Honda civic carb query

I have an old Honda Civic Shuttle, still giving good service,
including up and down the forest track that accesses my property here.
Recently, it has been cutting out intermittently, (can’t find a
constant pattern). I keep engine, plugs, ignition all in good state,
and the compression on all cylinders is good and equal. I overhauled
the head a year ago. Recently, it is occaisonally not easy to start,
again no pattern, but gives the typical symptoms of being too rich.
Carburettor was of course blown out and is clear of blockages. I
suspect it is something to do with a small solenoid valve which is
located at theback of the carb, as it seems to duplicate the symptoms
if I take the power off this. Does anyone actually know what this
device is and what it does?
I can see from the circuit diagram it is switched on by the ignition
switch, but I can find no other mention of it in my manual. Any
suggestions?

 

The device is a cutoff solenoid, added to more recent carburetors to stop
run-on. It could be the problem, but I doubt it.

My top suspect is the fuel filter, if it hasn't been changed recently. That
bit me so many times when I had carbureted cars it isn't funny. Most of them
gave trouble in the way you describe - intermittent, no good reason,
cutouts. However, they all refused to restart for several minutes to half an
hour as the fuel line pressure bled off past the blockage and the debris in
the filter could unplaster itself from the screen.

It would be good to determine whether the problem is fuel or ignition. I
imagine you pump the accelerator instinctively when the engine starts to
die, so the float bowl will be empty if the problem is fuel starvation. When
it dies, you can remove the air cleaner and look down the carb throat to see
if the accelerator pump squirts gas when you open the throttle. In the other
direction, if you find the throat already smelling of gas, I've had trouble
with the needle valve on one carb intermittently hanging and flooding the
engine. I cleaned at it a couple times without success, then got serious and
changed the fuel filter, polished the valve needle and seat with a round
toothpick, and had no more trouble. Dunno what the actual problem was, but
as long as it worked I didn't care.

Mike