Problem with air in fuel line. Pulse xt40

[flyeruk]

I have been flying my Pulse XT60 for a while now,but am having problems with bubbles in the fuel line.I had the problems when i had the Evolution 46nt and thought it was down to leakage in the card. I have changed the tank,fuel lines and clunk,pressure tested under water and it is completely air tight. I have now fitted a Irvine 53,hoping that would cure the problem,but its just the same. I am getting very good at dead stick landings.

I have just purchased a new tank from the usa,a pulse xt40 tank,fitted it in desperation,and still have the bubbles.The tank is a very tight fit in the fuse and its very hard to isolate it completely from the airframe. Yesterday,i took the tank back out of the plane,supported it on top of the fuse,no bubbles at all. I also held the tank at the same hight as it is when inside the fuse,as the pulse tank is well below the needle valve,and still have no air or bubbles in the lines.

I can only see the problem is vibration. It has been suggested that i use a sintered clunk as this is supposed to stop air getting into the lines.I have also balanced the prop.

Any other suggestions would be very much appreciated.

[NigelH]

A foam clunk may well sort out your issues. I had a Tribute 36 4D with the tank mounted right by the engine and it would be foamy but the engine always ran well with the clunk. Try one.

[Alan Cantwell]

the fuel is frothing, this is through resonace vibration in the airframe, balance the prop, and spinner, you can alsd add a few drops of Armorall to the fuel bottle, this will stop it frothing, it can be bought in car shops

Edited By Alan Cantwell on 20/05/2012 22:38:54

[flyeruk]

NMany thanks for that.How much Armorall should i add to gallon of bekra 10?As to isolating the tank,its just not possable unless i mount in even lower than it is.I have also ordered a sintered clunk.

[Alan Cantwell]

no more than few drops, but do try the balance the prop and spinner bit, sometimes, with a built up airframe, they can drum, this will transmit through the airframe, and into the tank, if its possible, cut away some wood round the tank, and pad it out with something better than cushion foam, i use pipe lagging, for smaller pipes, it comes with a slit down one side, easy to cut it to go over a formers edge

Edited By Alan Cantwell on 22/05/2012 16:53:44

[Steve Hargreaves - Moderator]

Are you sure there isn't a pin hole in the clunk tube at some point? This is a classic "GOTCHA" because when the tank is full the pin hole is submerged & the engine draws fuel as normal. As the fuel tank empties the pin hole is exposed & air is drawn in to the fuel feed.....

Do you get air bubbles as soon as you start the engine (ie with a full tank) if you do then its a leak between the tank & the engine & not down to the fuel foaming......

[Ben B]

It would sound like fuel foaming but as said if every xt40 had this problem it'd be well known, so something is particular to your installation. I'd have a long hard look at the prop balancing.... Maybe even the spinner. In my experience fuel foaming tends to be okay on the ground and only happens in the air (when on the ground the u/c dampens any vibrations I guess).

[Peter Miller]

Silicone car polish also works, just a few drops per gallon is fine.

[flyeruk]

Thanks for the replies. I fitted a sintered clunk this evening. I have run nearly a whole tank through it tonight,different throttle settings not a bubble to be seen

[NigelH]

Told you so

[Keith Miles 2]

To continue an old thread rather than starting a new one.....!

I have a Wots Wot with ASP80 FS. Of late it has developed a similar problem which, as yet, has not been cured. I will come to that shortly.

I have recently been reading up extensively about this issue in numerous forums and my view would be that (a) a leaking tank (often suggested) is highly unlikely to ever be a cause of this particular issue and (b) I would, personally, be very wary of messing with the chemical content of fuel as a fix and introducing “alien” substances into an engine!

In the first instance, I would have thought that only air in the fuel itself (the clunk being submerged in the fuel) would result in air being drawn along with the fuel. In fact, you NEED air to enter the tank to allow fuel to flow, hence the need, of course, to vent the tank or to pressurise it. Also, seems to me that even if fuel is leaking out, air cannot get in at the same location and a fuel leak should not affect fuel flow unless it was pouring out faster than the clunk could suck it up! When one drinks through a straw, the liquid is surrounded by air and, even if some spilled over, only loose lips or a hole in the straw would affect flow from vessel to mouth, yes?

I would suggest, therefore, that if air is seen in the fuel line at the carburettor then it is either being drawn from within the fuel, through a hole in the fuel line or via a leak at the needle valve.

In the case of my Wots Wot, I am witnessing not bubbles as such but rather small intermittent gaps in the fuel flow to the carburettor resulting in a fluctuation in rpm (especially acute at high throttle) and a very sensitive needle valve.

Having twice removed and reassembled the tank finding no problems with it ,even replacing all of the tubing, filling the tank and rechecking for leaks, the problem still exists.

Seems that vibration is often quoted as introducing a disturbance in the fuel (air bubbles/frothing). That said, the overwhelming majority of models do not lend themselves easily, if at all, to insulating the tank from vibration and most seem to have worked perfectly well for years, in any case, without having to take such measures.

My engine and carb have been previously inspected, tappets checked, carb parts cleaned etc. etc

As has been suggested, I am now going to try replacing the standard clunk for a sintered clunk filter (I’ve just ordered a couple!) which has, apparently, cured the problem in some instances, and also to then re-check the prop balance.

I am also aware that some people have had issues with SC/ASP needle valves but mine was fine originally, indeed everything was fine originally!

Hopefully, a step-by-step approach will identify the cause of the problem, and cure it, with the added bonus of educating both myself and others!

Watch this space!

 

Edited By Keith Miles 2 on 30/07/2019 15:32:44

[Kim Taylor]

Posted by Keith Miles 2 on 30/07/2019 15:29:33:

To continue an old thread rather than starting a new one.....!

I have a Wots Wot with ASP80 FS. Of late it has developed a similar problem which, as yet, has not been cured. I will come to that shortly.

In the first instance, I would have thought that only air in the fuel itself (the clunk being submerged in the fuel) would result in air being drawn along with the fuel. In fact, you NEED air to enter the tank to allow fuel to flow, hence the need, of course, to vent the tank or to pressurise it. Also, seems to me that even if fuel is leaking out, air cannot get in at the same location and a fuel leak should not affect fuel flow unless it was pouring out faster than the clunk could suck it up! When one drinks through a straw, the liquid is surrounded by air and, even if some spilled over, only loose lips or a hole in the straw would affect flow from vessel to mouth, yes?

Edited By Keith Miles 2 on 30/07/2019 15:32:44

No.

In order for the fuel to be drawn (there's your clue) up the tubing and into the needle valve assembly, the pressure in the fuel line must be less than atmospheric (certainly as the level in the tank drops).

Therefore it follows that a leak in the tubing between the tank and the needle valve won't necessarily let fuel out (when the engine is running) but draw (that word again) air in, causing bubbles.

Depending on the routing of the tube, these bubbles may come through as individual micro bubbles or if there's a high loop in the line, they may well settle at the high spot and then be pulled (different word, same meaning) through as a big bubble or void.

At least that's my experience.

Kim

[Don Fry]

Keith, if the model does not lend itself to insulating the tank from vibration, cut something away so it does, or fit a smaller tank, or plan the build round the tank. This is not an option. 3 to 5 mm of foam. Tanks do not touch the airframe. All the other air in pipe reduction techniques are adjuncts to a basic sound installation.

[Keith Miles 2]

Kim,

I believe that’s what I said. Apologies if I was not clear.

With the clunk fuel pick-up submerged in the fuel, only air in the fuel or, as you correctly state, air entering due to a punctured or loose fuel line between clunk and carb will cause both air and fuel to to be drawn at the same time and to appear at the carb inlet.

A leaking tank however, contrary to what I often see suggested, will not, itself, create air in a fuel line, hence my drinking straw analogy where the liquid being drawn up is entirely exposed to the air. Air entering a tank, even if as a result of a leak, only prevents a vacuum from being created and perhaps reduces pressure where pressurisation is necessary.

My Wots Wot tank has been both pressure tested and leak tested more than once with no faults found.

Don,

I agree that insulating a tank is probably a good idea, if it is possible to do without too much aggravation (as would be the case with many ARTF models) but repeat that there are numerous models flying, including my own, that do not have insulated tanks and they do not exhibit the problem that I currently have with my Wots Wot.

The reason I am going to try a sintered clunk filter is because somebody had this issue and eventually discovered that vibration was, indeed, the cause by running the engine in the airframe whilst holding the tank away from the airframe. On advice, he just replaced the standard clunk with a sintered filter type and the problem apparently disappeared. Much simpler, quicker and more convenient than hacking away for hours (or attempting to) at bits of airframe!

Like him, I would rather try the simple stuff first and leave any reconstruction as a last resort.

[Keith Miles 2]

Don,

One more point.

You say “Tanks do not touch the airframe”. Yes they do. ARTFs are usually supplied with tanks that are an exact fit to keep them in place including a hole in the front bulkhead through which the neck of the tank fits.

Perhaps you meant to say “Ideally, tanks should not touch the airframe”?

Apologies in advance if I misunderstood your post!

Edited By Keith Miles 2 on 30/07/2019 23:28:46

[Martin Harris - Moderator]

My experience with the ASP/SC engines has been that the needle O rings are of variable quality and may not last too long. Perhaps yours has gone hard - explaining why a previously successful installation has started giving trouble.

Unfortunately they are not easily obtainable standard O rings...they are square section, not round, and I haven't managed to find any proper alternative to the manufacturer's supplied ones. Round ones can be persuaded to work but the ones I've used successfully are extremely stiff in operation. With the demise of the manufacturer, if anyone knows of a source it would be useful to know!

I was always encouraged to pack the tank bay with pieces of upholstery foam to minimise fuel foaming by damping the tank vibrations - I can't say that I've ever experimented with not doing it so it may be an old wives tale - but worth considering?

Edited By Martin Harris on 30/07/2019 23:38:42

[Nigel R]

Ideally they do not touch. You may get away with it, if they do, etc.

A sintered filter clunk may well be a reasonable workaround. Definitely worth a shot before surgery.

[Keith Miles 2]

Martin,

Yes, the needle valve and “o” ring are on my check list. I have been given some silicone tubing that might fit snugly over both needle and carb body (a tip from a club member who had trouble with an SC carb)) so was intending to try that as well. Your thoughts were my thoughts as well regarding the change over time which, of course, could point to the needle valve as one possible cause of the problem. That said, it’s not that old in terms of hours flown so whilst the engine is fine, it would be disappointing if it was let down by premature needle valve wear especially now that ASP have left the market.

And if anybody says go electric.........😊

[cymaz]

I’ve found that ARTF fuel tanks are not to be trusted.

Poor quality bungs and leaking tanks necks are an issue.

I replace the tank with a Dubro item and these have never let me down. I’ve moved away from Slec tanks. I find that it’s difficult to plumb them and you can’t use any other clunk but the one that is supplied

Edited By cymaz on 31/07/2019 05:42:38

[Kim Taylor]

Posted by Keith Miles 2 on 30/07/2019 23:08:16:

Kim,

I believe that’s what I said. Apologies if I was not clear.

Yes, sorry I've re-read it and I misunderstood what you wrote.

Must pay more attention next time!!

Kim

[Keith Miles 2]

Posted by cymaz on 31/07/2019 05:42:02:

I’ve found that ARTF fuel tanks are not to be trusted.

Poor quality bungs and leaking tanks necks are an issue.

I replace the tank with a Dubro item and these have never let me down. I’ve moved away from Slec tanks. I find that it’s difficult to plumb them and you can’t use any other clunk but the one that is supplied

Edited By cymaz on 31/07/2019 05:42:38

Are you sure you had leaks at the neck?

I have never had a leak at the neck with ARTF tanks but have had leaks around the pipes and/or the clamp screw.

Here’s a tip!

Have totally cured any leak by either using slightly larger diameter fuel pipe or heat shrink sleeving around the pipe or screw where they pass through the holes in the bung.

Plastic pipes are crap, not as easy to bend as they claim and usually too short, so I usually bin those!

And I always test a tank before I fit it by fitting fuel tubing, stopping off all but one of the tubes in order to let air in, then standing the tank up with neck downwards, the open tube upwards (of course!) and leaving it for a few hours, or even overnight! If all seems well, I also then use the corner of a piece of tissue to check for any hard to see minor leakage!

My fuel bays are bone dry!

[Nigel R]

The SLEC tanks make really good use of space (which is really really handy if you've done something daft like making your tank bay too small, not that I would ever do such a thing). And consequently it is also easier than normal to surround them with foam.

Not sure why you say "it’s difficult to plumb them"?

The Sullivan slant tanks are also good, on a par with the Dubro I would say, if you want to stick with a standard bung arrangement.

[Martin Harris - Moderator]

I've certainly had leaks at the necks of ARTF tanks - usually caused by them splitting!

[Keith Miles 2]

Funny how experiences seem to differ!

[John Stainforth]

I have found that when I leave models for several months without flying, that were 100% fine before the hiatus, failures of the radio and tank system are very common when I return to the models. Neoprene and copper tubing, tends to deteriorate and crack, as do rubber bungs, and tanks do crack. I have quite a large pile of useless old tanks in the US and UK that have failed in these various ways. One thing I have learnt is to fill up fuel tanks very gently on the first fill of the day, since vigorous filling builds up pressure in the tank that can force out deteriorated bungs. Also: always fill up the first tank (at least) of the day with the canopy off so that one can spot a leak at the earliest opportunity - rather than suddenly seeing fuel poring out through the bottom of the fuselage!