rough Running 912 midrange rpm

by garfield langille in 912 / 914 Technical Questions 8 years ago

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Re: rough Running 912 midrange rpm

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

The balance tube is for rpms under 3500. Putting on a bigger one may not be as beneficial as you think. Once over 3500 rpm and running well above that at 5200 rpm it is worthless. Would one being 1" in diameter help? Good question.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: rough Running 912 midrange rpm

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

Here is what Flygas has to say.

The Intake Manifolds kit improves engine operating regularity Rotax 912 and 914 Turbo. The kit is complete with all accessories and relevant parts for assembly and installation does not require any modification.

FLYGAS produces effective solutions that improve the performance of your Rotax engines!

Why install the kit

The kit solves the problems of the Rotax 912 and 914 for power. The problems are the following:

1. The different opening of the butterfly valves which often is determined due to a different adjustment of the cables that determines
an imbalance between the cylinder banks generating annoying, worrying and especially highly damaging vibrations.
2. In the unfortunate event of a rupture of an accelerator control cable one you are forced to turn off the engine (for the same reason
described above) or it is forced to an operation only at full power.
3. Below 2500 rpm, during the heating phase, the engine has a degree of irregularity that forces at higher revs.

_____________________________________________________________________________________________________________________________________

My first comment on their reasons, Hogwash.

The different opening of a the butterfly is one reason we do a carb sync and if your carbs open unevenly then you failed to set them up correctly.

The second one on losing a carb cable and it goes wide open is set up that way on purpose. If a cable broke then if one carb goes open then you can advance the other to equal it and fly any place you want and into a pattern before you need to shut off the engine to land. If you have it to automatically close like some people have done then once one goes down to idle the other absolutely must follow of the engine will shake so bad you'll more than likely bend an engine mount and it isn't flyable to any airport to land. You will land peretty much in the terrain that is below you.

The last item about rough running below 2500 rpm while warming up is another mis-statement and even that somewhat depends on which 912 or even the 914 with different compression ratios. Running between 2100-2400 rpm at warm up and with properly synced carbs should not give you any more vibration than most any other time. Running too slow at warm can induce more banging on the gearbox due to the 30 degree spacing in the dog gears.
If you have your carbs on gauges and sync them and run them up to 4K-5K just to take a look and they are equal down at idle that pretty much shoots down their first two arguments. They should be pretty darn equal if you did a good job.

So as far as their 3 reasons I still have to say, Hogwash. (An American term)

I work on enough planes that have Italian part that I'm not impressed with any of their items so far.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: rough Running 912 midrange rpm

by Jerry B » 8 years ago

Jerry B

1 7 0

"The different opening of a the butterfly is one reason we do a carb sync and if your carbs open unevenly then you failed to set them up correctly".

I have found, with my engine installation, (Italian aircraft), the throttle cables are different lengths, and curves on the firewall side, getting the butterfly of both carbs to open simultaneous at low revs (where it is most important) to middle rev range is a compromise. I set them up to have the best sync between 2200 to 3000.
So at tickover 1600-2000 (idle jet) runs sweet. 2000 to 3000 is good. 3000 - 3800 bit of vibration, 3800 - full, all good. Rotax new there would be a problem with carb balance, hence the balance tube.
So question I ask, if the tube were bigger 1" or so (same as manuifold) so more of the mixture from both carbs could mix would this reduce the need for absolute butterfly alignment. Also if the balance pipe were bigger 1 carb could supply mixture to all 4 cylinders if one carb were to fail, (set to throttle close) but at less power. The carbs as fitted only supply mixture on alternate revolutions.

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Re: rough Running 912 midrange rpm

by Roger Lee » 8 years ago

Roger Lee

125 5.12k 4.53k

Hi Jerry,

"I have found, with my engine installation, (Italian aircraft), the throttle cables are different lengths, and curves on the firewall side, getting the butterfly of both carbs to open simultaneous at low revs (where it is most important) to middle rev range is a compromise. I set them up to have the best sync between 2200 to 3000."

Why not just fix the throttle problem? Not everyone has this issue so it should be very doable.

FYI:
The plugs fire on every stroke and not alternating strokes like some 4 stroke engines. This keeps the system simple, smaller and reliable. It is called a waste spark, but certainly makes sure all fuel gets burned. The carbs constantly feed fuel all the time. It just depends on which carb intake valve is open as to when that carb gets fuel/air.

One carb cost Horsepower and depending on the setup, uneven flows and temps. In the old days 2 strokes had one carb. To help increase HP and torque they went to two carbs. Then we (fliers and owners) were happy for a short time and then we wanted a lightweight 4 stroke so in late 1989 Rotax obliged.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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rough Running 912 midrange rpm

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