Post by dobrygomPaul,
Thank you for the follow up.
I checked the fuel cap agains creating a vaccuum - I even left it
loose.
When hot/stalled I even took the cap off, but engine would not start.
The engine idles fine and runs fine (even in high RPMs) until that
special warm/hot point.
Do you know if there is a way to disconnect that "choke" for testing -
is it possible to bypass it?
I don't know the design of this one. But on most modern scoots, it will
be an automatic one that depends on an electrical current and possible
coolant temperature to change its "position". If it is the automatic
enrichener kind it will likely look like a plastic tube several inches
long that is screwed or bolted into the carb body on the engine side.
It mayl have several wires entering it, one for battery power, one for
ground and one perhaps from a coolant hose. Often they have wax inside
which expands to heat them up which causes a small sprung needle to
either extend or retract. Whether it retracts or retracts varies by
design, if it retracts after a few minutes of battery power, it is a
design where the needle opens a tiny air passage, if it expands it is
the kind where the needle closes a tiny fuel passage. You might be
able to remove it, press the needle with your finger to make sure the
spring is working and then hook it to a 12 volt battery for five minutes
or so to see if the needle moves.
Usually, at least from what I've seen, in spring-time the most common
starting or idling problems are due to gasoline having deteriorated and
turned to gum or varnish. There are usually two jets on small scoots,
plus the float needle intake and several more tiny, tiny circuits that
end along the inside of the venturi. Such carbs depend on a very fine
balance of atomized mixture through the tiny openings. Sometimes the
only way to fix such problems is to get those passages clean. Sometimes
the various carb cleaners can help clean a carb in-situ and sometimes
the carb has to be removed. If you remove it, you could test it first
by spraying WD40 into all the orifices on the intake side and watch to
see if the fluid comes out from every tiny hold in the venturi. If you
try this you may have to block a few openings with your finger. There
will likely be one big hole rearmost on the air or intake side, which is
the main jet hole. Not likely this one is plugged but still possible
that the jet with a tubed insert below it is dirty (usally it's called
the emulsifier tube is made of brass and can be removed with extreme
care by a small drift of the right size).
At the engine side, there will be one or two holes in front of the
butterfly valve, one is likely the enrichener circuit and the other is
the idle jet. They are in front because they need to work when the
throttle is closed. A fraction of an inch behind the butterfly will
likely be two or three more tiny holes, these are the transition ports
and the inner passages that feed them usually originate near the main
jet. Sometimes you can get them clean by shooting carb cleaner through
the various ports, but be careful, that stuff is extremely toxic, that's
why I test with WD40 which is not quite so dangerous (and flammable).
The most certain way to clean a carb is to dismantle it but that's best
done with a diagram so you can put it back together right. The idle jet
usually unscrews.
At the top of the carb will be a diaphragm that controls the piston that
moves up and down to control vacuum as engine speed and throttle
position change. If the carb is extremely dirty, the piston may be
sticking, which can be tested with your fingers if the carb is removed.
Post by dobrygomI didn' t know one could adjust the float level in the carburator (I
thought there were only screws for LO and HI RPM mixture adjustments).
Is there another screw on the outside of the cabruator for that?
Some carburetors have adjustable floats, to adjust those the float bowl
has to be removed (some, like the Keihins on older Hondas, aren't
adjustable because the float "tabs" are made of plastic, not bendable
brass or such). Sounds like that's not your problem if you haven't had
it apart. But if the fuel mixture screw (if you have one it will be on
the engine side of the carb') has been OPEN'ed too far (perhaps to
compensate because warm-up enrichener or choke isn't working or because
the idle jet is partially clogged), then once the engine warms up it
would be likely to stall at idle. If this is the case, you might find
that when running it wide-open it doesn't stall after ten minutes, only
stalling when the throttle is closed.
I've only seen a couple of dozen carbs and there are many more designs
than that, but from what I've seen, the HI and LOw adjustments are
usually found only on very small two-stroke engines. I believe the SUNL
is a four-stroke, so apart from the choke, there will either be an air
adjustment screw or a fuel adjustment screw plus some kind of knob or
screw to adjust idle speed (all this one does is vary the tension on the
throttle cable, at idle the tension should be such that the throttle
butterfly is closed, it has a spring that keeps it closed).
On the more modern carbs, which I'd guess the SUNL has, there is a fuel
mixture screw very close to the engine side, probably on the bottom of
the venturi casting. It may be capped with an aluminum or plastic plug
because of EPA rules, if it is capped, that is a likely clue that nobody
has adjusted it, so either the enrichener/choke is bum or the passages
are at least partially blocked and need some kind of cleaning. If it is
not capped, then before doing anything else, GENTLY turn in the tiny
brass screw until you can feel resistance, counting the exacct number of
turns to do so. Wriite that number down for later, it will likely be
somewhere between 1 3/4 turns and 2 1/4 turns. This is also important
if you dis-assemble.
It's still possible that gum or varnish at the float bowl inlet is
restricting fuel flow into the bowl and after a few minutes stopped,
enough fuel drips in to get the engine started again. Sometimes you can
get away with only removing the float bowl, shoot some solvent through
the holes and then blow it out. Be careful not to damage the delicate
needle that goes into the float inlet. If you hold the carb at the
right angle you'll see the needle and may even notice sludge or dirt on it.
If you do remove the carb', drain the float first, there will a screw on
the float bowl to do this as well as an opening which should have a
plastic hose that is not connected at the other end or perhaps enters
the air box at some point. Otherwise, you might spill the float gas on
yourself or some small critter or child in the vicinity.
If the bike has sat for a year or more without running, the inside of
the carb will smell very sweet and will have green film or deposits on
the inside.
Please note my advice isn't complete, I only know this stuff from having
taken a few courses and helping friends with their engines, so my
suggestions aren't guaranteed to work!
Post by dobrygomThank you,
Martin