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Clunk!

How do I get around this problem?

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Gary Manuel25/03/2018 22:14:42
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1873 forum posts
1507 photos

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.

john stones 125/03/2018 22:27:26
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10500 forum posts
1480 photos

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

cymaz25/03/2018 22:28:56
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8584 forum posts
1161 photos

Have you thought about a Rotoflow tank?

Percy Verance25/03/2018 22:33:31
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8109 forum posts
155 photos

Hi Gary

Firstly Gary, it's been a while since I flew a petrol engine, but from what I remember it's not a good idea to completely empty the tank because (I was told) the diaphram in the carb will dry up and it won't function as intended.

It might sound like a silly question, but is there some sort of inlet to allow air into the tank? I assume you're not pressure feeding the tank, as the last thing you want is hot exhaust gasses going into a tank of petrol. It's simply a thought Gary, but if the carb is drawing fuel, there has to be some air going into the tank, otherwise fuel starvation will be result.......

Ron Gray25/03/2018 22:41:37
1432 forum posts
359 photos

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.

Gary Manuel25/03/2018 22:51:29
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1873 forum posts
1507 photos

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?????

Martin McIntosh25/03/2018 22:53:04
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2906 forum posts
1064 photos

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.

Percy Verance25/03/2018 22:54:00
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8109 forum posts
155 photos

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.......

A bloke I once flew with had a model with a tank that constantly used to throw the clunk forward on landing, resulting in endless engine starting and running problems. The cure was simple. Remove most of the clunk tubing from inside the tank, apart from a small piece at each end, and have solid length of brass tube in between. No more problems with the clunk. 

Edited By Percy Verance on 25/03/2018 22:59:10

Gary Manuel25/03/2018 22:57:14
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1873 forum posts
1507 photos

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.

Ron Gray25/03/2018 23:02:02
1432 forum posts
359 photos

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.

Gary Manuel25/03/2018 23:03:28
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1873 forum posts
1507 photos
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.

flight125/03/2018 23:26:29
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609 forum posts
33 photos

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

john stones 125/03/2018 23:32:56
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10500 forum posts
1480 photos

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 ?

Gary Manuel25/03/2018 23:49:24
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1873 forum posts
1507 photos

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.....

gwm_1528.jpg

...... and inverted (other clunk and tie wrap removed).

gwm_1529.jpg

Gary Manuel25/03/2018 23:53:43
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1873 forum posts
1507 photos
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.

iqon26/03/2018 00:31:50
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1452 forum posts
239 photos

It`s not floppy enough, just fly scale and you will be fine.cheeky

Chris Barlow26/03/2018 00:37:10
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1808 forum posts
1228 photos

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?

onetenor26/03/2018 05:52:19
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1900 forum posts

Is it down to fuel head / tank height in relation to the carb?

cymaz26/03/2018 06:24:49
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8584 forum posts
1161 photos

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.

Percy Verance26/03/2018 07:48:25
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8109 forum posts
155 photos

onetenor

It shouldn't make any difference where the fuel level or tank is. Petrol engines with Walbro type carbs pump the fuel through from the tank. That's why, when you are cutting your hedge with a petrol hedgetrimmer, it matters little what positon you use/hold it.

Edited By Percy Verance on 26/03/2018 07:49:12

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