Pilot Jet 35

Hi Bill,

Ref your comment; "The larger hole also means that debris up to a few 100 microns will not get stuck"

I thought Rotax recommended a fuel filter in the range 75-100 micron - I like the above point but you should not be getting 100 micron debris at your return line restrictor?

At 5 psi the flow through the .503 orifice calculates close to 7 L/hour, but there are some factors in the math that have to be guessed at. Surely your “collection of fuel” technique is going to be more accurate.  It’s unfortunate Rotax does not state the the optimum return flow rate, it seems that once you use an orifice other than theirs then you (or the aircraft manufacturer) are on your own.  They do say the that you must use a restrictor that ensures the fuel pressure stays within operating limits under all conditions.  This is likely to avoid liability, since they would not want to be responsible for the designs of others. But it also shows that the fuel pressure is of primary concern.  I have seen it stated in several builders forums that the return flow target is 2-3 L/hour, but have never seen that from Rotax. If you get the Rotax jet #35, measure it and post the actual size of the orifice.  

Just to clear up any misunderstandings.

Rotax does not specifically specify the construction of the return fuel restriction device (Orifice).
They state that the supplied ULS Manifold (if used) is furnished with a 0.5mm orifice.
All the other 900 series Installation Manuals state that ... 

" ... In this bypass line a restricted jet should be installed so that there is an ideal balance between short venting time and minimum fuel flow rate. The positioning and dimensioning of the jet is up to the aircraft manufacturer. " ...

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I described an alternative to the #35 jet composed of a Drilled bolt.
I was not suggesting that anyone use a 1mm "Orifice".
An Orifice is typically designed with a minimum thickness or is countersunk to a sharp edge and yields discharge Coefficients approaching 1.0
My Drilled Bolt has a flow path through it of about 10mm, greatly increasing the flow resistance over a thin orifice.
My Bolt restrictor has a discharge coefficient of less than 0.1 as evidenced by a flow rate at 45psi of less than 5 l/hr whereas a thin, 1mm orifice would have a flow rate of more than 50 l/hr at the same pressure.
As mentioned I have a 912is.
Its fuel pumps are rated at 120 l/hr; 5 l/hr represents a very minor bypass load.
A 1mm drilled bolt restrictor might need to be Enlarged if used at 5psi.

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Yes, I do have a coarse filter rated less than 50u (Microns) installed before the Fuel pumps and Bypass restrictor.
As such I should never see debris larger than 50u.
I was pointing out that a side benefit of a larger 1mm (1000u) bypass would be its ability to handle 100+u debris without any problem if needed.

FYI -

I have asked my local Rotax agent about PN 963-820. He tells me that this is the #35 slow/idle jet supplied by Rotax for use in the return fuel line. He does not know the ID of the jet but has promised to find out. I will report this when he gets back to me.

Further

PN 963143 is a carburettor jet, that my agents says is physically too large to use as the restrictor jet (that is it wont fit into most fuel manifolds). He does not know what the ID of the jet is, however he insists it is not suitable for the purpose under discussion.

Looking at the IPC, 73-10-00, page 6, that makes sense.  Illustration 25 is part number 973-820. It will be interesting to learn what the real ID is. As Bill points out, you would think it would be .5mm, if inline with the installation manual.  But, if I were a betting man I would go with .35mm.