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Canadian
Homebuilt
Aviation

  Mikuni Fuel Pump

The Mikuni impulse pump is widely used for two stroke engines on snowmobiles, ATV's and yes, even aircraft.  Just in case you wondered what's inside one of these, here are some pictures taken during the rebuild of mine.

mikuni pump 1.jpg (95022 bytes)
This is how my pump looked when I received it from a vendor on eBay.  It was a little grimy but otherwise in very good condition.
mikuni pump 2.jpg (106176 bytes) The pulse port line from the engine connects to the hose barb in the middle of the back cover.  The fuel lines connect to the remaining hose barbs on the pump - refer to the arrows to determine input and outputs.

Once the six cover screws are removed the unit comes right apart.

mikuni pump 3.jpg (76419 bytes)

Here are all the major parts; there are two gaskets, two diaphragms and two valve assemblies on the pump housing to replace.  It's very straightforward.  The rebuild kit is commonly available.

mikuni pump repair kit.jpg (59067 bytes)The kit I used is available online from Sirius Consolidated.   The stock number is 07-187-10 to fit the 27/16 inch diameter pump body.   There's also a kit available for the smaller 21/8 inch body - see the website for details.

I gave the parts a good scrubbing, and cleaned up the faces of the valve plate with some 600 grit wet&dry abrasive sheet (wet, of course) on my surface plate.  Only do this to the valve plate and not the covers - they have a small ridge that will help seal the gaskets.

valve_plate.jpg (149334 bytes)Here's the plate with the new valve discs installed.  Putting the rubber pieces through the holes in the valve discs and then installing them in the plate is the most difficult part of the reassembly.  I've got some dental picks that I've put blunt ends on for doing this type of thing and they work quite well.  You don't want to use a sharp tool to poke the rubber parts into the holes or you will most likely damage them.

It's amazing how much better engine parts look after they've been cleaned up.

The next step is to modify the pump to make it suitable for aircraft use.  One of the reasons that Mikuni pumps for Rotax engines cost more is that they have a bleed hole added to ensure that oil and/or gas doesn't hydrolock the pump.  This hole is very small, 1/64th inch to be exact.  There are drill bits available in this size but they are almost microscopic!  You'll need a pin chuck to hold the bit and a very fine touch to do the drilling.

The bleed hole is located on what will be the bottom of the pump.  You won't be mounting it on the side of the engine on those two bosses that are exactly the right spacing - there's a danger that engine heat will cause vapour lock, so forget that idea.   Instead, you need to mount it so that the impulse hose connector will be at the bottom.  The bleed hole is situated so that any liquid will drain to the hole and be blown out.

pin_chuck.jpg (174578 bytes)This photo shows a view through a magnifier of the pin chuck, drill bit and location of the hole.  The location of the hole was marked with an autopunch; you could also use a center drill to make a small indentation for the bit.  This is important, because these small bits will tend to wander instead of cutting into the material where you want the hole to be.

Thankfully, the casting is fairly soft metal and the hole isn't very deep.  You will need to "peck drill" this hole.  This means drilling a little and then pulling out the bit to clear the chips.  Then drill some more and repeat, until you're through to the other side.  If you try to push the bit all the way through it will most certainly break.  It does take a little time but you're saving a bunch of cash compared to the cost of the "official aircraft" version.

After the drilling is done the part is blown clean of any tiny metal bits.  The next photo shows the completed modification  - can you believe that such a small hole may be the key to a more reliable engine?

impulse port.jpg (215868 bytes) Here's a diagram showing the reassembly order:

mikuni_parts.jpg (71910 bytes)

Now all the parts are reassembled.  I used new screws and lockwashers and put a drop of Loctite in each hole to keep things from vibrating apart.  It's a beauty!

pump_assembled.jpg (137490 bytes)

My engine installation will use a single carb, so I need to add a wye connection on the outputs of the pump to combine them into a single feed.  I could also just block off one of the outputs and the result would be the same - they're fed by the same chamber.

This does allow me to change to a dual carb setup if I ever want to make things more complicated.