For those who aren't with a lot of experience with motorcycle carburators I'll list some info
that may help understand them better. Several forms are used. Some have the slide and float
with pilot jet & main jet like many of the HT's. Most motorcycles use these in the various forms
where small motors may rely on diaphram type carburators.
The classic motorcycle carburator uses a slide with a notch facing the intake opening. These
slides have numbers like jets and they contain the jet needle which has rings cut in it to adjust
it. The needle has a number just like a jet and so does the jet it rides in called the needle jet.
Then in the float bowl there is the pilot jet and main jet each with their own number and size.
At idle the air is taken into the carburator at the bottom vents and mixed with fuel via a pilot jet
and an air screw to help regulate the idle. When the throttle is opened the slide lifts and then
air begins to flow thru the venturi where the fuel is sucked up thru the main jet/needle jet, and jet
needle contained in the slide. As the slide moves further upward the venturi changes air flow
characteristics and more fuel is metered in from the narrower tip of the needle. At this volume
the idle circut will technically cease and the carburator is said to be "on the main jet".
See the illustration:
In the illustration below:
At different throttle positions, idle, 1/4, 1/2, 3/4, and WOT (wide open throttle) the graph
below shows which part of the carburator can be best adjusted to assure optimal performance.
At the right hand verticle of the graph is the word: "Effectiveness" indicating how and where
the most effective efforts can be made.
Again this applies to a motorcycle carburator with slide, jet needle & needle jet regulated fuel
supply, float/float bowl containing the main and pilot jets.
The Mikuni, Keihin, and Bing carburators are common on Japanese motorcycles but there are many other makers of similar carburators. Some of better design and with higher quality features.
Mukini carburators are popular and you can check out this tutorial if you are interested in fitting
on to your HT.
Mikuni carburetor operation and tuning
Some motorcycle carburators with slides use a large diaphram attached to them in what is known
as the CV system (constant velocity) where the slide & needle were attached to a large diaphram
which was covered by a dash pot and operated by vacuum. This design was decided to be better
than earlier "butterfly valve" carburators and more sophisticated.
For diaphram carburators such as Delloroto's, CNS, and Walbro's, the system they use works
somewhat differently. Power lawn mowing equipment has moved to diaphram carburation
years ago as it is simpler to produce although not quite as high of quality as earlier float bowl
carburators in earlier times.
In the diaphram carburator the diaphrams can be used to draw fuel from the tank and supply it
to the venturi where a "butterfly valve" will regulate airflow and other diaphram systems can
be used as valves to sense vacuum pulses from the engine to operate the fuel pump diaphram.
The fuel pump diaphram may also cycle the metering of the fuel supply into the venturi via a
regulating needle it works against with it's inward and outward movement.
Delloroto's downloadable ebook here:
Dellorto Motorcycle Carburetor Tuning Guide | Free eBook Download
Simple diaphram carburators such as those by Zama's or Walbro's may be seen on small motors
such as yard and lawn equipment. The HT motors may be refitted with different forms of these
and the link below will explain their inner workings with a well laid out diagram. It's desinged to
help with overhaul and theori of operations.
The link below contains the first animated illustration on this post and is a good further reading site:
There are many solutions to a problem and rarely is there only "ONE" right way. So it goes with
motorcycle carburation. I'm still with my inclinations towards a good classic & proven motorcycle
carburator design which features a float & bowl, slide, adjustments, and cable control that will
deliver a consistant air fuel mix ratio in all conditions..."where I live". Anymore than that....I'd be
specifying "closed loop" electronic fuel injection.
I hope for those who feel they know so little about motorcycle carburators that this will help. In some of the post members have shown "tweeks" they have made on their stock carb's to notch
the slide above a small hole drilled in the float bowl so that the float bowl will be vented to help it
run better. These little tricks can save an owner lots of money which could be spend on something
which could give better performance. (such as an expansion chamber exhaust system)