Nigel,
I couldn=92t agree more, but I don=92t think I would have been as kind.
Here in the States we have many =93Joe Bag of Doughnuts=94 modifications be
ing made and I am glad to see there are others in Europe seeing the same st
uff I get to see.
In a nutshell, a crossover tube on twin carbs is interesting. You Jaguar/T
R owners from the 60s had many =93experts=94 commenting on the dual carb se
tups.
Personally, =93if it ain=92t broke, don=92t fix=94 it applies here to the R
otax. Learning to balance the engine=92s carbs to a fine tune is more impo
rtant in my opinion than throwing hardware at it that is dubious. Worn car
bs do go out of sink. Gauges are not matched sets (always calibrate a gaug
e) and frankly, I learned to balance carbs (once initially set by mechanica
l/vacuum) by ear/vibration and sound. I had to put new carbs on 12AY becau
se one of them had a piston that just wouldn=92t work smoothly. I built a
vacuum setup for my Shop Vac out of PVC/Plastic and built an intake with ma
nifold with Rotax rubber to carb supports/seals for both carbs to test side
by side. I operated the dry carb throttles until I could see exactly what
was happening. The pistons would not operate in sync at any setting but i
dle and full. This gave a mid range vibration or shaking problem and barel
y made a 300 RPM mag check drop and only after the piston on the right carb
would finally move to a near proper position due to the vibration.
My new carbs now can be balance by ear/vibration observation of the ignitio
n boxes. The mag check coming back from Sun =91n Fun was 100 instead of th
e normal 50 drop from the annual in January. The 912S mag drop will be sli
ghtly higher than a 914 because of the plug firing difference of the types
---From being top/bottom L and R vs L and R top then bottom.
The large tube, in my opinion, is the American technique of just make it bi
gger (high rise manifold with three duce=92s) type mentality. Not really e
fficient normally, just easier. So much air could flow through the carbs i
t just muddled around the intake. It may allow some extra unmetered fuel a
ir to cross over (as the fuel mixing is done low in the manifolds and the t
ube is on top) and give you some leaning potential. How much, who knows.
They don=92t validate their claims. With no validation, I don=92t throw my
money or time at it.
We have the HacMan over here for leaning. It pulls intake manifold air and
applies it to the float bowl to reduce the fuel air mixture by decreasing
the bowl fuel pressure vs the venturi pressure ratio. It can work, is very
sensitive, and a sharp eye on the EGT (properly calibrated) and patience i
s needed to lean the carbs in flight. It does work, kinda, sorta to lean t
he engine and allow more efficiency for cruising with the 912 at 7500 to 10
,000 MSL. But it not a simple leaning knob/lever as on a late model Cessna
/Piper you could pull and lean by RPM/EGT/Kill the engine method.
Best Regards,
Bud Yerly
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Window
s 10
________________________________
From: owner-europa-list-server@matronics.com <owner-europa-list-server@matr
onics.com> on behalf of Nigel Graham <nigel_graham@m-tecque.co.uk>
Sent: Monday, April 16, 2018 6:21:41 AM
Subject: Europa-List: Re: Question re carb balance tube
.uk>
Jonathan,
There are some simple clues to judging whether a forum poster knows anythin
g about engines or not. If you spot the keyword =9CRPM=99s (wit
h an =9CS=9D) you know straight away they do
n=99t have a clue! You see it all the time on boy-racer forums for wh
om =9Cstage one=9D tuning involves installing a Mega Base, putt
ing their baseball caps on back-to-front and describing everyone as a
=9Cmud flicker=9D ( .I think that=99s what they
=99re saying).
The links you posted are classic examples.
To understand what the balance pipe does, you need to know how the engine w
orks. A petrol engine is an air pump =93 at anything less that wide o
pen throttle (WOT), it is working at below atmospheric pressure. The RPM (n
o =9Cs=9D) and hence power, is regulated by the throttle butter
fly which acts to close off the inlet tract (that=99s why it=99
s called a =9Cthrottle=9D) for idle, and successively open for
full power (atmospheric pressure). When you fly in the cruise, you typicall
y use about 75% power =93 meaning the throttle butterfly is partially
closed and the manifold pressure is below atmospheric.
The Rotax engine is a =9CBoxer=9D configuration meaning two cyl
inders are on the left and two on the right and the designers decided that
the simplest way of feeding fuel mixture to them was to use two separate in
duction manifolds and carburettors.
Whilst the =9Cpackaging=9D issues were simplified, this introdu
ced the problem of ensuring that both systems were perfectly synchronised s
o that all cylinders produce equal power. Any difference and the engine wil
l vibrate.
When you balance the carburettors, you measure the (partial) mean pressure
in each manifold and adjust them to be the same. If the mean pressures are
the same than there will be NO FLOW through the balance pipe, so there is n
o need for a 1.5=9D drain pipe between the manifolds.
So why do they fit a balance pipe? Carburettors have been developed over th
e years to feed the right amount of fuel throughout the changing rev range
and changing atmospheric pressure =93 but small differences in airflo
w mean that the two manifolds are almost never exactly matched throughout t
he flight profile. The balance pipe allows the small pressure differential
to equalise, and smooth out any vibration that would otherwise result.
The contributor of the second of your referenced forum postings, Billy-Bob
P Sledgehammer III, is frantically calculating how much extra flow he could
get though his balance pipe, not realising that he should be aiming to min
imise the airflow through it!
My advice would be to leave it exactly as Rotax supplied it =93 the p
ower unit is surprisingly powerful for its size and very reliable.
Read this topic online here:
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