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.

   I've seen the Keihin 51 on ebay for $460 I think.

   -- Gordon.


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