When the tank is empty you loose the head of fuel on the carb line, but you still have full muffler pressure, so it will be leaner.

With the three line setup: when the tank is full the fuel head pressurizes the carb line the same as before, but also depressurizes the muffler pressure by the same amount as it is pressurizing the carb line since it enters is at the bottom of the tank. When the tank is empty then the fuel head is gone but the muffler pressure is 100% now, so the pressure stays constant from full tank to empty tank.

Now if the tank is too low to begin with then the engine will have a harder time pulling fuel up with the uniflow system (at full tank) than the two line setup because it has a little less pressure with the muffler entering at the bottom of the tank, but that (low) pressure will still stay the same throughout the whole tank of fuel. With an almost empty tank, the pressure to the carb should be the same with either system.

I have this setup on two of my planes and there is no rpm change between full tank and near empty tank, it stays constant the entire tank.

Jim Thomerson

Try a fixed uniflow line at the top of the tank, if the tank is too low. That might work. The engine thinks it is receiving fuel from the submerged end of the uniflow and is unaware of other factors. I had to set up a control line stunt tank with the uniflow at the top of the tank, right where the outside wedge began. It worked to suit me.

Sport_Pilot

Using a normal tank won't be any better. The advantages of uniflow are only there when the tank is set where the needle is somewhere between the tank and bottom. However for uniflow the ideal position is about center of the needle. For a normal two line tank the needle should be about 3/8" above the center.

pe reivers

Uniflow has a well defined edge over any other system, except pump and regulator, considering constant mixture settings. It just needs to be set up well.

freakingfast

My Feedback: (3)

Agreed.

I run this Uniflow variant in almost all of my non YS engines : http://www.rcuniverse.com/forum/fb.asp?m=9045226 The tank placement rule still apply.

1QwkSport2.5r

That is one heck of a setup, but holy crap is that a lot of plumbing!

Sport_Pilot

I think it is an effort to keep the fuel from spitting out through the vent which happens with uniflow, especially with a full tank and short muffler vent line.

1QwkSport2.5r

I don't know about anyone else, but I plug my vent line when running the engine. I do see fuel in the pressure line sometimes, but like you say, at full tank and usually coming back to idle abruptly. I don't ever see fuel spewing from the muffler so I don't worry about it.

I have an OS. Bubble free clunk (which isn't bubble free) and it seems to work okay but the price is too high. I try to support my LHS where possible, so I paid $18 for it. My felt clunks from the hardware store cost $3.49 and work far better than the OS bubblefree clunk. They don't flow enough fuel for my .90 though.

freakingfast

My Feedback: (3)

Yes it is extra plumbing! Update: the check valves cant use springs, just soft flapper and seat type as shown.


Your mostly right.

After the throttle is chopped there's stored muffler pressure in the tank, this takes a while to equalize since the fuel takes longer to exit than air. This reduces momentary rich idle/ loading up as well as dumping fuel into the muffler.
Didn't you know OS flies them to the moon and back so they can charge more for it Yes, felt is another way to go. The OS clunk will easily flow an 8 oz. tank out in three minutes, but the Du-Bro "R/C tank filters" I tried had trouble flowing a 55AX so I chucked them. You can tell when you need to open the needle a bunch or it's not sensitive from a known setting, this is always a bad sign.

1QwkSport2.5r

I could remove the needle from the carb and plug the opening for the needle valve while the engine was running as it was still lean. Took out the felt clunk and replaced with a regular one and it needles properly. It only happened on the .90.

Sport_Pilot

I hope you are plugging the fill line. It cannot work as avent if plugged up. The vent line goes to the muffler, unless flying control line stunt where it would simply be pointed into the wind.

1QwkSport2.5r

We are on the same page, I just have a different name for it. Vent is 3rd line, vented while filling and plugged while running. Feed line is to the carb, I fill through this line. Pressure line is from the muffler to the tank. I have easy access to the tank and lines since I run airboats. My airplanes will probably have to get setup differently.

pe reivers

Good system. However, never fill through the feed line. This line needs to be super clean always, so should only be used by the carb to draw fuel. Unless you take reliability of engine running and carb tune less seriously of course.

1QwkSport2.5r

I have 3 fuel filters on my filling station, including a felt clunk at the bottom of the fuel jug. I take fuel system cleanliness seriously. I am not concerned with debris in the fuel tank.
I tried filling the tank from the pressure line clunk once but the vent couldn't keep up and forced fuel into the carb with the throttle fully closed. The 90 likes a fat prime so it's not a big deal on that engine, but my .29 would be flooded to high heaven.

It may be unconventional, but it works for me.

freakingfast

My Feedback: (3)

Good point Pe.

But I've also been where 1QwkSport2.5r is with leaking carbs while fueling.
On my system, I use an inline filter. To fill/drain, I pull the line between the filter and tank, never between the carb and filter. Or, one could pinch off or plug the carb line, open the vent line and fill/drain through the pressure clunk.


All of the hand crank, gear up-sped, impeller pumps eventually generate debris that will clog a carb or needle. The filler tip has a filter in mine and I clear it from time to time.