> Quirks of the engine seem to preclude being able to balance the carbs
> across the rev. range; another compromise needed. However, if tickover speed
> is to be low enough to land into short strips, the balance needs to be good
> at idle speed.
Duncan, I would disagree with this.
An engine runs at cruise power almost all the time. So the speed at which
you want the best balance is cruise speed - 5000 rpm.
With a car it is possible to balance at tick-over (idle to US folks) and at
a specific rev count, as there is usually a flexible bit in the linkages.
On the Rotax the idle is defined exactly by the position of the throttle
cables (as they are tight at idle) and so a balance at 5000 defines the
balance everywhere else.
> I've got no further with finding out what it is that causes the variation of
> balance; it may be tolerances in the pressed steel throttle arm and the way
> it is tightened on to the throttle spindle; I'll explain further if you
> haven't nodded off at this point.
I think the variation from day to day is caused by the movement of the
throttle cable within the outer cable as it goes round the circuituitous
route required. Somehow a shorter and straighter route is required. Or a
single cable connected to a linkage across the engine. Unfortunately the
carbs are offset and so a cross linkage would be difficult to do.
It is a shame that Rotax didn't put on a single bigger carb with a central
location like "Lycoming". No carb balance problems then !
On the other matter discussed this morning - carb ice :
I have occasionally (normally after 40 minutes or more) had small hiccups in
the running of my 912S. It also happens when reducing power for the descent.
My (totally unproven) theory is that this is small amounts of ice forming in
the carb, then it breaks off and the engine coughs, spits out steam and then
continues. Although it is not nice I have ignored it to date. It has however
been cold recently so I fitted a temp probe to the outside of the carb body
(blutak !). It went below freezing at one point and so I decided to act. The
SkyDrive kit is now on - I am waiting for the PFA sign-off inspection
(hopefully) tomorrow.
This seems to be an XS firewall forward thing. (More totally unproven
arguments follow). With the classic fwf the radiators are at the front of
the cowling. This probably generates enough hot air to keep the carbs warm.
In the XS fwf a good chunk of this heat just passes along the cooling duct
underneath the engine, leaving the carbs colder.
Thus in winter the Classic is a better arrangement in respect of carb icing,
and in summer the XS provides better cooling !
So I will be trying my carb heater kit soon, and I hope the annoying slight
misfire will be eradicated. If not it will have been another 200 thrown
away on this plane :-(
Richard
Richard F.W. Holder 01279 842804 (POTS)
Bell House, Bell Lane, 01279 842942 (fax)
Widford, Ware, Herts, 07860 367423 (mobile)
SG12 8SH email : richard.holder@avnet.co.uk
++++++++ Please note new email address ++++++++
Europa Classic Tri-gear : G-OWWW, High Cross
PA-28-181 : Piper Archer : G-JANA, EGSG (Stapleford)
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