do carbs ever need to be tuned?

   Just wondering?  I replaced a sparkplug on my Honda Reflex with a
new NGK Iridium plug.  The mechanics checked it before sale but to my
eyes the old one looked slightly corroded, sort of reddish colored.  I
noticed the idle speed went up 200 RPM, so I had to adjust the screw
downward.  I know on alot of motorcycles/scooters you cannot easily
adjust the lean/rich mixture screw.  But if you take it into a
mechanic, is it possible to get a carb "tuned", or do they just set
them at the factory and forget about it for thousands of miles?

  If it matters, the scooter does have a catalytic exhaust.  I'm not
sure how that affects the air/fuel mixture.

 
Most m/c's have an idle mixture screw. Not only can you "tune" carbs
with resized jets or resetting idle speed, but you can tune fuel
injection bikes, too, and they don't have carbs. Mostly, though, when
I think of tuning the carbs, I think more of rebuilds or cleaning due
to lack of use, gas gumming up the carbs.

As for your scooter or any other bike, stuff gets set to factory specs
at the factory, but time, mileage, and wear can work together to force
you to set back to specs.

A factory manual will tell you what the idle speed should be.

Greg

 Magnulus wrote:

Iridium plugs run at really high temperature, your old one was probably
oxidized.


That will happen when you put a new spark plug into an engine, the voltage
required to fire the plug decreases, so the timing advances a bit.


Carburetors will gum up if you store the motorcycle and don't ride it for
months at a time.

Mechanics at the dealership will tell you that they will get fined for
readjusted the idle mixture screw, but it's right out there if you want to do
it yourself.

The correct setting is probably around 1/4 of a turn open, since your
carburetor has a rather large idle jet. If you turn the idle mixture screw
out too far, the mixture will become too rich and the idle RPM will slow down,
so you would think that the solution is to turn the idle speed screw up to
make the engine idle faster.

Then it won't idle down after you blip the throttle. The key to tuning idle
mixtures is understanding that
when you turn the idle speed screw up too far, it uncovers a pattern of three
transition ports that are there to give the motor extra fuel for accelerating
away from a stop.

So, if you mess with your idle mixture screw, don't open it up much further
than about 1/4 of a turn.

http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id"55375

Item #5 adjusts the idle mixture

Item #6 adjusts the idle speed

Item #42 is the main jet, which can be changed

Item #43 is the idle jet, which can be changed

The catalytic converter is there to burn any excess fuel that didn't get
burned in the engine.

--
Message posted via MotorcycleKB.com
http://www.motorcyclekb.com/Uwe/Forums.aspx/bike/200706/1

 wrote:

   The old one wasn't iridium, though.  It may have been a colder
plug, but it had the standard shape of the regular NGK electrodes (not
tapered and thin like an iridium plug).  I found a cold plug under the
seat.  Probably a colder running plug for extended high speed
running?   I just bought the standard heat range iridium.  I can't
guess yet what kind of riding I'd eventually do, though I'm pretty
sure a warmer plug would be useless/harmful as it is never all that
cold in Florida (the lowest temperature I have ridden in is 38 degrees
for 1 day).

  I've had pretty good results with an iridium plug in the past and
they are only 6 dollars so I figure why not.  Next one to get a plug
replacement will be the Vino 125, just to see if it cures the startup
problems I've been having (which are actually not "problems" so much
as the way it was designed- it takes 10-12 cranks to start).


  It was my understanding you needed vacuum gauges and gas sensors to
do it properly?

   I'm not interested in increasing the power just so much as making
sure it isn't running leaner or richer than was intended.  Oh, and on
the Honda machines it does indeed have an anti-tamper cover over the
mixture screw(s).


  Does this catalytic converter work assuming a lean or rich mixture,
though?  My guess is it is setup to be slightly rich?  What are the
downsides to having a leaner running engine?  Theoretically it should
give better fuel economy, right, but I'm guessing it will also lead to
hotter plugs and engine parts?

  The rich/lean screw thing is definitely something I'd have to
research more before tampering with it.  But it seems to me as there
are no O2 sensors or things like that, that it is possible a carb
could be running in a configuration it is not suited for?

   You could approach it from the flip side and just figure out which
fuel it runs best on, and vary the grade according ot the engine,
rather than trying to vary the engine according to the fuel.  Probably
easier.   I did get much better mpg one time I used 89 octane instead
of 87 on an older scooter, but this was a sample size of 2-3 fillups.
Not enough for me to really say "higher octane is better".

 


Not exactly.  It's there to convert noxious byproducts of combustion to
compounds that are less noxious.


Nope.  Quite the opposite.  They work over a rather wide range of
lean/rich......up to a point.
If the mixture is TOO rich, the leftover excess fuel will burn IN the
converter and ruin it rather quickly......and if even richer yet, you have
smoke and fire.

Overall mixtures are usually set on the lean side to give extra margin
against the above AND because some noxious byproducts are lessened with a
lean mixture.