Why Seal Bee Tanks?

JayeLK Mon Jun 03, 2013 3:55 pm

If the tanks of the Babe, Golden, Black Widow, etc. are open to the air, why are some recommending sealing the tank back screw heads and back/tank edge? I can't understand, except for the tank leaking and affecting run time, why sealing these areas would change the way the engine runs. Venturi, crank case gaskets, and needle seal I understand affecting performance, but the tank already has a huge hole in it, what difference does a little leak around the edge make?

I ask because I bought a NOS tank and back, and when I put it on an engine and pressurized it with a syringe and alcohol, I made a small geyser coming from the tank edge seal. The tank has a burr in it that prevents it from sealing well. I may have to try to buff it out and see if it helps. In this case, the burr may keep the venturi from sealing well, but this is not the point.

Thanks,

Jaye

Ken Cook Mon Jun 03, 2013 4:47 pm

It makes a huge difference, The tank itself isn't open to the air. The only air allowed to enter the engine is through the venturi and that hole is only open to the crankcase when the engine itself is ready to accept it via the reed valve. The fuel enters through the venturi from the backplate the same way a siphon spray gun sprays paint. If you have air leaks, you have poor fuel delivery. These engines are plagued with a minimum amount of vacuum pressure as is, so any air leak is ultimately going to affect the run. This isn't going to allow the engine to needle or feed properly. Air leaks around the o-ring, tank to crankcase, backplate to tank, and air leaking through the threads of the needle or the insert that the needle is threaded into all contribute to poor runs and troublesome engines. The biggest problem I was faced with was cracked backplates, leaking screws. The screws themselves can be extremely problematic and can not only cause air leaks but short run times. The engine will run somewhat fine until the fuel level drops below the upper screws and then the engine is now sucking air into the tank through the leaking screws vs distributing the fuel load properly through the needle. Ken

JayeLK Mon Jun 03, 2013 6:27 pm

Ken Cook wrote: It makes a huge difference, The tank itself isn't open to the air.....Ken

????

Ken,

Scratch my head in confusion and call me obtuse, but isn't the filler nipple on the top of the tank a big hole to the air? I'm just don't understand why having another opening to the air just around a screw head or around the outside edge of the tank, aside from the tank not holding fuel very well, would have any affect on engine performance besides run time. I understand that if the leak (fuel or air) is anywhere into the fuel path downstream, or engine side, of the pickup tube performance is degraded, but I can't grasp performance issues with air leaks into the fuel reservoir upstream of the pick up tube.

I just didn't want to spend a lot of time trying to find some way to seal this piddly little fuel leak if it is only a slight problem of run time and only really happens under the abnormal condition of tank pressurization.

Jaye

Ken Cook Mon Jun 03, 2013 7:06 pm

I stand corrected in my response and I should've made that more clear in terms of the leaks in the tank. Obviously we need to fill the tank, and the tank also needs to be vented or else we could never displace the air within it to fill it nor would it run very long. . I suppose that what I'm trying to convey is that you can't have a leak within the fuel volume. The result is like trying to suck liquid through a straw with a crack in the straw. As long as the crack is submersed, one can draw the liquid, but when that area is exposed, it doesn't work. There obviously is a correlation here. Although the vents are allowing the atmosphere to enter within the tank, the vacuum is within the fuel while the head pressure is on top of the fuel. The leaks break the vacuum thus resulting in a shotty run.

The same thing happens when you have a hard tank, any leak, even the slightest leak is going to cause fuel delivery problems. The system regardless if separate or intergral needs to be free of leaks. Ken

JayeLK Mon Jun 03, 2013 8:16 pm

Thanks Ken,

I think I get what you're saying. I'll probably be spending a little time trying to remove that burr to improve the seal. Hope I don't ruin the tank trying to operate on it.

Jaye

**RknRusty** Mon Jun 03, 2013 8:41 pm

Jaye, have you tried using a piece of dacron flying line or thread? I usually cut a piece to the length of the tank's circumference plus about a 32nd" of overlap. Either wet it with spit, or even better, castor oil, to help hold it in place, then carefully press it into the groove around the back edge of the tank. It makes a good tight seal between the tank and backplate.

And that was a good question about why a leak in the tank matters. It's been nagging at me too and I'm glad no one asked me.

pkrankow Mon Jun 03, 2013 8:45 pm

I hypothesize the screws can cause bubbles that affect the pickup. If the engine is not peeing fuel I consider the screws minor. A tiny dab of anaerobic sealant under the screw heads guarantees there won't be leaks.

I have never had thread work properly for me. Filling the groove with anaerobic sealant, striking it off, clean the inside of the tank, assemble then clean the outside. Dissassembly shows the sealant only where it should be for me using this method.

Phil

**RknRusty** Mon Jun 03, 2013 8:48 pm

Yep, I've used sealant for the tank back and crankcase gasket too. Works fine, just a little more to clean up next time you open it up.

dckrsn Mon Jun 03, 2013 9:09 pm

Jaye, I've used this technique a few times for the
tank-backplate seal. Works for me. Read down for details.
Bob
http://www.rcuniverse.com/forum/m_9820380/anchors_9833987/mpage_3/key_/anchor/tm.htm#9833987

dirk gently Tue Jun 04, 2013 4:50 am

I was thinking the same thing. As a test, I took a Baby Bee with a custom tank that was leaking fuel like crazy due to the mounting screws not being tight enough. The engine did run fine, even though half of the fuel ended up on the test stand.

With another engine that I have, I sealed the tank-backplate join, as well as the tank screw heads. I used silicone sealant. I measured the RPM before and after this operation. There was no difference. Perhaps the engine was not leaky in the first place.

What is critical for engine performance, or in fact, it's ability to run at all, is (based on my very limited experienced) the quality of the seal provided by the venturi gasket.

Kim Tue Jun 04, 2013 6:39 am

Yeah, I always suspect the veturi gasket or pick-up tube's fit on the backplate nipple whenever an engine runs fine for a few moments, then gets weird with it's mixture. Of course, the case/tank gasket can mess with things if the screws are loose or stripped...

I've also extended a tanked engine's run by sealing the vent tube and running a fuel line from an external tank...that was just messing around...but it DID depend on a good seal all around with the tank...

Why Seal Bee Tanks? 1_18

pkrankow Tue Jun 04, 2013 7:54 am

That would be a topping tank, I believe, in helicopter lingo. I could be wrong on the lingo though.

This concept is used for helicopters and for some twin engine configurations (well, at least one that I have seen in person). It provides a benefit of extended run with a small tank that will remain free of bubbles because it is "vented" to another fuel feed.

Phil

andrew Tue Jun 04, 2013 9:21 am

Kim wrote:Yeah, I always suspect the veturi gasket or pick-up tube's fit on the backplate nipple whenever an engine runs fine for a few moments, then gets weird with it's mixture.

Ditto!!

I think the venturi gasket is one of the most overlooked parts for replacement or repair and can contribute to more aggrevation in tuning than most any other part on the reedies. As soon as the fuel level drops below the fuel pickup tube or venturi, air gets sucked in and your finely tuned machine turns into a t**d.

duke.johnson Tue Jun 04, 2013 11:04 am

I just put all new gaskets and sealed up a black widow last night. It was running bad and I messed with it a little, then it wouldn't run at all. When I did get it to run at surged and wouldn't run ou the tank. Hopefully after sealing it as Paul does for his mouse racers it will run great tonight.

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