Pilot Jet 35

I now have an orifice measurement (sort of)  for the restrictor/jet fitted to my Rotax.

Sort of, because I used oxyacetylene nozzle cleaners/wires to do the job.

I am imperial measurement challenged, so I hope you can make sense/convert the following;

Oxyacetylene Cleaner No 22 (Drill 55-54) fits. When I say fits,  the non burred (smooth) end of the cleaner fits snuggly, the burrs do not.

Cleaner No 24 (Drill 53-52) is loose and this includes the burred section.

The above, are found on the back of the cleaner kit - got no idea what they mean to a metric person?.

I assume you meant to say the #22 (Smaller) cleaner was loose and the #24 (Larger) was Snug.

Ref. Numbered drill sizes.

No.   Inch      mm
55   0.052    1.32
54   0.055    1.40
53   0.0595  1.51
52   0.0635  1.61

Reverse engineering says that Orifice has had a 1/16", (0.0625", 1.5875mm) drill bit put through it!  Waaay! too large.

A #76 (0.020", 0.51mm) drill would be more appropriate.

 - - -

I created my Orifice by Drilling an 1/8" (3mm) hole through a screw and filling it with solder, Easier to drill.
Then, a #60 (1.0mm) hole was drilled through the solder plug.
It is not so much an orifice as it is a short tube.
The Length of the tube compensates for the larger hole.  Same results.
The larger hole also means that debris up to a few 100 microns will not get stuck.
This has been working fine for me.

Hi Bill,

At the risk of making a fool of myself this is what I have found;

Using my very old micrometer screw gauge (metric)

Cleaner 18 for drill 56, measures 0.510 mm

Cleaner 22 for drill 55-54, measures 0.503 mm

Cleaner 24 for drill 53-52, measures 0.450mm

Cleaner 26 for drill 51-50-45, measures 0.365 mm

The larger the cleaner number, the smaller the wire/orifice. My cleaners go up to 6 for drill 77-76

So, if my gauge is correct, the diameter of my jet is about .503 mm.

My 1mm drill bit, did not fit.

Jeff B's earlier observations are starting to come close. If my jet is a .503 and my flow rate per Hobbs hour 7+L/hr it may be I need a smaller jet.

Sean,

I have been operating under the assumption that you are NOT using the Rotax fuel block with the Rotax provided restrictor.  If this is correct, then why not just order the Rotax pilot Jet #35 (part # 063-143) and use that, or at least use it for reference.  

It’s unclear to me if the #35 jet is .35mm, or .5mm, but it should not matter.  By my calculation, at 3.5 psi, a .35 orifice would yield a fuel return flow of about 2.8 L/hour, and a .5mm orifice would yield about 4.3.  Assuming a fuel burn of 27 L/hour at full power, then either of those would keep you well within the pumps minimum delivery rate of 35 L/hour. 

If you are truly getting 7 L/ hour return flow, it would seem like your restrictor is larger than .5mm, (probably about .7mm). Maybe you should recheck the return flow rate just to make sure. I would do that with the aux pump off since that’s the condition you are experiencing low fuel pressure.  

Lastly, if I’m not mistaken, the 1.0mm restrictor Bill H refers to is installed in the vapor bypass line on his 912iS, which is a different fuel system.  I would think 1.0 mm is too large for your 912ULS fuel return line.  

Jeff Blakeslee wrote:

Sean,

I have been operating under the assumption that you are NOT using the Rotax fuel block with the Rotax provided restrictor.  If this is correct, then why not just order the Rotax pilot Jet #35 (part # 063-143) and use that, or at least use it for reference.  

It’s unclear to me if the #35 jet is .35mm, or .5mm, but it should not matter.  By my calculation, at 3.5 psi, a .35 orifice would yield a fuel return flow of about 2.8 L/hour, and a .5mm orifice would yield about 4.3.  Assuming a fuel burn of 27 L/hour at full power, then either of those would keep you well within the pumps minimum delivery rate of 35 L/hour. 

If you are truly getting 7 L/ hour return flow, it would seem like your restrictor is larger than .5mm, (probably about .7mm). Maybe you should recheck the return flow rate just to make sure. I would do that with the aux pump off since that’s the condition you are experiencing low fuel pressure.  

Lastly, if I’m not mistaken, the 1.0mm restrictor Bill H refers to is installed in the vapor bypass line on his 912iS, which is a different fuel system.  I would think 1.0 mm is too large for your 912ULS fuel return line.  

Hi Jeff,

You are correct - the fuel distributer block/manifold, in my aircraft, is a custom creation.

The restrictor jet fitted, was purchased/recommended from the Australian Rotax agent. There is no record of the size/name/brand of the jet. 

The return line fuel flow was determiner by collecting the fuel delivered (separate from other fuel carried) over several test (new aircraft) flight hours (Hobbs). 

Your fuel pressure of 3.5psi is below what my Dynon says is being delivered  @ 4.5 -5.0 psi This would increase the flow rates above your calculations.

The 7+L/hr resulted from the normal practise of running a boost pump at TO/Landing, switching off when cruise altitude reached. The boost pump has been run for a longer period, than that which I would call normal, due to the low fuel pressure warnings the pilot (my Son & I )have been getting/seeing during climb out.

1mm restrictor would be far to large. The 0.503mm restrictor fitted now, is delivering 7+L/hr.

I will ask my local Rotax agent if he has PN 063-143 available.