X-Mozilla-Keys:
Ken Powell wrote:
Finn,
I will make an attempt to explain the various fuel circuits in a slide
carb, but I really think you would be better off to use Tracy's EFI.
It takes a lot of practice and patience to jet a new set of carbs
(ones that you do not have base line jetting specs for). I have been
jetting these types of carbs for about 25 years and it is always a
frustrating experience.
With that being said here goes: (note that the following will be
based on the amount of slide opening)
1. main jet - 1/2 to wide open
This jet screws into the needle jet and is the first jet you see after
removing the float bowl.
2. needle jet - 1/8 to 3/4 opening
This circuit contains 2 parts: the needle and the needle jet. The
needle is the long thin rod sticking out the bottom of the slide and
it slide in the needle jet. The needle jet is removed by removing the
float bowl, unscrewing the main jet (the main jet is screwed into the
bottom of the needle jet, which has been pushed into the carb body
from the venturi side of the carb), then pushing the needle jet up
thru and into the venturi from the float bowl side of the carb body.
Both can be changed; note that the taper on the needle allows
variable fuel flow into the venturi at different throttle settings. A
larger or smaller needle jet will affect the needle jet in a linear
manner. Note that the needle jet slides up / down in the needle jet.
Some of you may call the needle jet a 'spray bar'; I never have
figured out why. This is the really hard part of the jetting because
there are so many variables. You will 'cruise' on the needle jet, so
it is really important.
3. Idle jet - from closed throttle to 1/8
Affects starting and idling - since the float bowl is off, you can see
a jet screwed into the center and forward part of the carb body.
Also, if you look into the carb throat from the front of the carb, you
will see a hole in the bottom of the throat before the needle and
needle jet - the idle jet is dirrectly below this hole.
4. Slide cutaway -
affects starting and idle. The throttle slide has a cutaway on the
ottom of the back side. You usually replace the entire slide, but you
can grind the opening larger. This very seldom needs to be changed.
Note that all circuits affect the others except wide open throttle
(and even this is a little affected by the idle jet). But what will
drive you nuts is the needle jet circuit. Only attempt this if you
have lots of patience. An EGT will help, but be sure to take lots of
plug readings to quantify the EGT reading. Yes, I admit that I trust
plug readings more than EGT readings for this particular job. You can
bet that I will be taking plug readings even with Tracy's EFI to
verify what is happening in the engine. Maybe not neccessary with the
rotary, but at least I will feel better.
I know that a lot of the guys on the newlsletter can also help since
many have raced motorcyles and snowmobiles.
One last suggestion. Order a catalog from California Power Systems.
They are mostly an Ultralight company, but the catalog gives a great
explanation of how slide carbs work (with pictures). call them at
510-357-2403. I hope that this is still a valid number. They know
what they are doing. There pictures will be of a Bing carb, But a
Mikuni works the same way.
If you would like to talk to me, call me at 501-220-8622 (office) or
501-847-4721. My email is Ken.Powell@Alltel.com.
Ken Powell
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