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| I found one of the first things to
break on my LA .15 engines is the needle valve. One good hard impact
on the engine, and the needle valve gets sheared off. Instead of
purchasing another backplate/needle valve assembly at $14 a piece,
I came up with a way to repair them using some 1/8" brass tubing
and a 1/8" "Bullet Point" drill I purchased at Home
Depot. The repair consists of drilling shallow 1/8" recesses
in the brass assembly and soldering some 5/8" long 1/8"
brass tubing pieces into these recesses. |
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| Usually if the needle assembly gets
broken off of the backplate, it's still usable. However, most of
the time not only does the needle valve get broken off, but one
or both of the plastic nipples will get sheared off as well. This
will make the assembly useless. |
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| The "Bullet Point" drill
bits have a small pilot which will act as a guide while drilling. |
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| First, remove the needle and the spring
from the assembly. The ABS plastic casing will have to be removed
from the brass valve assembly. I use a thin cut-off wheel in my
Dremel and make 3 scores in the plastic. I use a small bench vise
to hold the assembly while I score the plastic. |
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| Be careful not to cut into the brass! |
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| Once you've scored the plastic, use
some side-cutting pliers and break the plastic away from the brass. |
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| I find it's easier to use a drill press
to drill the recesses. I hold the brass assembly and push it up
against the turning drill bit. It seems easier to control the depth.
The valve assembly will have two small holes where the fuel enters
and exits the valve. We want to drill these out to 1/8" but
only deep enough to create a shoulder to hold the brass tubing in
place while we solder it. |
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| This operation can also be done with
a hand drill. Just be careful not to drill too deep. Once you're
finished drilling the assembly, flush it out with some 409 or solvent
to remove any drill shavings before we solder the tubes in place. |
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| Cut two pieces of 1/8" brass tubing,
5/8" long. Make sure the end you're going to solder is clean.
You can remove any oxidation from the brass by cleaning it up with
some very fine sandpaper or steel wool. Usually, you can press the
brass tubing into the recesses you've drilled and they will sort
of swage in place. Use some sort of vise or clamp to hold the assembly
while you solder it. I have had good results using my propane torch
to solder the tubes in place. I use standard electrical solder and
set the torch very low. If you have a large solder gun, it will
probably work just as well. Just apply the heat to the valve assembly
at the base of the brass tubes. Once you've got it hot enough, you
can "sweat" the solder around the tube just like sweating
copper plumbing. |
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| Make sure and flush the assemblies
out with some 409 or solvent. Install the springs and the needle's
and they're ready to go! |
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| The needle assembly can be mounted
to the airframe or just inserted into the fuel system. I use a tie-wrap
to wrap around the assembly and screw the tie-wrap to some rigid
part of the fuselage. You could also tether the valve to the fuselage
so that in the event of a mid-air, you won't loose the valve. |
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