I will present my decompressor installation on my
Walkerjet. Hope somebody finds this useful.
This uses the "old style" Solo decompressor from Mojos, but I built a link
to automatically operate it every time the throttle cable is pulled, so it
doesn't have to be manually pushed every time.
The above picture (decomp1A) shows the modifications to the head. The
cylinder head must be tapped M14x1.25 (which is the same as the spark plug
thread). The existing boss in the head is only rough cast and must be
milled flat before drilling or the valve won't seal. Rather than tapping
directly into the soft aluminum head I installed a helicoil insert (while I
was at it I helicoiled the spark plug hole, too). I replaced the cylinder
head bolts with studs, 3" long for the front and back and 2" long for the
other four. The extended studs provide a place to mount the decompressor
mechanism. I would have used even longer studs and made the arm simpler,
but 3" was the longest I could fit in the deep socket for my torque wrench.
Note that all the nuts are all metal (not nylon insert due to the heat)
On the back stud I mounted a small aluminum plate to anchor the cable.
The lever arm is attached to the front stud. Both these parts are 1/8 x
5/8 aluminum strip, which is flexible enough to allow the lever arm to bend
when the cable is pulled. This eliminates the need for a spring to pull
the arm away from the decompressor valve when it's released. There's a
small tab riveted onto the arm since the valve isn't centered directly
between the studs.
The cable assembly is the leftover housing from an automotive choke
assembly, but I replaced the original solid wire with 3/64" braided cable.
Other similar items could be used. One end of the housing is attached to
the lever arm, so that end of the housing moves with the lever, the cable
itself being anchored to the cylinder head. Both ends of the cable are
terminated with nicopress sleeves and thimbles.
The second picture (decomp2) shows an overall view of the whole thing.
The third picture (decomp3A) shows the other end. The cable housing
terminates in a block attached to the frame. On the end of the cable is a
small marine pulley which the starter rope passes through. It's set tight
enough to pull the cord in a "V" away from a straight shot between the
starter housing on the engine and the fairlead on the frame. When the cord
is pulled, it pulls straight, pulling the cable, and opening the
decompressor valve. As soon as the engine starts, the tension is off the
starter cord, allowing the cable to pull back, and the engine compression
closes the valve.
I made a significant improvement on my decompressor
since the version shown on above. The two attached pictures
I eliminated the lever arm after the first one broke due to metal
fatigue. Instead, I installed a block on the back (actually the front of
the paramotor unit) of the cylinder head to terminate the cable
sleeve. There is a angled hole drilled through the block for the cable to
pass through, which is counterbored to accept the cable sleeve. The cable
then runs down, through a plastic block which is held onto the decompressor
stem with a setscrew, and back up to another block where it is
clamped. This not only eliminates the metal fatigue caused by vibration,
but allows the cable to be adjustable since it is only clamped at the free end.
I can't claim credit for the idea, since I saw it on an old ParaPlane first.