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Power Loss Van's RV12 Rotax 912ULS

n233va

Member
POWER LOSS VAN?S RV12 ROTAX 912ULS

Today 3/14/2017 a friend flew a RV12 to Williams Arizona, KCMR. Airport elevation 6691 ft. outside temperature was low 70?s. Flight in was normal. When he went to leave, just after becoming airborne the engine stumbled and lost significant power. An aborted landing was successful. An A&P from Prescott, AZ was contacted. The A&P had built this RV12 as well as a second one he flies. The A&P flew the second RV12 to Williams. He checked the carburetor bowls for debris on the first RV12 as well as doing a full run-up. All seemed normal and no debris was found. A second attempt to take off was made by the first RV12. Again the engine stumbled and failed to produce full power. The decision was to leave the first RV12 in Williams and fly the second one back to Prescott, planning on returning in the morning. The second RV12 did a run-up, which was normal, and started the take off roll. The second RV12 experienced a similar engine stumble and extreme roughness. The second RV12 could not develop take off power. Both aircraft are Rotax powered with 912ULS engines. Both are identical in the fuel system and carburation. Both also experienced fluctuating fuel pressure during the attempted takeoffs.

Both aircraft were burning 91-octane auto fuel with ethanol, purchased locally in Prescott Arizona at two different locations. It appears that both aircraft experienced vapor lock while applying full power for take off. The weather has been much warmer in Northern Arizona than usual for this time of year. We believe the fuel, which is in all probability, a winter blend, has too high a vapor pressure, thus causing the vapor lock.

For those of you RV12 drivers using auto fuel at high elevation airports in warm temperatures BEWARE. Both RV12?s will be flown out of Williams tomorrow morning when the temperatures are expected to be in the high 30?s to low 40?s.

Until or when we can determine that the refineries are producing summer blend auto gasoline, we will be changing to 100ll. And may continue with 100ll during the summer months to avoid the possibility of vapor lock at high elevation airports, regardless of the auto fuel blend.
 
Thank you for the informative report. What kind of Fuel Pressure readings did you see (download flight data log?) during the climb-out and subsequent rough engine?
 
Until or when we can determine that the refineries are producing summer blend auto gasoline, we will be changing to 100ll.

N233VA - I remember reading somewhere here in the forums where a gentleman did a little experimenting with the auto fuel/vapor lock issue and determined that by adding 30% 100LL to the auto fuel that it solved the vapor lock issue. This would save you the hassle of draining all the fuel in both aircraft.

Unfortunately, I can't put my mouse on that thread at the moment.
 
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Is there a mixture adjustment on the Rotax? This sounds very similar to power loss when power is advanced with lean mixture...
 
N233VA - I remember reading somewhere here in the forums where a gentleman did a little experimenting with the auto fuel/vapor lock issue and determined that by adding 30% 100LL to the auto fuel that it solved the vapor lock issue. This would save you the hassle of draining all the fuel in both aircraft.

Unfortunately, I can't put my mouse on that thread at the moment.

Would it happen to be post #9 in this thread?
 
Installation?

:confused:
I'll volunteer my dumb opinion. It must be some installation problem. I've been flying behind a Rotax 912ULS for the past two years. 100% of the time premium auto gas 91 octanes with 25% ethanol. Field at 3400 ft MSL, median temp around 25C/77F. Never experienced vapor lock. It's not a RV-12, it's a Paradise P-1, High wing.
 
Sergio,

I believe the bigger issue here in the states is EPA directed to strive for better emissions for automobiles, where by, there are two formulas used here ... one for winter and one for summer. The winter formulation is designed to improve the fuel evaporation during the cold temperatures of the winter months.

The vapor-lock issue is a real concern here when getting into spring and summer while still using the winter blend because it vaporizes much easier with the increase in ambient temperatures coupled with increases in altitude.
 
Do both of these airplanes operate with electrical fuel pump running 100% of the time?
No switch to shutoff electrical fuel pump - correct?

Electric fuel pump in this application provides a vital function to reduce vapor lock. When engine is shut off the residual heat under the cowling can boil off the fuel in the carb bowls. If engine is warm for the restart it is best practice to allow electric fuel pump to circulate cool fuel for some period of time (maybe 5 minutes) prior to start and departure.

Note approximately 2 psi fuel pressure with electric pump running by itself and then about 5 psi when engine starts and mechanical pump is working in conjunction with the electric pump.
 
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Both aircraft were flown out of Williams AZ this AM. Temps in the low 50's. No problems experienced. Both had fuel pumps running the other day. It is clearly a fuel blend problem. Winter blend in the states. Did not drain the tanks prior to takeoff and pressures remained steady.
 
Thanks for the info Bob, I think I'll add a little 100LL next time I add some gas. Do you know when they change to the summer blend here in Az?
 
May 1 is the final date for the refiners to switch and ship.

You might see it sooner.

I started topping off with 100LL whenever I can.

Burning 100LL will help with low fuel pressure warnings.
 
IF you can get it; summer blend (when it gets warm)91 octane auto "with out alcohol" is the best fuel for a Rotax. With alchol is next best. 100LL should be the last fuel of choice. Lead will cause problems with the starter clutch sprag, gear box, slipper clutch and in the Oil tank under the baffle. The engine is much happier without lead.
 
Rotax fuel

The Rotax engines DO NOT like 100LL. They are very different from other aircraft engines, 92 or better octane premium unleaded with no alcohol is the preferred fuel. If you need to run pure 100LL always treat it with a lead scavenger, TCP or Decalin. Also with 100LL the only engine oil allowed is Aeroshell Sport 4 plus, and with 100LL you need to change the oil every 25hrs. If you exclusively operate with 100LL the gear box needs to come off sooner, 600hrs instead of the normal 1200hrs. Russ
 
I've never experienced vapor lock in the 5 years I've flown my 12 in AZ including airports like Sedona and Valle and Williams in high country. What I have experienced is slight roughness when I pull back the throttle to descend and land. My engine has also developed a habit of starting and dying once before a second successful start. Full power and cruise are smooth as silk. I suspect a dirty idle jet, and I cleaned the carburetors during my ongoing annual. When I finally finish the annual it will be interesting to see if the issue is solved. If not, I guess I'll bite the bullet and pay to have a ROTAX shop overhaul them.

Rich
 
It?s not the seasonal vapor pressure blend of the auto fuel, but rather the residual heat after engine shutdown that causes the fuel to boil off in the carburetor float bowls as I described in my post #10. Using 100LL doesn?t fix the problem.

The RV-12 has a tightly cowled engine and when you shut it down the heat remains for a long period. If the engine cools and you restart at a later time ? no problems. Cool fuel and cool carburetor bodies allow fuel flow with no harmful vaporization. Engine starts and you?re on your way.

What you have to watch out for is restarting a warm engine that has sat for some time. In this case there is likelihood that fuel has vaporized from the float bowls and the carburetor bodies are still warm. That is why it is best practice to allow the electric fuel pump to operate for approx. 5 minutes to recirculate cool fuel through the upper distribution manifold and return to the fuel tank.

I experienced this phenomenon just like others have. I was hoping rides with frequent start/stop and had the engine stumble on several occasions. I follow the hot-restart rule now and haven?t had a problem. I run 93E10 auto fuel as recommended by Rotax. 100LL as mentioned in above post is undesirable in the 912 because lead spins out and deposits in the gearbox and also presents as sludge in the oil tank.

The only concern running unleaded auto fuel is the octane rating can be reduced if stored improperly or for long period of time. I use fresh fuel and fly often. I have an early model 12 and elected not to install the fuel tank vent. I stayed with the vented fuel cap which minimizes air transfer in the fuel tank when the plane is stored.

I didn?t mean to write a book on this subject ? just trying to keep us safe?
 
Both aircraft were flown out of Williams AZ this AM. Temps in the low 50's. No problems experienced. Both had fuel pumps running the other day. It is clearly a fuel blend problem. Winter blend in the states. Did not drain the tanks prior to takeoff and pressures remained steady.

When I was flying with Subaru engines and mogas, I tested fuel before take off with the Hodges vapor tester, found winter blend way below safe flight reading one time and drained fuel.

At that time the Hodges tester was sold by Petersen with the mogas STC kits for various engines.

I found that ethanol eventually destroyed the tester, it had plastic tube. I called Petersen about it and he said he would consider having it made with glass tube but I don't think anything came of it. In fact I believe he stopped selling the tester, maybe because of liability issues.

I've considered running 91 octane mogas with the Superior IO360 180 HP engine but when I could not find another Hodges tester, it was factor to stick with 100LL. The other of course is the engine driven fuel pump, its internal materials probably are ethanol ok, like the AFP system, but manufacturers of fuel pump will not verify it.
 
Sounds like another good reason to use Subaru with factory fuel injection controlled by
SDS FUEL/IGNITION CONTROL !
 
IF using Decalin or TCP, does that mean you can change oil at non leaded intervals, and none of the bad things from lead will happen to your engine?

The Rotax engines DO NOT like 100LL. They are very different from other aircraft engines, 92 or better octane premium unleaded with no alcohol is the preferred fuel. If you need to run pure 100LL always treat it with a lead scavenger, TCP or Decalin. Also with 100LL the only engine oil allowed is Aeroshell Sport 4 plus, and with 100LL you need to change the oil every 25hrs. If you exclusively operate with 100LL the gear box needs to come off sooner, 600hrs instead of the normal 1200hrs. Russ
 
I am sure Petehowell has posted this on VAF, but here is a link (access Dec2013) to his graph of 100LL and mogas mix. Also, note his test apparatus for Reid Vapor pressure. This should be of interest for those flying with a fuel system without tolerance to high vapor pressure fuels. Carry some test equipment and know your temp/altitude limits.

The RV drivers need to know what the limits are for the fuel/fuel system combination. It is just too risky "hoping" it is ok. Especially if locked into using mogas.
 
At that time the Hodges tester was sold by Petersen with the mogas STC kits for various engines..... In fact I believe he stopped selling the tester, maybe because of liability issues....

I understand that general reductions in the volatility of US fuels resulted in reduced demand for the testers to the point where Petersen decided to send their remaining stock back to Dr Hodges in Australia.
 
I've seen some boiling point comparisons of winter mogas and some were as low as 105F at SL- lower at several thousand feet. Food for thought on a warm engine with heat soaked carb float bowls.
 
It?s not the seasonal vapor pressure blend of the auto fuel, but rather the residual heat after engine shutdown that causes the fuel to boil off in the carburetor float bowls as I described in my post #10.

Actually it is both.

The vapor pressure of the fuel definitely has an influence and is a variable so operators need to keep it in mind in warmer temps in the spring early summer.
Particularly with specialty fuels (non ethanol) that could have a low sales volume and have been sitting in the retail tank for quite a while.
 
IF using Decalin or TCP, does that mean you can change oil at non leaded intervals, and none of the bad things from lead will happen to your engine?

No, the same leaded fuel maint. intervals are recommended regardless of whether you use a lead scavenging additive or not.

BTW, use of 100LL doesn't cause any "bad things" to happen with a Rotax, but if you are looking for something to push the Viking higher on the "benefits" chart I guess this might be one. Though with so few engines in service, does anyone actually know what the long term (many hundreds of hours) effects of 100LL is on a Viking installation?
 
I understand that general reductions in the volatility of US fuels resulted in reduced demand for the testers to the point where Petersen decided to send their remaining stock back to Dr Hodges in Australia.

Robert,

Is the Hodges test kit available in Australia?
 
BTW, use of 100LL doesn't cause any "bad things" to happen with a Rotax...

Oil change intervals are reduced using 100LL and Rotax techs tell me that gearboxes on engines using 100LL are pretty gunked up with lead, even with the shorter oil drain intervals.

Lead isn't good for a Rotax but the characteristics of the 100LL fuel itself is superior in hot and high conditions from vapor lock formula consistency perspectives.
 
Thanks for the info Bob, I think I'll add a little 100LL next time I add some gas. Do you know when they change to the summer blend here in Az?

Although I have a friend here who works at the local refinery and gives me heads up (I'm in MInn), the best way to know when they've switched to the summer blend is when your car picks up about 4-5 mpg on a tank.
 
This might apply in this situation. It might also apply when carb ice is suspected.

A few Rotax 912S powered Europa aircraft have suffered similar power loss. The problem eventually was traced to a small exhaust leak at the cylinder head exhaust pipe joint or from a joint in the exhaust system allowing hot exhaust to hit the carb bowl causing the fuel to evaporate. You may see a tan soot line from the exhaust to the carb bowl. This will only happen on 912 engines that are not fitted with carb bowl drip trays. Often the engine will run fine after the throttle is reduced since the exhaust is cooler and the carb bowl has a chance to refill.

The solution is to fit carb bowl drip trays. They are a standard Rotax part or you can fabricate your own. They are also a good idea in general since should the carb bowl overflow for any reason, the gas may drip on to the exhaust pipe!

Jim Butcher
 
Oil change intervals are reduced using 100LL and Rotax techs tell me that gearboxes on engines using 100LL are pretty gunked up with lead, even with the shorter oil drain intervals.

I am aware of the specifics related to operating with 100LL (one of our RV-12's has been operated for years using it exclusively).
I guess I disagree on that that translating too to "Bad things will happen".
 
And the springs driving to WOT, I don't like it.

The 912 series engines need to be looked at as if they are two separate power plants coupled to a single crankshaft. This is because each “half” has its own carburetor and induction system unlike traditional aircraft engines where all cylinders share a single carburetor.

Reason for springs on 912 carbs is for safety. If throttle cable fails on one side of the engine you will have partial throttle – perhaps zero to something approaching 100%. Say the other side is still functioning correctly with its half of the throttle cable intact. Worse case scenario would be 0% power on one side and cruise or full power on the other side. Engine would probably shake itself loose from it’s mounting.

The springs are designed to advance power to full if throttle cable breaks. With opposite half of engine already at controlled cruise power the engine continues to run smoothly and makes sufficient power for continued flight. In this case the pilot will need to judge height for safe landing and then kill engine by shutting off ignition to prevent the power imbalance described above.

The springs are there for safety…
 
I am aware of the specifics related to operating with 100LL (one of our RV-12's has been operated for years using it exclusively).
I guess I disagree on that that translating too to "Bad things will happen".

Nothing bad is likely to happen other than you may end up taking apart the gearbox before TBO and at overhaul, more pieces inside that gearbox may have to be replaced. Those are perhaps small prices to pay compared to an engine out caused by vapor lock and winter mogas. The best scenario will be when proper unleaded avgas is widely available.

Then again, some folks could ditch all these problems by replacing carbs with EFI as some others have done on the 912s.
 
A bad thing to me is taking off at an 8000' Dalt with a family member or friend and seeing nothing but pine trees, forest roads and homes after an engine failure due to vapor lock....putting up $800-1500 for a professional gearbox rebuild a little early is simply the cost of doing business.

Switching to 100LL for the majority of your fueling needs in the hotter climes of the southwest when "watered down" winter gas with ethanol is being pumped is also just the cost of doing business.

My 2 cents.
 
A bad thing to me is taking off at an 8000' Dalt with a family member or friend and seeing nothing but pine trees, forest roads and homes after an engine failure due to vapor lock....putting up $800-1500 for a professional gearbox rebuild a little early is simply the cost of doing business.

Switching to 100LL for the majority of your fueling needs in the hotter climes of the southwest when "watered down" winter gas with ethanol is being pumped is also just the cost of doing business.

My 2 cents.

Couldn't agree more with you....I'll be mixing 60/40 % (40 % LL) during Winter/Spring months to minimize lead build up. Even in Summer @ high altitude airfields....just for peace of mind. A sputtering engine would freak my wife out!!
 
There is little mystery in this story. Once fuel is in a hot float bowl at atmospheric pressure, if fuel temperature exceeds the boiling point of the lightest fraction (usually Butane in winter mogas), bubbles result, those bubbles are pulled into into the main jet and you have a very lean mixture.

Winter mogas has about double the vapor pressure of summer mogas. This is a good reason to be careful using winter mogas on a warm day or at higher altitudes where pressure and therefore boiling point is lower.

Like I said, some winter gasolines will start to boil at just over 100F at sea level. That's easily exceeded on a heat soaked engine with carbs. Be aware that gasoline formulations are molded around the fact that most cars have been fuel injected for a couple of decades and EFI picks up cold fuel in the tank and pressurizes it before it reaches the engine. This drastically reduces the chances of vapor lock issues compared to carbs.

All this being said, mogas was not designed for aviation use. It may work fine if you're careful. Here is a good graphical representation of the vapor pressure variances in mogas vs. avgas: http://www.bp.com/content/dam/bp-air/en/Files/Fuel/Air BP Avgas leaflet.pdf

Personally, it would give me the willies to run a carbed engine on winter mogas. I run it in my Subaru but it has EFI and I use up the winter mogas before it gets warmer. I've drained it out of the -6 and put it in my car on occasion because it wasn't safe IMO. I bought a Peterson tester when I switched from 100LL to mogas and that was pretty eye opening. There's a big difference between the 2 fuels.
 
Many of us on occasion have experienced low fuel pressure warnings in the RV12/Rotax which I've seen described as a "Rotax gremlin". I've never had a rough engine in flight, but in warmer months I've experienced these warnings almost always when I pull back on the throttle to reduce altitude usually after a long cruise. These temporary low pressure readings eventually stabilize and the pressure recovers, but obviously it really gets your attention. Last spring it happened one time when I was defending over NYC at 8,000 ft and boy does your heart rate go up until it recovers.

Can't recall ever experiencing these events when I've had at least some 100LL in the tank. I almost religiously avoid using 100LL, but with spring approaching I think its prudent to mix fuels for upcoming flights until I'm sure summer blends are available at the local ExxonMobile station.
 
Yeah... we've got a Young Eagles even this weekend, and I need to fill my tank. Think I'll mix in some 100LL just to be on the safe side.
 
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