The first two minutes of my test flight went very well, although the slight "hiccup" when doing a roll or any other high(ish)-G manouver was still there.  And then - DISASTER !  It started sounding very unhappy and almost died so I switched ff so I tried to land. But the bird wasn't ready to land - she wanted to keep on flying ! The engine could not be brought to idle. So I did another approach and switched off the ignition to do a dead stick landing.

I didn't have my thermometer there but could feel the engine was EXTREMELY hot. There REALLY isn't anything more I can do for cooling - the cowl is already full of ducting / guides and a proper baffle setup. So I thought perhaps I set the needles waaay lean. The same thing happened with every subsequent flight now matter how rich I set the needle. The symptoms all looked like fuel starvation to me, and looking at the colour of the spark plug it was obvious that the engine was indeed running way too lean.

So I took everything apart AGAIN. There was NO dirt inside any part of the carburetor, and fuel flow from the tank was unrestricted.
One thing I did however notice was how much force it actually took to make the dome / bubble of the fuel pump membrane move against that spring. Perhaps the membrane hardened a bit over time but I can't get a service kit for this carburetor anywhere to check if that's the case. I decided the fuel pump is a poor design because the negative pressure pulses from the intake manifold are pretty weak especially at wide open throttle - can't see how that can really be sufficient to properly pulse that "fuel pump".

So I did a modification - blocked off the little channel under the carb that goes to the slot in the phenolic spacer whereby the low pressure pulses from the intake manifold is supposed to suck that dome against the return spring. I used epoxy for that. Then I drilled a hole halfway through the intake manifold flange and the phenolic spacer, right underneath the hole that goes to the dome of the fuel pump membrane on the carburetor. Next was another hole drilled from the side of the intake flange, just deep enough to meet with the first hole. I then press-fitted a piece of brass tube with some green high strength thread locker into this second hole. The return spring that pushes back the fuel pump membrane's dome was removed and tossed away, and the carb was bolted to the intake manifold flange again.
So now I was able to apply pressure to the fuel pump membrane's dome / bubble from the outside via that piece of brass tube sticking out.

Next step was to remove the rear engine cover and drill a hole to fit a second nipple (same as the one used for the vent, but with a larger hole through it). I then replaced the rear cover and connected this second nipple to the brass tube I attached to the intake manifold flange under the carb with a length of tygon tube. This mod makes the fuel pump work the same way as the normal Walbro carbs on the 2-stroke engines.

I tested this mod by turning the crankshaft with an electric starter, and the fuel inlet to the carburetor sucking fuel from a container 1 meter below. The new fuel pump setup pumped like crazy !  Next I ran the engine on the ground, and had to set both needles slightly more lean which tells me the carburetor was actually getting more fuel pressure from the pump.

I forgot to mention - another mod was to carefully cut off about 1mm length of the pop-off valve spring to slightly reduce the pop-off valve reseat pressure. In Walbro's service manual it says pop-off pressure must be 20 - 30 psi, but most importantly the pop-off valve reseat pressure must be 10 to 12 psi. Before cutting the spring, I tested about 15psi. After cutting 1mm off, it was just under 12psi.

I couldn't believe how good the throttle response was now (ZERO hesitation at any point), and how smooth it ran right throughout the range from idle to wide open throttle. I also picked up the plane and shook it around with the nose pointed upward at full throttle, and not a single cough or hesitation from it !  In other words - it now seems to be running almost PERFECTLY.

BUT as they say - "the proof is in the pudding" - so I can't say that all these mods have taken care of my overheating problem - will have to fly it first. One thing I CAN say however is that this is my very last attempt at getting this engine to actually run like an engine. You may ask why I'm doing all this effort, why not toss this piece of junk and get a proper engine ?  Well, I like a challenge like this and I will not give up until I can say I have tried EVERYTHING that I can think of.  Also, I'm learning a lot about gas engines in the process too !