Carb Synch doesn't hold thru RPM range

by Robert Hirschfield in 912 / 914 Technical Questions 12 years ago

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Re: Carb Synch doesn't hold thru RPM range

by Ron Parr » 12 years ago

Ron Parr

5 11 0

Mark & Roger,

How much out of sync is too much for the equalization tube to handle?

Thanks,
Ron

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Re: Carb Synch doesn't hold thru RPM range

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.53k

Hi Ron,

I don't have an exact number for you, but I'll ask my friend at Rotax.

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

Thank you said by: Ron Parr

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Re: Carb Synch doesn't hold thru RPM range

by Ron Parr » 12 years ago

Ron Parr

5 11 0

Roger,

Thank you for the reply. I have a Titan Tornado II which at first I bought to keep me flying, while my son and I build a couple of airplanes together.

My Tornado II has a Rotax 912S and has a throttle cable system quite different from the split cable being described by other pilots on this forum. Both carbs actually are connected to the same throttle cable, which makes a loop through housing and attach points from the rear seat throttle quadrant (the front and rear throttle quadrants are connected via a push-pull tube). The cable is attached at the top of the throttle lever at the back seat and returns to an attach point near the bottom of the throttle lever. The throttle lever pivots between these to attach points.

I am getting into more detail then what I intended, but wanted to prepare the 'ground' a little by stating that both carbs react simultaneously to throttle inputs, because they are attached to the same cable.

BTW I installed new throttle cable, and this was one of the reasons for performing the syncronization.

Syncronizing the carbs with this arrangement is more interesting (challenging?). I may have some questions later, but wanted to comment on syncronizing at 3500 RPM vs. 2500 RPM. I have noted exactly what you pointed out about syncronizing the carbs at 2500 RPM and then losing the syncronization at higher RPM's.

I started syncronizing at 3500 RPM a couple of days ago, and stopped because it was getting dark, and also to see if anyone else had done this before I proceeded.

I left my vacuum gages hooked up, my intent now is to complete the process as soon as I can pull the airplane out of the hangar when it stops snowing and raining up here here in Northern Utah. At present the left carg is pulling about 1/4 inch more power than the right at 3500 RPM.

This has been a long explanation leading up to my original question about how much out of sync can the equalization tube handle. I'm interested because I'm not sure what it will look like at 2500 RPM when I finish syncing at 3500 RPM.

Anyway, if you don't mind asking your friend at Rotax I would greatly appreciate it, and thank you again for the reply.

Ron

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Re: Carb Synch doesn't hold thru RPM range

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.53k

Hi Ron,

.25" of vacuum is easy to adjust out unless you have run out of cable adjustment. Then you would need to either adjust the other side or shut down and change the throttle cable length by about 1/16". If you are only .25" out you might as well make it perfect and then you won't need to worry about how much the balance tube can correct. After you have done a few syncs its a piece of cake. I do at least a hundred a year and after a while you can just look at the gauge and know all kinds of info and exactly how you want to proceed.
Your time and frustration will get much shorter too.

As far as throttle cables some are thicker in the sheath and have a little friction which doesn't allow them to always go back in the exact place, even if they are attached to a single cable like yours. Some throttle systems also have a little play which causes the carbs to not return in the exact set point. I have grown to prefer small diameter cable that slides smoothly through the sheath and handles bends in the sheaths much better.

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

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Re: Carb Synch doesn't hold thru RPM range

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Roger

"As far as throttle cables some are thicker in the sheath and have a little friction which doesn't allow them to always go back in the exact place, even if they are attached to a single cable like yours. Some throttle systems also have a little play which causes the carbs to not return in the exact set point."

Now I understand why you have problems setting the carbs the way Rotax recommends. I would be talking to the aircraft manufactures about redesigning the throttle cable set up. If the cables are of poor quality and design you are fighting a loosing battle trying to balance them if they do not return to the same place when you close the throttle.

These are the underlying problems I mentioned in my previous post.

Mark

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Carb Synch doesn't hold thru RPM range

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