Considering the Rotax 915

BEN S

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In my experience if you use but do not abuse a good Rotax 2 stroke engine (buy new) on a regular basis they will operate exceedingly reliably.
Let the engine warm up before giving it steam.
Check all your rubber hoses and boots for dry rot or cracking,
Keep an eye on the simple stuff like plugs and cables.
set prop pitch to the correct EGT ranges
check bolts for loosening occasionally
Use the exact oil and fluids needed, don't cheap out and substitute.
start and use the engine preferably once a week
Stay away from the cold (always good advice) your 2 stroke will reward you with a very long trouble free lifestyle at a good power to weight to price ratio.
The TBO times are there as well as the maintenance schedule, but I agree with Fara on this one, if it ain't broke don't fix it.
 

loftus

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The 200 hour carb rebuild by Rotax is a big CYA and usually results in problems because the guy building it is just not as careful. I never rebuild my carbs till I reach close to 500 hours if everything is working. I leave it alone
In a sense one can take this approach to all recommendations. In my case there was corrosion which was missed, so yes the guy building it did not do his job, but the problem may well have happened anyway, so not blaming the problem on the rebuilder, just the fact that he missed it. There's no absolutes regarding preventative maintenance, TBO recommendations etc but I tend to follow them for a couple of reasons. First I don't pretend to know better than the manufacturers and I operate from an abundance of caution, and recording manufacturer recommended preventative maintenance in the log books can be an important part of the resale value. Same with using Rotax recommended and pricey parts etc. You know what they say about the cost of things that fly, float etc and probably better to rent.:)
 

Abid

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In a sense one can take this approach to all recommendations. In my case there was corrosion which was missed, so yes the guy building it did not do his job, but the problem may well have happened anyway, so not blaming the problem on the rebuilder, just the fact that he missed it. There's no absolutes regarding preventative maintenance, TBO recommendations etc but I tend to follow them for a couple of reasons. First I don't pretend to know better than the manufacturers and I operate from an abundance of caution, and recording manufacturer recommended preventative maintenance in the log books can be an important part of the resale value. Same with using Rotax recommended and pricey parts etc. You know what they say about the cost of things that fly, float etc and probably better to rent.:)

I generally agree with that sentiment. Carb rebuild at 200 though is one of those items that even many aircraft manufacturers don’t require in their SLSA schedule. Many allow 400 hour rebuilds that have to be done at a service center like Lockwood by sending the carbs in. Nothing in the carbs is rocket science but it has to be done thoroughly and in detail.
 

Vance

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I was recently surprised to see the corrosion and debris in a client’s low time 914 carburetors on a Cavalon.

After seeing that; I can better understand the reasoning for the 200 hour service.

I sank the floats on the Cavalon I was flying (Puff) at around 400 hours and things still looked pretty good on the inside.

I suspect the urgency of the service depends on the fuel available.
 

Abid

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I was recently surprised to see the corrosion and debris in a client’s low time 914 carburetors on a Cavalon.

After seeing that; I can better understand the reasoning for the 200 hour service.

I sank the floats on the Cavalon I was flying (Puff) at around 400 hours and things still looked pretty good on the inside.

I suspect the urgency of the service depends on the fuel available.

It depends on how often the engine is used and run. If it sits around, you can get corrosion fairly quickly
 

loftus

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It depends on how often the engine is used and run. If it sits around, you can get corrosion fairly quickly
Abid, any thoughts on preventative measures to take particularly here in Fl short of full winterizing protocols, but where the aircraft may stand more than a week or two from time to time? I'm an ACF-50 guy for the externals, what do you like as a preventative measure for the fuel system?
 

Abid

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Abid, any thoughts on preventative measures to take particularly here in Fl short of full winterizing protocols, but where the aircraft may stand more than a week or two from time to time? I'm an ACF-50 guy for the externals, what do you like as a preventative measure for the fuel system?
Stabil. Phil Lockwood uses it all the time. 1 to 2 weeks though should not too much. A month or so I would do something. For over a month I would use up mogas and use a little Avgas and make sure its in the carb bowls. Its much less prone to absorbing moisture. Ethanol as you know likes absorbing moisture so when it separates out standing around in carb bowl it creates issues
 

loftus

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Stabil. Phil Lockwood uses it all the time. 1 to 2 weeks though should not too much. A month or so I would do something. For over a month I would use up mogas and use a little Avgas and make sure its in the carb bowls. Its much less prone to absorbing moisture. Ethanol as you know likes absorbing moisture so when it separates out standing around in carb bowl it creates issues
Thanks. I stick with Swift Fuel as much as possible.
 

wolfy

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Can you explain how having exact egt readings enables you to have exact idle mixtures on a carburreted Rotax? What is your procedure to adjust idle mixtures according to EGT readings? Do you have EGT readings for each cylinder?
I always thought my 912's ran pretty smoothly, until I installed EFI. Other advantages of EFI are much easier starting even when it's cold and no adjusting throttles (just leave at idle) when starting, better fuel consumption, and a little more power, supposed to be about 5% more.
Personally I am looking forward to never having to balance carbs again, because even if one gets the balance perfect, they don't stay that way. I still think having 1 carb instead of two would have been simpler, eliminated the need to balance the carbs, and not sure what Rotax were thinking with 2 carbs.
And then of course not worrying about floats not floating, needle valve sticking etc. (This happened to me 2 weeks after the 200 hour carb rebuild by Rotax dealer.)
While I agree on most of the advantages of fuel injection, I just don't like electronic crap on engines. But that is only my opinion.
A 9 series rotax is essentially two engines sharing the same crank, so balanced carbs and balanced fuel mixtures (egt) are required for smooth running.
I have just 2 egt probes, one on each side's back cylinder, idle mixtures are adjusted by the idle air bleed screw to balance idle egt.
Needle, needle clip position and needle valve for mid range egt, and main jet for top end egt.

wolfy
 

Burrengyro

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I have a Montgomerie Bensen gyro with a Rotax 532 twin-carb engine. I have been advised to get a dual carb manifold and a single carb to reduce the twin-carb 532 engine's tendency to power surge at higher throttle settings. My regular gyro is my ELA07S with the Rotax 914 turbo. Would a single carb with a 4 pipe manifold be more useful in the Rotax 912 and 914 engines and eliminate the carb balancing job entirely?
 

loftus

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I have a Montgomerie Bensen gyro with a Rotax 532 twin-carb engine. I have been advised to get a dual carb manifold and a single carb to reduce the twin-carb 532 engine's tendency to power surge at higher throttle settings. My regular gyro is my ELA07S with the Rotax 914 turbo. Would a single carb with a 4 pipe manifold be more useful in the Rotax 912 and 914 engines and eliminate the carb balancing job entirely?
A number of people much smarter than me have speculated on exactly this point and question the logic of the Rotax two carb design which is probably a holdover from older motorcycle engine designs. Most horizontally opposed aircraft engines with carbs are designed with a single carb.
 
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helipaddy

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A number of people much smarter than me have speculated on exactly this point and question the logic of the Rotax two carb design which is probably a holdover from older motorcycle engine designs. Most horizontally opposed aircraft engines with carbs are designed this way.
I think the issue with the 532 is the exhaust porting which makes the engine a bit peaky with the power coming in all at once. The single carb reduces this by reducing the engine efficiency. The 582 has different exhaust porting which makes the power curve a bit more linear.
 

Burrengyro

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Hi Lotus and Helipaddy, I was just wondering if a single carb conversion for the 912 and 914 Rotax engines was realistic to eliminate carb balancing issues purely from a theoretical point of view. Commentators have said the 912iS engines are more efficient than the 912ULS engines and that the fuel injected version will be the only version available in the future. Happy Christmas, John H
 

Abid

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Hi Lotus and Helipaddy, I was just wondering if a single carb conversion for the 912 and 914 Rotax engines was realistic to eliminate carb balancing issues purely from a theoretical point of view. Commentators have said the 912iS engines are more efficient than the 912ULS engines and that the fuel injected version will be the only version available in the future. Happy Christmas, John H

Seriously I do not understand what is so difficult about carb balancing. It would take about 30 minutes once every 100 hours of flight.

One thing I learnt very very fast after getting into flying is if you are not somewhat technical and mechanically inclined, you better learn quickly. Otherwise you will
1) Be in danger because you won't know when something is wrong till it really goes wrong
2) Spend a lot of money on trivial maintenance tasks paying someone knowledgeable to do them
3) Can't find people to do things even if you have money because aircraft are not serviced by Jiffy Lubes around the corner. Even if you find mechanics you have to have enough knowledge to at least know what they are doing is right because a lot of them don't know what the heck they are doing themselves

It is nothing like owning a car experience at all no matter what's in your bank account.
 

Tyger

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Burrengyro

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Perfect answer, Tyger. Abid, 100% agree. I just ran out of beer. Happy Christmas. John H
 

MilesW

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Hi Lotus and Helipaddy, I was just wondering if a single carb conversion for the 912 and 914 Rotax engines was realistic to eliminate carb balancing issues purely from a theoretical point of view.

Definitely doable. Four cylinder engines have been fuelled with a single carb since forever. Inline, Vee and Flat. VW and Subaru (flat 4's) have both used a single carb. There have also been numerous motorbike conversions from dual and quad carb to single (with mixed success). As already pointed out above, a single carb is the norm on many horizontal aero engines, including the Jabiru.
Technically, it is probably neither inferior or superior to dual carbs if well executed. Very careful runner design to ensure fuel is distributed to all cylinders evenly is the goal. Harder than it looks, but doable.
A big factor in choosing dual carbs would have been the packaging. Every single carb setup I have ever seen takes up a lot of vertical real estate. Definitely not cowl friendly. Not such a problem on the back of most gyros though.
 

MilesW

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from the above;
"Power was the main consideration.
redundancy was also a consideration; the engine will continue to run on only one carb, with enough power to keep the typical aircraft in the air."

I wonder if the gentleman who made that claim has ever tried it?
 

Burrengyro

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By all accounts, the 915iS is a great engine for gyros. It must be great fun to be able to take off and climb out in seconds rather than minutes.

MilesW's point about hearing from anybody who experienced flying a Rotax 912 or 914 on just one carb would make interesting reading. This will not be a problem for the 912iS or 915iS engines. With the advent of the iS engines, will carbs soon be a technology of the past?

To Abid's point re maintenance: most cars and trucks cannot be maintained by your typical handyman anymore unless he/she can connect the vehicle with the latest computer software to tell what's wrong. Usually, the answer is to replace such and such a part rather than fix anything followed by a reboot of the system. Everything is moving towards greater complexity and deliberately short design life rather than durability and ease of maintenance. I am a big fan of stripping away as much "technology" as possible so that emergency maintenance is possible with the minimum of hand tools. The 912 Rotax will still run if your battery dies. The 914 will go silent. What about the 915iS and 912iS? For me, half the fun of gyro flying is maintaining a flyable machine. Simple and robust systems make maintenance easier and safer. (Remember the story of the ballpoint pen designed especially for space flight versus the pencil?)

Right now I am happy to have a 914 Rotax in my gyro. I am looking forward to reports on experiences with the 915iS engine from Jungleman Pete and others on the forum. Safe flying for now and 2022.
 

loftus

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Hi Lotus and Helipaddy, I was just wondering if a single carb conversion for the 912 and 914 Rotax engines was realistic to eliminate carb balancing issues purely from a theoretical point of view. Commentators have said the 912iS engines are more efficient than the 912ULS engines and that the fuel injected version will be the only version available in the future. Happy Christmas, John H
I have heard the same thing about Rotax phasing out carbs. It makes sense considering carbs have been pretty much phased out in cars. Having flown long distance
Seriously I do not understand what is so difficult about carb balancing. It would take about 30 minutes once every 100 hours of flight.

One thing I learnt very very fast after getting into flying is if you are not somewhat technical and mechanically inclined, you better learn quickly. Otherwise you will
1) Be in danger because you won't know when something is wrong till it really goes wrong
2) Spend a lot of money on trivial maintenance tasks paying someone knowledgeable to do them
3) Can't find people to do things even if you have money because aircraft are not serviced by Jiffy Lubes around the corner. Even if you find mechanics you have to have enough knowledge to at least know what they are doing is right because a lot of them don't know what the heck they are doing themselves

It is nothing like owning a car experience at all no matter what's in your bank account.
It's not that it's difficult just an unnecessary PITA in my IMHO. Even a single solid cross linkage between the two carbs would be a simple fix over the variability of two separate cables. The fact that you have to do do it every 100 hours and in my experience they always change during that time, means that the carbs are probably at least a little out of balance for a good part of that 100 hours. I've also seen slow-motion video comparing the smoothness of the carb Rotax engines with injected engines and the difference is quite dramatic. I'll ask if I can get a copy from the person and post when I can. Improved starting at all temps is another major advantage, significantly better fuel efficiency, approx 5% more measured power, better adjustment for altitude, no carb icing, maybe less vapor lock potential etc. Carb balancing is just one additional issue that is unique to Rotax. Few people would buy a car today with carbs, basically carbs are simply old technology.
 
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