As we understand it this is basically correct.
Balance the engine at the operational RPM 4000-5000RPM. At this speed the
carbs should be feeding off the needle jets (as opposed to the main jets
which come into play at full throttle).
This usually leaves the carbs slightly out of balance at idle but any rough
running can be corrected using the mixture screws. The problem is adjusting
these while the engine is running. I have manufactured a tool that enables
me to connect a small socket spanner to the adjusting hex and turn the screw
without having to disconnect the carbs or remove the lower cowling.
Carl
G-LABS
From: "Richard Holder" <rholder@avnet.co.uk>
Subject: Re: Europa-List: Carb balancing - Differential presure gauage ?
>
>> 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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