On Fri, 15 Jun 2012 01:02:31 -0700 (PDT), Richard Savage
wrote:

[snip - my original reply]

It's a 1970s or more likely early 80s Saxon pedestrian flail mower
with a Robin EY40B pull start engine fitted with the separate coil and
KTR unit. Both the switches are push to make, the clutch switch makes
on clutch engagement, the gearbox switch makes on reverse gear
selection only. As far as I can work out, they were originally wired
in series as that's what the cable remnants seem to determine.

Pullstart complicates things, because that means you don't have 12V to
drive a relay. Also means there isn't a starter relay (or start
inhibit relay). Since the switches are closed when the selectors are
in the "shouldn't start" position, it's probable they're intended to
provide an alternate ground path for the igntion (kill). If simply
wired to the ignition kill, as your friend found, it'll still kill it
when the engine is running and you go to engage the implements as
you'd expect to be able to in order to utilize the tool.

Does the engine have an alternator winding? Might be a little
bullet-like connector on the side of the flywheel assembly that may
have a fuse in it (I'm not familiar with the Robin engine, though I
could go look it up in a reference book here) - that'd provide 12V
when the engine is running, and could be used to actuate a relay.

Here's a basic primer on relays:
http://www.the12volt.com/relays/relays.asp

Check out Figure 1 - that's the terminal diagram for a very common
SPDT relay type used for automotive applications (you could pop the
bonnet on amost any car at a wreckers and pull several of these out).
An SPST relay would serve as well, since you don't need to switch a
different signal in - just disable the interlock once started.

You'd wire 85 to chassis ground, 86 to the alternator output from the
engine (all of this assumes it is present), 30 to the ignition kill
line, 87a to the control switches (in parallel, so either one provides
a ground path), and 87 needs no connection.

Now, when the engine isn't spinning (or not not spinning fast enough
to really generate 12V, such as when you very first pullstart it), you
have no 12V, so the solenoid isn't energized, and 87a (normally
closed) has continuity through to 30 (common). As a result, if either
lever is set to a no-start, the ignition system is grounded, just as
if the kill were active. Take them out of their no-start positions,
and there's no ground, and the igntion can fire. Once you've started
the engine, the alternator produces 12V, and the solenoid engages,
switching from 87a to terminal 87 (which has nothing wired to it) -
now, the position of the two levers won't kill the engine, though the
existing kill switch still will (which isn't wired through the relay).
Kill the engine with the kill switch (or because it runs out of
petrol, or whatever), and the 12V goes away, solenoid disengages, and
terminal 87a is once again selected, and the levers will inhibit
starting.

Absent a relay configuration as described (and a necessary alternator
tab), it makes me wonder if perhaps the switches are in place for an
electric start model/option (though electric start would need some way
of charging the battery, which would be the alternator tab). On a
small engine, the alternator can be wired inside the flywheel - it
doesn't need to be a separate belt-driven apparatus like you see on
cars. The original engine on my tractor had a generator - when the
engine was spinning it, it generated voltage, and when you applied
voltage to it, it spun to start the engine.

You're really capable of getting the thing pullstarted when the
implement is engaged? I wouldn't dream of expecting to get my tractor
pullstarted with it either in gear or the PTO tiller engaged.