Gary Manuel Posted March 25, 2018 Share Posted March 25, 2018 I have been test flying a new model this weekend and have come across a problem. I think I understand what's going on but I don't know how to fix it. I'll give the detail first, then ask a more specific question. My model is a Weston Capiche 50cc with an AGM-55 2 stroke petrol engine. The fuel tank is a conventional "Du-bro" style with extended front lip. I have plumbed it as 3 pipe with 2 clunks. 1 clunk feeding the carbureter as normal, 1 clunk feeding a fuel dot to allow the fuel tank to be completely emptied, 1 bent copper pipe from close to the top of the tank as a fuel overflow / vent. For normal flight there is no problem, including inverted flight climbs and dives. The problem appears when the tank is about half full (or more importantly, half empty) when the model is nose down and inverted for more than a couple of seconds - e.g. inverted spins. The first time it happened, I put it down to the engine just being new. Fortunately I had plenty of height and managed to dead-stick land it. After a few more uneventful flights, I tried a few more aerobatic manoeuvres and found that for the first few minutes of flying there was no problem, but after 10 minutes or so - admittedly, trying to replicate the original problem at a safe height, the engine would cut whilst doing sustained inverted spins or in one case during a slow rolling circle (whilst inverted). My gut feeling was that the clunk may be too long and could be trapped in the bottom corner of the tank. I stripped the tank and the clunk line did look marginal, so I cut 1/2" off t's length to make sure it could move freely from top to bottom. I gave it another go and exactly the same thing - fine for the first 10 minutes, the the engine then cut during an inverted spin. I have now convinced myself that its due to the standard practice of allowing the clunk to flop between the top and bottom rear of the tank, but not flop towards the front of the tank for fear of getting it stuck by being bent double. What I believe is happening in my sustained spins (with the nose in a downwards attitude at about 1/3 throttle to maintain airflow over the surfaces - and keep the partially run-in engine running) is that the fuel is below the clunk, which cannot droop down into the fuel, resulting in fuel starvation. So how do I overcome this problem? Is there an alternative way of plumbing that allows the fuel pickup to be submerged at all times? I'm sure that this issue must have been raised before because it's an obvious problem if you think about it. Suggestions would be most appreciated. I'm still unsure why it is happening on this particular model, but I'm pretty sure that this is what is causing it. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted March 25, 2018 Share Posted March 25, 2018 I get what you're saying, you using the thin tubing for your clunk line ? never had this problem myself, also can you use a heavier clunk ? Plus is there a lot of pipe (not fuel tube) inside the tank, not allowing it to flop forwards ? Edited By john stones 1 on 25/03/2018 22:29:18 Quote Link to comment Share on other sites More sharing options...
cymaz Posted March 25, 2018 Share Posted March 25, 2018 Have you thought about a Rotoflow tank? Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 25, 2018 Share Posted March 25, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted March 25, 2018 Share Posted March 25, 2018 Use a large capacity in line filter between the tank and the carb, that way, if the clunk is in the air you have a bit of fuel reservoir to draw on until the clunk is submerged again. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 25, 2018 Author Share Posted March 25, 2018 Wow, Some fast readers and responders on here - Thank's fellas. John - It's flopping well, but not doubling back on itself (it's doing exactly what it's supposed to do). Cymaz - Yes I thought about them, but I don't think they would overcome the problem of sustained nose-down attitude, so would be no benefit over what I've got. Percy - Tank is over half full when the engine has cut. The vent / overflow pipe is open to the atmosphere (under the fuselage) so airlocks isn't a problem. I've been reading up a bit more since I started this thread and I think THESE GUYS might be discussing a similar problem but the solution might not be the same. I'm wondering whether a better quality / larger felt clunk with a bit of a reservoir built in underneath the felt might help. The one I'm using is heavy enough, but there's not much capacity beneath the filter. It's something like THIS one. Why is everything so complicated????? Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted March 25, 2018 Share Posted March 25, 2018 I was flying a re engined 20 year old fun fly the other day when it started to do exactly the same thing, vertical downward rolls OK but cut in negative spins. Same old tank but a different motor. It turned out to being caused by a lean top end which was critical because I was only using straight fuel. This is of course glow but the same may apply to petrol. Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 25, 2018 Share Posted March 25, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 25, 2018 Author Share Posted March 25, 2018 Ron - I was typing when you responded, but I'm thinking on similar lines to you - add a reservoir in the line somewhere. The jet guys add an air trap to the fuel line to avoid flame outs, which can be more dramatic than a petrol engine spluttering. Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted March 25, 2018 Share Posted March 25, 2018 I’ve only had problems with inline filters on glow engines, not on any of my gassers. On these I always run with a felt clunk and an in-line between the tank and carb, as I said above, if they are large capacity then you have a reserve ‘tank’. Regarding the filters blocking, that’s why they are there, to prevent the crap getting into the engine and they can be cleaned! Go for it, it’s a cheap thing to try out. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 25, 2018 Author Share Posted March 25, 2018 Posted by Percy Verance on 25/03/2018 22:54:00: My own experiences with inline filters were generally not good. They're usually more trouble than they're worth. They gradually block up, beginning the day they're fitted, and they are a potential air entry point. Remember, if it can go wrong, then at some point it will....... Similar reservations here. The clunk will suck whatever is offered to it, whether that is air or petrol. Eventually, that will find it's way to the carb. I'm thinking that an air trap is what is needed, but why isn't the problem so widespread that everyone is doing it? I need to think about this a bit more. Keep the suggestions coming fellas - much appreciated and good food for thought. Quote Link to comment Share on other sites More sharing options...
flight1 Posted March 25, 2018 Share Posted March 25, 2018 use a small header tank with the pipe to the carb drawing from the centre of the tank( fixed pipe not moving) and the feed for this tank from the main tank clunk out let , this way the draw point will allways be in fuel untill the header tank becomes half empty a proven system i use on a couple of models. or get a fuel balancer like this kwick fire one Edited By flight1 on 25/03/2018 23:27:19 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted March 25, 2018 Share Posted March 25, 2018 I'm thinking...all your others are o.k...why is this not, all on same method fuel tube ? you got air traps on others or filters or other gizmos ? Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 25, 2018 Author Share Posted March 25, 2018 Thanks all. Looks like I'm not barking up the wrong tree if other people have had the same trouble and there are solutions out there. I though it was just me (again). I'll have a read up on the various suggestions and let you know what I decide to do. What's puzzling me at the moment is why I'm having the problem with this model but not other similar ones. Maybe it's because I was deliberately keeping the nose down in the spins rather than flattening them out, in case the engine cut? who knows? For completeness - here is a photo of how my clunk hangs in the upright position..... ...... and inverted (other clunk and tie wrap removed). Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 25, 2018 Author Share Posted March 25, 2018 Posted by john stones 1 on 25/03/2018 23:32:56: I'm thinking...all your others are o.k...why is this not, all on same method fuel tube ? you got air traps on others or filters or other gizmos ? This one should actually be better than the others, which are generally 2 line systems with a "T" piece in the clunk / carb line for a fuel dot. No gizmos in any of them. Quote Link to comment Share on other sites More sharing options...
iqon Posted March 26, 2018 Share Posted March 26, 2018 It`s not floppy enough, just fly scale and you will be fine. Quote Link to comment Share on other sites More sharing options...
Chris Barlow Posted March 26, 2018 Share Posted March 26, 2018 Gary, does the engine run for 100% flight times if you don't go inverted and just fly circuits? I'm wondering if the inverted/spinning is a red herring and the problem is else where. Is it the inverted part or the spin part? Will it behave the same just flying inverted circuits? Quote Link to comment Share on other sites More sharing options...
onetenor Posted March 26, 2018 Share Posted March 26, 2018 Is it down to fuel head / tank height in relation to the carb? Quote Link to comment Share on other sites More sharing options...
cymaz Posted March 26, 2018 Share Posted March 26, 2018 Check the gauze filters in the carb. Even take the carb off and thoroughly clean it through. I helped a club mate a month or two ago. He has a evo10cc that refused to run more than a couple of minutes. Turns out the carb was filled with manufacturing crud. Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 26, 2018 Share Posted March 26, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Andy G. Posted March 26, 2018 Share Posted March 26, 2018 Posted by Gary Manuel on 25/03/2018 23:03:28: Posted by Percy Verance on 25/03/2018 22:54:00: My own experiences with inline filters were generally not good. They're usually more trouble than they're worth. They gradually block up, beginning the day they're fitted, and they are a potential air entry point. Remember, if it can go wrong, then at some point it will...... Percy... isn't that the point of fuel filters, to stop crud finding it's way from the tank and into the engine? If you get the dismountable type and clean them from time to time they are usually fine.. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 26, 2018 Author Share Posted March 26, 2018 Posted by iqon on 26/03/2018 00:31:50: It`s not floppy enough, just fly scale and you will be fine. Oh yeah - I never though about just flying scale-like. Posted by Chris Barlow on 26/03/2018 00:37:10: Gary, does the engine run for 100% flight times if you don't go inverted and just fly circuits? I'm wondering if the inverted/spinning is a red herring and the problem is else where. Is it the inverted part or the spin part? Will it behave the same just flying inverted circuits? I can't honestly answer that - I'm hoping to cure it by next time but if not I'll give it a try. Posted by onetenor on 26/03/2018 05:52:19: Is it down to fuel head / tank height in relation to the carb? As Percy says, it shouldn't make a difference on a petrol engine as the carb pumps the fuel. Also, it shouldn't make a difference whether the engine is inverted or right way up if the clunk is correctly drooping to where the fuel is. I've found THIS video, which shows exactly what I think is happening. The comments to the video discuss sustained nose-down flight and it's effects on the engine. I'm still reading up on how to solve the problem effectively. Edited By Gary Manuel on 26/03/2018 10:31:11 Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted March 26, 2018 Share Posted March 26, 2018 This was discussed one time Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted March 26, 2018 Author Share Posted March 26, 2018 Thanks Denis. That's a very good and appreciated solution - but to a different problem of diaphragm pressure regulation. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted March 26, 2018 Share Posted March 26, 2018 Tis one of lifes great mysteries, when you've drawn a conclusion, can you tell me why your other petrols behave, given you fly same style with those. Quote Link to comment Share on other sites More sharing options...
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