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Old Dec 14, 2012, 02:49 PM
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Someone once stated a reason for the bubbles and a way to fix it, I think. It might be in this thread someplace.

I had seen the pump body ooze a little fuel when the engine is running. It sort of accumulates oil residue on the body when the engine is running.

If you see tooling marks on the pump body mating surfaces... Some guys use a piece of plate glass and some extra fine valve lapping compound to ensure the pump body mating surfaces are smooth and fit together. A tiny pin hole in the diaphragms could do it. But probably the best source for the bubbles might be the fuel fittings being slightly smaller with single rounded barbs are letting air leak into the fue lines. You can try using some small Zip Ties as clamps or carefully use some thin steel wire to clamp down on the fuel tubing more too.
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Old Dec 14, 2012, 02:56 PM
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What I did was glue a short piece of plywood onto the side nose of the plane to serve as a shelf and put some padding on it and loosely held it in place with a length of velcro. This allowed me to move the fuel pump closer to the carb on the engine. The shelf keeps the pump from moving around too much causing needle changes. Later on a different plane, I found that I could place the pump behind the engine, if I kept it padded and loosely held in place too.



Later I found that the pump works Ok behind the engine, if it is padded and loosely held in place with a velcro strip too.
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Old Dec 14, 2012, 11:28 PM
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Brutus,
OK I understand to close the carb inlet fully when priming. I will observe where the bubble originated and update it here.
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Old Dec 14, 2012, 11:50 PM
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I am using 2.5mm tubes plus zip ties already. the bubble generated from the pump, i think mainly due to the vibration. I can see that much more bubbles are splitting out from the pump when the vibration is big, but when there is little vibration, there is no bubble. The bubble causes rpm to jump as earlwb said, but it doesnt kill the engine, so maybe this is not a critical issue since the plane will still fly.

earlwb, when you placed the pump on the shelf by the nose, wouldnt the wind current generated by the prob cause the pump to vibrate a lot?
when you placed it behind the engine, eventhough there is padding, but since somehow the padding must be touching engine body, and the vibration from
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Old Dec 14, 2012, 11:55 PM
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when you placed it behind the engine, eventhough there is padding, but since somehow the padding must be touching engine body, wouldnt the vibration from the engine being transfered to the pump?
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Old Dec 15, 2012, 12:00 AM
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when I had the pump mounted with padding behind the firewall, it constantly vibrates. I think the souce of vibration comes from the vibration of the firewall and the fuel tubes.
when I am having it now hanging loosely by the 3 fuel lines and also situated behind the fire wall, it seems to vibrates marginally less, but the amount of bubbles dont seem to decrease.
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Old Dec 15, 2012, 12:01 AM
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earlwb,

i will try to do the 2 different methods that you used, and see what result they yield.
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Old Dec 15, 2012, 02:08 AM
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Quote:
Originally Posted by kentli22 View Post
I am using 2.5mm tubes plus zip ties already. the bubble generated from the pump, i think mainly due to the vibration. I can see that much more bubbles are splitting out from the pump when the vibration is big, but when there is little vibration, there is no bubble. The bubble causes rpm to jump as earlwb said, but it doesnt kill the engine, so maybe this is not a critical issue since the plane will still fly.

earlwb, when you placed the pump on the shelf by the nose, wouldnt the wind current generated by the prob cause the pump to vibrate a lot?
when you placed it behind the engine, eventhough there is padding, but since somehow the padding must be touching engine body, and the vibration from
Bubbles=leaks. Nothing to do with vibration. Plain and simple. You cannot cause a liquid to form vapour from just shaking it, that is just basic physics. Of course, if your engine runs, it runs and it is up to you if you want to do something about it.

Vibrations can influence the pump efficiency because theoretically it has effect on the opening and closing of the valves, but the regulator part should be able to cope with that. Bubbles tend to influence the pump as well, because liquid is (virtually) not compressable, but bubbles are. Unfortunately, a leak tends to be not constant, thus it can cause your engine to cut out under circumstances and you will be looking for a reason forever, because you ignore the leak "as it was always there anyway".

Just to save yourself a lot of trouble, check the surfaces of the pump-parts and smooth them out (or ask somebody to do that for you if you do not have the technical skills or possibilities to do it yourself).

Brgds, Bert
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Old Dec 15, 2012, 03:30 AM
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Originally Posted by Brutus1967 View Post

Bubbles=leaks. ...snip...
Not necessarily, Bert.


The early version of OS.91FX engines had a rear-mounted needle-housing that was made of heat-conductive metal.

Most owners encountered bubbles in the fuel-line, between the needle-housing and the carburettor fuel nipple. Virtually everyone blamed it on an air-leak through that needle's thread/O-ring and suggested ways to seal it, Etc..

The reason was actually heat conduction from the crankcase to the needle assembly; that caused the methanol component in the fuel to evaporate upon contact with it.
This in turn caused the mixture to become too lean for the engine to run consistently.


After seeing that this was indeed the reason, OS did the responsible deed and offered a free-of-charge retrofit of a plastic needle assembly, that solved the problem outright.
The following batches of the engine had that plastic item fitted as standard.
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Old Dec 15, 2012, 04:56 AM
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That is hair splitting and does not help Kentli any further....

The pump of the NGH 9 is not subject to heat transfer, and the assumption of Kentli was that vibration might cause bubbles.

Remember the last PM I sent you? basically you are doing the same here....

Brgds, Bert
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Old Dec 15, 2012, 06:27 AM
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I am sure there is a reason for it. But if I mount the pump/regulator onto the back of the engine using the nifty mount that was included with the engine, I get a lot of bubbles coming out of the unit. if I let the pump/regulator float free, no bubbles (after I clear out the bubbles inside it though).

I had two ideas on where the bubbles were coming from. First at the time it was summer, and high heat outside over 100 degrees Farenheit, so the fuel had a tendency to vaporize into a gas inside the pump/regulator unit. You have a high pressure zone on the pump side, going into a relatively low pressure zone on the regulator side, causing that condition to occur. When I go flying and the temperatures are well over 100 degrees Farenheit (110 degrees F or more) all of my gas engines (not just NGH either, all of the other engines) have a tendency to vapor lock and stop running on the landing approaches when I throttle back to low throttle for landings. But when it is cooler outside no problems at all.

Second the engine vibration aggravated the condition and caused the gas vapor bubbles to occur. I did notice that if I used the SEF or TRU-Fuel pre-mix gasoline (with extra oil added) that this didn't occur as much. But it may be that our USA blends of gasoline with all those extra chemicals such as butane and propane in it tends to be more sensitive to it.

So then the question is what inside of the pump/regulator unit causes the gas bubbles to form? Then is the problem fixable or not? The Walbro Carbs do not appear to have the problem, but then the carbs are all integrated and we can't see inside them, so we don't know for sure.

I remember some of the guys, who really know Walbro carbs inside and out, know the answer, but unfortunately those people who are extremely annoying drove them off before they could answer it.

Here is a video showing the bubbles coming out of the pump/regulator when it is mounted to the engine. If I let the pump float free the bubbles go away.
NGH 9cc gasoline engine fuel pump getting bubbles in the fuel line during engine test run (0 min 18 sec)



Now then with my planes and engines. The first engine run of the day at the flying field requires a ritual of me running the engine and moving the plane all around at different angles to ensure all of the air bubbles inside of the pump regulator are removed. As I run the engine and angle the plane about I can see some air bubbles going to the carb. After I do this for a while the bubbles go away, and I can then fly the plane. it takes maybe a minute. It isn't a big deal, as I do something similar with glow engines to ensure the main needle is set too, by pointing the plane's nose up and check to make sure it isn't too lean.
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Last edited by earlwb; Dec 15, 2012 at 06:34 AM. Reason: add more information
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Old Dec 15, 2012, 06:59 AM
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Brutus,
it is absolutely possible that there is air leak in my fuel supply system. I am tracking it down step by step.

With my new set up for the pump, there is virtually no more bubble feeding to the carb, except rarely 1 or 2 bubbles. but most of the time there is no bubble at all from the pump to the carb, and also no bubble from the back of the engine to the pump.

However, there is plenty of bubble and constantly from the fuel tank to the pump, but no bubble coming out of the pump to the carb. I am already using the bubble-less clunk (not the felt clunk type), but I can see that my tank is vibrating so much, so I wonder if there is something to do with this.

My new set up for the pump is like this: it is packed inside a huge foam block positioned behind the firewall and not touching the firewall. In order to do this I have to extend the fuel line to be around 1 foot long. The fuel supply doesn't seem to be affected by the longer fuel line. I assume with the vibration is much less as the pumb is stocked inside the foam block and somehow the bubble was almost gone.

I think with the bubbles entering the pump from the tank and/or from the leakage of the sealing of the pump itself, vibration of the pump somehow causes the bubble to be fed to the fuel line to the carb.

Later I will do another experiment to let the pump hanging by itself without being stocked in the huge foam block, to see if this yield the same result or not. Since I had tried to tighten the screws on the pump befoe I put it in the foam block, so I will do a control experiment to see if the foam block is actually being a factor of the reduction of bubble.

Aside from the bubble issue, there is another issue that I am experiencing, ie, when I have the RPM to reach near the 6000 neighborhood, the rpm will raise soon and eventually reach so high that it quit. Of course I assmed it was too lean, so I gradually open up the HSN until it was 2 and a half turn out, it then seems to be able to hold the RPM at a little bit higher position but will soon raise rapidly then die. Then I also open up the LSN another half turn until I reached 6 and a half turn out, but it seem to be not curing anything. Below 6000rpm, rpm was stable in a way that it just keep jumping up and down within a 500rpm range, idle is stable in the same manner but with a smaller range.

Could this simply be the behavior it is prior to its fully breaking in? Or I should richen the needles more? Or what else?
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Old Dec 15, 2012, 07:04 AM
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Brutus,

I am not capable of fixing the pump if there is a leak or any problem. I can only go buy a new one if I must suspect the pump has a problem. I don't think anyone will fix it for me, and I am not sure if the manufacturer has any interest to fix or repair anything.
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Old Dec 15, 2012, 07:52 AM
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I just tried taking the pump off the foam block and just let it hang freely in the air by the 3 fuel lines, there was some bubbles resulting from this. I packed the pump back into the foam block, there was not more bubble except 1 or 2 but rarely.

I richened the HSN another quarter turn more, the higher rpm seems to hold better, but now HSN is 2 and 3/4 out.

I couldn't figure out where the air leak from the fuel tank was if there was any. But inside the fuel tank , the fuel moving violently, very much like boiling water. I wonder from the fuel tank is created that way.
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Old Dec 15, 2012, 07:53 AM
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I wonder bubble from the fuel tank is created that way.
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