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Bruce Tuncks

Fuel Pressures

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There is a risk, but more likely it can get back into sump id have thought

 

Theres a small vent tube on the side to stop this but yes fuel goes overboard

 

Jabiru now have a little tin catch tray with drain, its a certification addition id reckon

 

 

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Jabiru now have a little tin catch tray with drain, its a certification addition id reckon

NOW ? I don’t know when but (at least on factory built examples) the tray has been standard for over 10years, when I purchased mine. At some time over the period they changed from fibreglass to tin (aluminium).

 

 

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Yes they had a composite one since J230 id say, Also prevents fuel pump being bolted up tightly. Was required for LSA certification i think. Also had low fuel pressure switch and light which was pretty random.

 

More recently chanaged to a folded steel version, quite expensive i think

 

 

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I still have not got an answer to whether it is possible for the electric pump flow to go through a possible diaphragm hole into the engine sump instead of into the carby. I don't think it is possible, because if the fuel pump is driven by a pushrod which is a close fit in the engine wall, and there was no other opening, then any fuel would have to get through the running-gap between the rod and the engine wall.

 

Having never taken a fuel pump off, I don't know this for sure.

 

 

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I'm not sure about the Jab fuel pumps, but I think it was possible with the Rotax fuel pumps. They put out a modified fuel pump some time back because of that exact problem.

 

 

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Yes the plunger rod is a firm fit but there has to be loose clearance so fuel could get in there

 

BUT

 

The little nipple on the side of the pump, might be a mod Jabiru do to it, lets fuel vent out rather than too much entering sump

 

The hose on this should be attached to oil carch bottle vent

 

 

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Bruce, my case has a small hole,1/16th inch perhaps, to allow excess oil to drain out if too much makes its way past the shaft on the mechanical pump. You must be careful when applying sealant (518 from memory) not to cover it up.

 

Ken

 

 

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If engine stoppage can result from a diaphragm failure, it sure would be a bad bit of design negating the backup fuel pump. I have never heard of this happening with a Jabiru, although I was told it was possible with a Gypsy engine.

 

If the pushrod to the sump clearance is fairly small, then most of the fuel from the electric pump will get to the carby. That little nipple which vents the fuel pump must not let too much fuel drain out though, because that alone could cause fuel starvation in the event of a diaphragm failure.

 

In most cases I guess the hole would start real small and the first indication would be some fuel in the pump drain catch bottle. I would just like it confirmed that no sudden tear in the diaphragm could cause the engine to stop if you turned on the electric pump in time.

 

 

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I reckon an electric fuel pump would push fuel through the diaphragm into the engine if the diaphragm was faulty.

 

The cam lobe pulls the diaphragm away from the fuel and the spring pushes it towards the fuel, producing the fuel pressure. Fuel enters the pump through a one way valve valve, so electric pump pressure will open that valve.

 

 

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These small holes are called "weep holes" and are designed to let you know early on, you have a small leak or holed diaphragm before it becomes major.

 

The electric pump will continue to feed fuel up to maximum flow and/or maximum pressure. It doesn't know where the fuel goes.

 

So with a holed diaphragm the fuel can choose one or both routes.

 

1, to the carb and 2, out the holed diaphragm.

 

A lot of variables come into play when looking at where the fuel is going to go at any given time.

 

Mainly the fuel requirements of the carb.

 

For example, at full throttle system pressure is reduced (due to high fuel flow to the carb) and this MAY reduce the leak at the diaphragm.

 

Conversly at idle with high system pressure, the electric pump may be actively pumping fuel out the diaphragm.

 

Note: I'm talking about a small split, leak in the diaphragm. A major leak will result in low pressure/high flow.......and a rapid loss of fuel.

 

If bad enough the carby's will starve and the engine may stop.

 

 

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I have had 2 mechanical fuel pump failures on my Camit 3300 both in the first 2 hours of operation. In both cases the electric was enough to get me back without trouble. Not sure of the failure mechanism but probably stuck diaphragm due to the weaker spring or mis alignment. No troubles since and I now have 100 hours. Occasionally there is some venting when I have both pumps on for landing and no power is used. I usually turn off the electric on landing now to prevent it.

 

 

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That's a good reason for a fuel pressure gauge graham. You are not the only one to land with only one pump. There was a Jabiru at Gawler which had the engine stop in the circuit when the electric pump was turned on. I think this was a rare happening and training is done by the book with no further events.

 

It was not a Camit engine. It would be interesting to know what the fuel pressures were when that happened.

 

Here's an experiment needs doing... cut a big slit in a diaphragm and reinstall it like that and see if the electric pump still works the engine without too much fuel going into the engine.

 

 

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That experiment may need doing, but I am certainly not going to try it.

 

Any L1 or even anyone with any mechanical knowledge can work out what will happen, so why be stupid?

 

 

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Bruce, the weep hole is between the pump flange and the case. It is only for excess oil that makes its way up the pump push rod to allow drainage. In my motorcycles with cv carbs (bmw and yam) I have only had one diaphragm failure in 40 years. It would idle but not have any power on one cylinder, but when I changed it and then the good one to make sure, it pretty well disintigrated in my hand as I removed it. Very easy to inspect but you will probably damage it when you do.

 

Ken

 

 

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If a big diaphragm failure actually happened, and the output from the electric pump couldn't get past the mechanical pump, a 2 way valve which diverted fuel from the electric pump past the mechanical pump would be necessary if you wanted to be sure the engine could run on just the electric pump.

 

I have never heard of this being done. Surely it would have been done if there was any real need. But Ken, you have me a bit worried about old diaphragms.

 

 

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There is a great video about the guys who resurrected a Catalina in Spain and flew it back to Longreach. They were dogged much of the way by a problem which I think was due to a fuel valve vibrating to the partially closed position. ( They did a lot of fix attempts and wiring this valve in the open position was the last fix in the story.) One of the engines had repeatedly lost power till this was done.

 

Now this problem would have been picked up easily with a fuel pressure gauge I reckon.

 

I don't know if the Catalina has fuel pressure gauges... surely it should have, a big aircraft like that?

 

 

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Should have a flowmeter. It's the same engine as a DC-3 (P&W 1830) It has engine driven fuel pumps but I can't find info on any other. It's possible there are none. But unlikely. You have tank selector valves and X feed for engine out and fuel balancing. The DC 3 has an electric pump so I think the Cat would also. You need to be able to shift fuel around on the ground/ water.

 

Nev

 

 

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You are right Nev. Looking up the flight manual online, there are 2 flowmeters and 2 pressure "gages" listed among the engineer's instruments.

 

So now I am more mystified about the trouble they had. The video is " Return of the Catalina" and well worth seeing.

 

 

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