Fuel return Line for a 912UL

Sorry psi

Bill,

A set of carb jet drills are on order.  I'm going to fix this one way or another.  

Thanks again.

A set of #61 through #80 drills are amazingly small and easy to loose.

Looking at a #80 through a 10x magnifier shows it to be a perfect lookiing microscopic drill bit.

It amazes me how they can even exist.

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These drills bit are Easy to break with very small pressures.

To drill the jet you will need to chuck either the Jet or the drill bit and hold the other piece in your hand.

You will never be able to use a conventioal drill press.

Option 1.

Chuck the Brass ftting with the Jet in a drill press or a Hand Drill in a vice.

Chuck the drill bit in a Pin-Vice and guide the bit into the jet by hand.

This will be like tickling a butterfly behind the ears.

Option #2.

Chuck the Drill Bit in a Pin-Chuck and then Chuck the Pin-Chuck in the Drill Press or a Hand Drill in a vise.

Hold the Brass fitting with the Jet in your hand and guide it delicatetly onto the Drill bit.

If you do not already have the tools, you will need to get a Pin-Vice and a Pin-Chuck while waiting for the bits to arrive.

Go for the smallest one.  0.0 to 1.0mm will be fine.

Genival Pacheco wrote:

Dear Bill

How is the pressure regulated to 2.8 bar?

 

 

A mechanical Fuel pump's pressure is regulated by the spring pushiing on the diaphragm.

As long as you do not exceed the flow capacity, the spring will push fuel at a constant pressure.

The  pressure produced by an electric AUX pump is fairly constant and limited by the impeller size.

The pump might produce 2.8 psi at 2gph and 2.5 at 5gph.

A totally blocked fuel system might yield up to 3psi.

As long as the flow rate stays between zero and ~7 gph, the pressure remains relatively stable.

It may be Poorly regulated but is is definetly NOT Un-Regulated.

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When operating Both the Aux Pump And the Main Mechanical Pump in series, The pressures do NOT Add together.

Whichever pump produces the highest pressure is the pressure you will see.  2.8 + 3.2 = 3.2psi

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The actual Fuel pressure in a carburetted engine is of little importance.  Flow is what really matters.

As long as the pressure is high enough to fill the carburettor faster than the engine can use the fuel, more pressure adds nothing until the point is reached that the carb floats can no longer block the fuel entering the carbs because the pressure is too high and flooding occurs.

There is a very wide acceptable fuel pressure range.  Anything between ~1psi and ~7psi will usually work.

3 to 5psi is a nice safe middle ground.  The Goldilocks range.

Bill,

I worked my way through college as a foreign car mechanic.  I started out in a VW dealer and graduated to a Porsche Audi dealer.  Back in the early 70s, the Audi 100LS had a driveability problem.  From a stop, the carburated engine would stumble and sometimes die.  Some cars got hit when they died or didn't move fast enough from a stop into traffic, so it was a big issue.  Notwithstanding accelerator pumps or timing or any other problem, Audi's solution was to drill out the main jet larger.  Your right those jet drill bits were tiny and very hard to work with.  I remember the tools were kept in the parts department and handed out on an as-needed basis not allowed to be floating, no pun intended, in the unit room where all the engine and transmission special tools were kept.  The set of bits I ordered come with a pin vise and there is a pair of each drill bit size.  It is truly amazing how something that small can be made.