***** Hi Gordon,Â
 WOT is usually takeoff and climb, where we want "Full Rich"Â
Â
Thinking some more about mixture control for the Mikuni; I think IÂ
prefer the "in- carb" mode; the carb leans itself and the failure
mode is back to "normal" ie , no lean.Â
There is something natural about balancing venturi pressure against
fuel pressure within the carb that appeals to me. And all for 160Â
bucks!Â
Â
Bill Jepson,Â
we have spoken at length on carbs, and "things" these past few years;Â
i would appreciate your valued comments.Â
BR, Dave McCÂ
WOT is not necessarily where you want full rich.Â
Usually people run WOT at 7500 feet and above where power is
down to 80% or less. You need to be able to lean it there.Â
Â
In fact you should be able to lean it an any altitude so IMHO
a leaning feature is mandatory. However if fuel burn and MPG
are not issues with one then one could go with the stock carb.Â
Â
-- Paul Lamar ...No rotor no motor.Â
Dave and Group,
the carb concern is that we want full rich at WOT at takeoff, but
we need to lean significantly above about 3000' and certainly above
4000'. The advantage of a vacuum slide carb, (or constant velocity
carb)
is that the lift of the jet needle is caused by the airflow. In less
dense air
the carb is "self-leaning" to a certain extent. That is why they use
them
on all the ultralite 2 cycles even though the straight slide throttle
usually
produces max HP at sea level. The "flat slide" carb is a design that
really
mimics a slide throttle. The sides of the venturi are generally faired
into
the sided of the venturis bore. There is still a reduced diameter
venturi,
but the lead-in and out are carefully contoured to allow max airflow
for a
given diameter. Great for power, but crappy for high altitude unless you
stay a size down to keep up the vacuum signal so they will still
pull fuel
from the float bowls. The vacuum-slide carb is a natural for icing
BTW. The
openings are small and they (VSC's) all have a butterfly downstream
of the
slide that is the actual throttle control. The vacuum slide acts solely
due to
vacuum and isn't physically connected to the throttle butterfly.
Paul Dave et al, Mikuni used to have available remote jet kits that
replaced
the center throttle cap and replaced the main jet. If you want to
run carbs
this would be "the way to do it" as the system could be replaced with a
needle
valve that would allow leaning. The runs of tubing can't be long, less
than 3" if
memory serves, but that would be enough to allow replacement with a
variable-
orifice system.
Bill Jepson
Bill,
Thanks for chipping in. I don't think my carb experience comes close
to yours -- and I have zero experience with the flat-slide carbs.
I can say that the vacuum-slide carbs (constant depression, constant
vacuum or constant velocity) work very very well on the Rotax
4-strokes and these would be my choice for the rotary as well.
Keihin makes a 51 mm as an aftermarket part for the Harley and I
would take this carb over the Mikuni flat-slide.
All the Rotax 4-strokes that I've flown have had the high altitude
compensation system, which works extremely well. The manual system
is even better in my opinion because it lets you dial in the exact
mixture based on your EGT gauge.
All the Rotax 2-strokes have the round-slide carbs, which are not
vacuum slide (the slide is operated by the throttle cable). These
can also be fitted with either the automatic or manual leaning
system. These would be a good budget choice for rotary aircraft
engine (Mikuni VM 44).
Btw, what is that remote jet kit? Is there a diagram showing how it
works? I don't quite understand how it works.
Regards,
Gordon.
Altitude compensation is nice but it is not enough. We need to lean at
any altitude for best MPG.
What are the 51 MM Keihin going for price wise? I stopped in at a local Harley
dealer today and their price for a Mikuni 48 mm was $450 each.
Paul Lamar ...No rotor no motor.
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