Fuel pump

Got it. It seems like we need to install a return line and see if it solves the problem. What I don't understand is that we never had this issue with the old pump, only with the new ones.

Sure, Dino, that really seems strange.

just one more thought on vaporization. In our region fuel is sold in two versions, winter and summer fuel. They are said to be optimized for the respective season with different “boiling (vaporization) temps (points)”. Sounds good. However, when one flies winter fuel in warmer conditions the described effects of vaporizing occur at OATs we regard as normal. A hot engine with no airflow under the cowl can easily increase the temp under the cowl to 70 -80C (until it temp gets back down after a while of non-operation). Surely it takes while for the heat to creep through fire protections sleeves and the tubes to actually heat up the fuel sitting in them but winter fuel does not even need 70C to vaporize (I recall something in the 50s).

Perhaps you like to try an extensive cool down phase with the engine idling bringing the engine temp down before you switch it off, just to see if it makes a difference. Turing your plane’s nose into a light breeze can make a big difference. 
My bet is that it’s not the newly purchased pumps, too.

Got it. here in New Zealand, we do not have winter/summer petrol, but petrol with different octane. with Rotax, we use the 95 octane petrol. we strictly avoid the 91 and no need for the 100.

I believe you guys are right, it does happen only after landing, with the engine off and, of course still hot. after a while, he problem is not there anymore.

Next landing ill take of the cowling in order to coll the engine down faster and see if the problem is still there.

Meanwhile I'll see what i need to do to install a return fuel line.

I'll keep you updated!

Dino,

You will find a lot of information on fuel return line setup/installation in this Forum.

What's important is the

 # Location of the return line fuel in - usually in the fuel distributor (where the fuel goes to each carburettor)

 # You match the return line restrictor valve/aperture to the fuel pressure being delivered by the Boost pump / Boost+Mechanical pumps / Mechanical pump alone ie if restrictor too large there will be a concerning pressure drop. If too small pressures may be too high and "scavenging" of  fuel vapour take too long.

What I find curious is that this never happened before changing the old pump.