914 Barely Starts and Won't Get Above 780RPM (Video)

Hi Ken - my point is; The burning of leaded fuel (AvGas) leads to the symptoms so described.

The cure is to minimise / avoid the use of AvGas.

Decalin merely treats the symptoms of a self induced problem- the use of AvGas.

As you have found it is not 100% effective.

Using AvGas is your prerogative - its not forbidden by Rotax, just advised against.

You will pay more for AvGas, for the servicing/maintenance and as you have found, likely diminished reliability PLUS the cost of Decalin.

Where is the upside?

(My apologies for proselytising)

Three reasons I don't use MOGAS:

1. I experienced pretty violent engine vibration at altitude with MOGAS, which was attributed to the usage of MOGAS.  Switched to AVGAS and that hasn't happened since.

2. There are several variables with MOGAS I'd rather not have to deal with: Winter mix vs. Summer mix, ambient temperature, and altitude.  Use a Winter mix in the early summer, when it's hot and high altitude and you're way more likely to encounter vapor lock.  I just don't want to have to think about that.

3. Availability: My airport doesn't carry MOGAS and I think there are only one or two airports that do in my state.  I don't feel like tankering it in.

So, it's AVGAS with Decalin for me.

Craig -

The decisions to use whatever fuel is entirely yours.

In Australia we (VFR pilots/aircraft) are limited to 10,000ft AMSL. I almost never use AvGas,   I have often flown at this altitude & never had an engine problem.  I can not see any reason for ULP (of the correct octane) to cause vibration at any altitude. There is another factor causing this problem.

True " vapor lock" in hot weather is a characteristic of Rotax 9 engines. This characteristic can be managed, in several ways, including the use of AvGas. Personally I have management strategies that do not include AvGas. 

Few airfields carry ULP, so most Rotax 9 pilots do "tankering it in" I respect your choice not to do so - but as with all choices, there are ramifications.

May your pockets remain lined with $$$$$

In my opinion, the decision to use or not to use Decalin with Avgas is less clear with a turbo charged engine (see attached data sheet from Decalin).  With Decalin, lead oxide is converted to lead phosphate, which is more easily expelled through the exhaust.  However, this lead phosphate then leaves deposits on the exhaust components, including the turbo charger and wastegate. I guess the turbo charger is somewhat easier to service than the heads, but you still have to pick your poison.   

Its also easy to dismiss the problems with lead under the guise that conventional aircraft engines do just fine with it. But with a Rotax engine you have a couple things working against you. First, you have twice the ignition cycles, which can theoretically double the speed of lead accumulation.  Second, the water cooled Rotax heads run at a lower temperature than conventional air cooled heads, and this cooler temperature promotes lead precipitation.

With the complications of winter/summer fuel blends, and the fact that Mogas is often not available at airports, the case for using Avgas is easily made, but that decision will come with higher maintenance requirements.   It will be a good day when unleaded Avgas is available at all airports. 

Jeff -

I would dispute "the decision to use or not to use Decalin with Avgas is less clear with a turbo charged engine" 

on the basis that Rotax have consistently, over a long period, recommend against the use of AvGas - the use or not, of Decalin does not change this.

Further; this Forum sees lead (AvGas) related contamination of engine/gearbox questions, on at least a monthly rate, if not weekly. This suggests to me that the routine use of AvGas in Rotax 9 engines is far from being a trouble free decision, even with the use of Decalin. 

The decision to use AvGas will significantly increase (x2?) the service/maintenance cost of your Rotax 9. Decalin will just further add to this cost, as will the higher, than ULP, cost of AvGas.

This is now a very old debate - all points for/against (AvGas/ULP) are well known (by most). It is now in the realm of personal preference - I would liken it to smoking, with its known health/financial/social costs verses non smoking - your choice, please don't smoke in an enclosed space with me.