Very high fuel consumption

If you have fuel coming out of the vent pipes then this is due to an incorrect fuel level in the float chamber. Causes of this can be:

1. Faulty floats (too heavy and sinking).
2. Faulty float valve. Check the viton tip of the float needles and check the valve seats for damage or contamination.
3. Bent/broken/incorrectly adjusted float arm.
4. Poor carb balance.
5. Fuel pressure too high.

Given all of the problems that Rotax have had with the floats I would start by weighing them and see if they are within spec. I would then inspect the valves. If you have an electric pump fitted then you can pressurise the fuel system without running the engine. If you see fuel coming out of the vents then it may be easier to trace the fault.

What Kevin said,

"1. Faulty floats (too heavy and sinking).

2. Faulty float valve. Check the viton tip of the float needles and check the valve seats for damage or contamination.
3. Bent/broken/incorrectly adjusted float arm.
4. Poor carb balance.
5. Fuel pressure too high."

Weigh the floats and carb sync now. Carb sync should always be done at the annual and or checked during any other odd fuel based issue. Use gauges and not a electronic device. You can diagnose with gauges and not the elctronic ones. Once you get some experience you can tell a lot in about 30 seconds with gauges. 

Thank you very much Kevin and Roger!

I have another question please-

Why isn't the extra fuel flow out of the vent lines don't go back to the fuel tank like from the fuel manifold??

Best regards,

Barak.

I have a single seat gyrocopter with rotax 912uls on it. 

I experienced same problem on one carburator where after shut down fuel was flowing out of vent tube. 

I took both sets of floats out of carbs without removing carbs from engine. One set was way over 7 grams. This was the one set from carburator where fuel was coming out of vent after shut down. I am sure probably also on occasion fuel probably was coming out during flight especially maybe during low power during landing. I use 100LL avgas, aviation fuel all the time.

Yes floats are expensive. I believe 150 dollars a pair. I replaced both sets since the other set though within weight limit was getting close to probably getting heavy. 

Suggest as others have advised start with the floats and see if this will resolve your problem as it did mine. They claim the new version floats are better but rotax does not make carburetors. Carburetors are manufactured by Bing for them and parts. This float problem has been an issue and I don't believe a scientific explanation has been provided yet.

The person I bought my gyrocopter used none alcohol car gas. Car gas is very unstable and when aircraft sits for a while especially during winter this may be the reason floats get heavy. I may be wrong on this. Suggest running aircraft on aviation 100 low lead for the last flight if one puts aircraft away during winter or other reasons. 

Because of car gas people usually run there lownmoyer empty before putting it away for the winter. This is not the case regarding aviation gas 100LL. You lawnmower will start even if aviation fuel is left in it for several years without touching it. 

Don't use aviation fuel all the time in 912 unless you are willing to follow suggested procedures since engine oil is shared by the gearbox. This means changing oil more often, cleaning oil tank, using chemical in gas to scavenge lead like a product called TCP or similar, using oil that is not all synthetic, and changing plugs more often. 

I personally change my oil every 15 hours and clean the tank. I know it's overdoing it but oil is cheap compared to engine and gearbox maintenance. I use aeroshell recommended oil but probably am considering using pure, no synthetic oil in it, like motorcycle Pennzoil 10w40 since I only use 100LL aviation fuel. Synthetic oil and lead In gas don't get along.  In my state none alcohol fuel is very hard and far to buy. 

As stated in maintenance issues, start with the simplest and see if it resolves the problem. Hope it's the floats though parts expensive labor not so much. 

Barak wrote: "Why isn't the extra fuel flow out of the vent lines don't go back to the fuel tank like from the fuel manifold??"

The vent lines are not intended to be overflow fuel lines.  They are air vents to ensure the pressure in the bowl is the same as at the carb intake.