Dual carburetor Rotax

[backcountry]

These carburetors get out of synchronization verified by measuring the manifold pressures.
Whenever I get a rough engine it?s the first thing I have to check and most of the times that?s what it is.
I was just looking at the Garmin G3X manual and it looks as though it can be configured for multi engine aircraft. With that in mind could it be possible to monitor both manifold pressures constantly. If that?s possible then I would know any time the carburetors are out of synchronization or possible a dirty carburetor.
My question is has anyone looked into this before?

[DHeal]

The two carburetors are connected downstream by a balance tube whose purpose is to roughly equalize the manifold pressure seen by the two carbs. On the 912 the system manifold pressure of the twin carburetors is sampled through this balance tube at a single point.

[RFSchaller]

Dave,

I thought the same thing, but then I thought if the imbalance was bad enough to cause rough running the balance tube capacity is being exceeded so if taps were properly placed it should show which induction pressure manifold is high and which is low.

Rich

[Piper J3]

Maybe another way to discern difference L-R would be to compare EGT's. I usually see good EGT correlation at low and high end and discrepancy in the mid-range. I think this is pretty typical...

[DaveWelch]

Originally Posted by RFSchaller

Dave,

I thought the same thing, but then I thought if the imbalance was bad enough to cause rough running the balance tube capacity is being exceeded so if taps were properly placed it should show which induction pressure manifold is high and which is low.

Rich

That might work if you clamp to off the crossover tube at one of the short rubber hoses that are one each end of the crossover tube. Can anyone think of a downside? It would eliminate the need for vacuum gauges or manometer, but at what cost?

[Piper J3]

Originally Posted by DaveWelch

Can anyone think of a downside?

The engine is designed to operate with an intake manifold crossover tube. The tube helps equalize manifold pressures L-R. I certainly wouldn't suggest circumventing this design feature.

[DaveWelch]

Originally Posted by Piper J3

The engine is designed to operate with an intake manifold crossover tube. The tube helps equalize manifold pressures L-R. I certainly wouldn't suggest circumventing this design feature.

You are correct.
I was thinking only of the Carb Balancing procedure. Not full time operation. Should have been clearer.

[pilotyoung]

Jim,

What kind of temperature differential do you see on the EGT's at cruise?

Thanks.

John

[Piper J3]

Originally Posted by pilotyoung

What kind of temperature differential do you see on the EGT's at cruise?

Cruise shows EGT's tracking very close, usually within 10-20F. Idle and off-idle is similar. Mid-range is something like 80F deferential.

[RFSchaller]

From what I remember from the Rotax course the imbalance issue is most critical at low RPM where the butterfly valves in the carburetors are nearly shut. Once you exceed about 3500 RPM the butterflies are near full open and the manifold pressures are less sensitive to minor differences in throttling on each side. That would seem to be born out by the differences in EGT you noted at low and high RPM.