Fuel system woes

Header tanks are a good return source like in the 912iS. Tecnam returns it into the left wing, but if you're not careful and don't keep the fuel level down it can over flow. 

Marc, surely shutting down the engine by closing the fuel valve will work. Besides, I do agree that a return line does not cure the problem you are experiencing. However, I am stunned that all measures did not help so far.

I've heard some operators open the oil check door on the top cowling.  This apparently let's the hot air escape.

Hi Mark

"I'm not sure though if a return line would really h"elp. The heat soaked carb filled with residue fuel would not get replaced by cooler fuel and it would not evaporate through the fuel line, right?

WRONG!

I fear you do not really understand the problem or its management.

When the fuel is heated to vapour, the carburettor can no longer control the air fuel mixture (stops working). In addition the fuel boost & mechanical pumps, designed to pump liquid, loose considerable efficiency.

The heating, of the above engine fuel lines, is unavoidable but can be minimised by using insulating sleeves (fire sleeves).

The heating of the float bowl & its contents, by the rear exhaust system, is also unavoidable but can be minimised by using some form of heat shielding between/on exhaust pipe & carburettor.

Pointing the aircraft into wind & opening inspection doors in top cowling (removing top cowling) will all increase cooling airflow to reduce the effect of the hot engine on the fuel system.

You could use AVGas as this fuel vaporises at a much higher temperature (read up on the management of  Rotax using AnGas) 

All of the above can be done - Insulating/shielding are usually permanent fixtures on Rotax 9's that have an ongoing benefit, apart from addressing fuel vaporisation.

Rotax (in line with a whole range of automotive manufactures) have found removing the hot fuel/vapour, is the most effective management tool.

Removing the bulk of the hot fuel & vapour, replacing it with cool liquid fuel, is the most convenient method of addressing this occasional (hot weather) phenomena.

The fuel is removed through a fuel return line, located as close to the carburettors as practical (minimising the amount of hot fuel remain).

Combining the fuel return line, with a boost/auxiliary pump being turned on, forces the hot fuel/vapour unto the return line, back to the tank , replaces the hot fuel with cool, 

The incoming cool fuel, cools any remaining hot & the fuel supply system itself.

Liquid fuel enters carby float bowls, forcing out any remaining fuel vapour.

The carbys now have a chance of functioning as designed and you can start your engine.

Fit a fuel return line & boost pump, to enable best management of vapour lock.

"I was thinking of opening the throttle fully after shutting down the engine, perhaps that will give the vapor from the carb an escape path?"

Certainly cranking the engine, may assist in removing vapour, from the inlet system but will have minimal effect on replacing vapour, in the hot fuel supply system, with liquid fuel. Net effect will be to flatten a weak battery - no start, possibly overheat/damage your starter - no start.

"Also, I could try closing the main fuel valve before shutting down the engine and wait until the fuel has been depleted from the float chambers. I'll give these options a go."

You persist in focusing on the carburettor float bowls, to the exclusion of the greater volume hot fuel/vapour in the fuel supply lines.

The fuel lines must be flushed of hot fuel/vapour, only then will liquid fuel enter the float chambers.

The whole system - fuel supply & float chambers, must be managed together IF you are to have any hope of managing this problem effectively.

Fit a fuel return system. Any design will be better than non.