I have been doing some final run-in and prop testing on a motor that I intend to fly for the first time this weekend. Prior to today's run, the motor had held, solidly, needle valve settings and had run very steadily at any setting. Today, however, right from the start, it would not hold a setting and required an unusual number of turns either to lean or richen the run.
As is my habit, before the first run I had checked all backplate, head, and mounting bolts, as well as the plug, for tightness and checked the fuel line and the exhaust pressure line for snugness.
The second run exhibited the same inability to hold a setting and difficulty in changing the run with the needle valve. So, I again checked all of those things that I had previously tested and, in addition, checked to see that the NVA was properly secured.
Having found nothing loose, I ran it a third time and had the same problems. This time I focused on the clunk tank. The first thing I found was that the nylon backer for the rubber fuel plug had stripped so I could not tighten the screw. I was able to back the screw out and, upon removing the plug, found that the wire used to secure the in-tank tubing to the copper fuel tube had cut that tubing on one side. This is certainly not consistent with a leak free fuel system.
So, tonight I will replumb the tank and, instead of using wire, I will follow the suggestion that I think Nils made and use heat shrink tubing to secure the in-tank tubing to both the clunk and the copper fuel tube.
Has anyone else experienced this particular problem? And, just out of curiosity, does anyone have a suggestion other than Nils' to prevent this from happening?
Thanks in advance for any comments or suggestions.
I have used clunk tanks since about 1970 in various applications. Best I can remember, I have had two problems with split lines or lines coming off the tube. Sand the end of the tube with fine wet or dry paper until the end is clearly rounded. That will stop the metal tube from cutting the rubber tubing from the inside as it flexes around. I really question the value of clamping the rubber hose on the tube. The clamp is just one more place for the tubing to get cut. I don't think I have ever had one come off on its own. Just be sure the hose is on the metal tube about 3/8th of an inch, and is a snug fit. You should have to work a little to get it on. I can't imagine one coming off if it is a tight fit. The loads aren't large enough and aren't from the right direction.
I use the shrink tubing to hold the uniflow line to the feed line so they move together. I've never had trouble with the clunk tubing coming off the copper. I used to bell the end of the copper line slightly but I found that unnecessary. By all means smooth the end of the copper after you cut it.
Nils
Wow heat shrink the uniflow line to the feul line. I've been using clunk tanks since I moved to Canada becouse I just cant justify(to myself) spending what it costs to mail to the US to buy a feul tank, so I taught myself how to solder (believe you me that was no easy task) never thinking to heat shrink the two lines together, thanks NILS you just made my life a whole lot easier( well maybe not but at least I dont have to solder any more)
Thanks Dwayne
I have used very small zip ties on the outside feed line of my uniflow tank. I don't see why it wouldn't work on the inside of a clunk tank. They are wider than the wire and should clamp with less tendency to cut the tubeing. Hope this helps.
Bob
TigreST · Sep 13, 2001 12:58 PM
RE: A Lesson Learned: In addition.#5 sourceOne of the lessons I learned over approx. 3-months in 1986 was to check the condition of the internal silicon tubing as stated above. In my case there was a micro-hole in the internal flex line between the clunk and the brass tubing at the tank plug/stopper. As long as the tank was better then 3/4 full performance was fine, but as soon as the micro-hole was uncovered (read: above the resident fuel level/in the tank vapor space) the system started to suck air, ..this was followed by the resulting erratic engine run, ..which ended with short duration flights with fuel still in the tank.
Up front everything looked fine, even after close inspection of all components. But once the flex line to the clunk was hyper-extended in very short radius bends, the hole in the silicon tubing could then be seen. A cursory glance was not enough to pick this up during the initial investigations though. FWIW.
kdheath · Sep 14, 2001 08:56 PM
RE: A Lesson Learned: In addition.#6 sourceAbsolutely right, Tony. Anytime an engine starts running oddly, a check for leaks is a good idea. It can be the rubber hoses, the copper or brass tubing itself, or the backplate of the engine. (In addition, of course, to the other possible causes.) But sometimes a cracked line is the answer to a maddenning problem with an engine.
Thank you, gentlemen, for all the helpful comments.
Nils: Sorry for misrepresenting the generous advice that you have given me.
Further on the subject of clunk tanks, does anyone use surgical tubing instead of typical fuel tubing inside clunk tanks? Surgical tubing is so much more flexible.
Thanks, again, for your help.
I have always used silicone lines exclusively. The older surgical tubing was a latex product that eventually dissolved in fuel. I made an awful mess once or twice with the stuff years ago. But with the latex allergy issues, the tubing material has probably changed. Actually, the tubing in a CL tank likely doesn't need to be as flexible as it does in an RC model that moves in more directions. Maybe Nils can tell us more about the issue-he has much more experience with the clunk tanks in CL than I do. Nils?
>Actually, the tubing in a
>CL tank likely doesn't need
>to be as flexible as
>it does in an RC
>model that moves in more
>directions.
I became frustrated with setting up a 2oz clunk tank since the short silicon lines flex very little. Have to admit - it is very cool to watch the stream of bubbles from the uniflow line.
One way to avoid some of these problems is to use the stiff plastic tubing which is supplied by some tank mfgs. or use the material which RC flexible throttle is threaded through. If you carefully heat the end of the tube, it will make its own knob at the end to slip the silicone tubing over. It is also permanently bendable with heat. It takes very little practice to do this. John
If it doesn't stink, it's not a diesel.
LAST EDITED ON Sep-16-01 AT 01:35 AM (CDT)OK, let's see-
Dwayne- You're welcome. But I'm glad you didn't find out about the shrink tubing until after you learned to solder, learning to solder is a handy skill. 
Peter- No need to apologize it's no big deal. Btw, kdheath is right, surgical tubing will work for a while but eventually fuel will eat it. I've found that the tubing that comes with the tank has always been long enough to make both lines if cut carefully.
Steve- I have a Ukey 15 with a 2 oz plastic tank plumbed the same as the larger ones, & you're right, they don't bend much. I fiddled around with it for a while & I finally just said the heck with it & tried it & it works just fine. I've had rectangular metal tanks with no "V" at all in the outside wall & they worked fine as well, I think the tanks are so thin vertically it just doesn't matter as long as the uniflow vent is at the right height compared to the NVA.
Dzlstunter- I like the stiff plastic tubing too. I've used it on my last several tanks. Besides being easy to bend & being self flaring with heat it's a lot lighter than copper.
Just for general info- I flew my old profile Oriental 7 times yesterday afternoon. The airplane has well over 800 flights on it. The tank is on it's second stopper. The first time I set it up without a solid fuse mounted remote overflow & uniflow tubes connected with fuel tubing. After about 500 flights the stopper started leaking air. I'm reasonably sure the cause was all the wiggling it got every time I fueled it & capped & uncapped the vent. After I added the solid mount it has somewhere between 300 & 400 additional flights with no further problems to date.