Re: Keihin carburetor tuning problems

 

Hey, thanks.  Yes, in that blow up, the mixture screw is labeled part
number '8'.  I should have mentioned above that even when the mixture
screws are turned in nearly all the way (when fully seated the engine
just putters down and dies) and I ride it for a bit, I still get
carbon fouling!  It's quite boggling, really.  Also, my choke is not
on.  That's the first thing to check, of course.

A factory spec-sheet that I found said 3/4 of a turn, which I've
tried.  I've also tried, a full turn, 1 1/2 turns, etc.  It shouldn't
have to be quite so accurate, anyhow.  I don't even have different
grades of carbon fouling, no matter how I adjust the carbs, if I ride
it for about two miles and pull the plugs, they are coated in carbon.

 



Clockwise.


Idle mixture screws stop having any effect on the idle mixture
delivered to the idle mixture port after about three turns
counterclockwise.

A site called oldmanhonda.com used to have a list of jet sizes and
starting points for setting the idle mixture screws and air screws of
all the first and second generation Hondas, but I can't find it.

I recommend that you screw your idle mixture screws all the way in
until they just gently bottom out, then turn the idle mixture screws
counterclockwise one full turn and see how that works.

If you turn each idle mixture screw clockwise 1/4 of a turn and give
the engine a chance to burn off any rich mixture and then the idle RPM
increases a little bit, turn the idle SPEED screw a little bit
counterclockwise so the engine slows down to the specified RPM in the
manual.

Then try turning the idle mixture screws clockwise another quarter
turn and let the engine use up any exces mixture and see where the
idle RPM goes.

Some manuals recommend that you use a precision tachometer to
determine when the idle RPM drops off 50 RPM as you reach the critical
adjustment on the idle mixture screw.

When it drops off 50 RPM, you tweak the idle mixture screw
counterclockwise
again to get that "perfect" idle mixture (for the particular weather
on that particular day).

 

Nichts wrote:

I see some of the big guns haven't answered yet, so I'm curious whether
the "fouling" is the only apparent "problem"?  Does it start easily in a
second or two when cold?  Does it respond well at different amounts of
throttle?  Does it smoke?  When starting from cold, when you put your
hand on the headers for the first minute or so, do the cylinders seem to
warm up equally?  Are the plugs new and was the bike not ridden very
fast in recent history (if the bike has a history of mostly being ridden
slowly, it may take a long time for the plugs to burn off some of that
carbon).  Sounds like  the idle jets are clear, could the float levels
be way out of whack or the piston needle be set very high up?  Not
asking for answers, maybe one of those questions might trigger an idea.

 



When you ride a motorcycle with CV carburetors, you *are* riding
around on the idle mixture most of the time. That's why it's so
important to get those idle mixture screws set right.

 



Needle height really isn't all that important, but amateur tuners will
often stack a few 0.5mm washers under the head of a non-adjustable
needle.

They think they are really smart when they do this, but what they are
doing is just causing the needle to allow gasoline past the needle jet
slightly sooner, they aren't affecting the peak power of the engine,
they are affecting the midrange, and it's possible that the engine
isn't producing enough vacuum at lower RPM to lift the slide enough to
take advantage of extra needle height anyway.

www.factorypro.com has some *really* good tips about CV carburetor
tuning and troubleshooting that the manufacturers of competing jet
kits don't want you to know.

For instance, there are pictures of needles that are badly worn and
needle jets that
are *subtly* worn in an egg shape so that gasoline can get past them
at small throttle settings, like less than 1/4 of the slide lift