I usually use the metal wedge tanks.  On occasion, I have used the plastic clunk tank but only on a full-fuselage plane, not on a profile.  I followed this sketch for the plumbing and this system works well.  I did not use muffler pressure.

Hi all, thanks for the feedback.
Finally I had the chance to fly without muffler pressure. It was amazing. Easy starting, steady run from start to end. Very easy to needle. Not critical at all.
In flight the engine appeared to "understand" the load conditions much better.
So I can stop worrying about the issues with muffler pressure. Without muffler pressure, the engine behaves much better.
I really like the fast 4 stroke and the extra power were needed.
I also sealed the Tank compartment better against the engine heath. Perhaps it helped, perhaps not.
Now main focus on more trimming, the shapes, the bottoms and the sizes...
Thanks again.

      Well this is good to hear. I don't think you mentioned before that the fuselage is open behind the engine. A former there add much strength and it is also called a "firewall" for good reason!   Fly the model some more with no changes to make sure it is cured. Then, if you like, you can test the firewall heating the tank theory with a few flights back on muffler pressure and no other changes. If the problem returns with the first flight, then you may conclude that pressure is the problem, for what ever reason. If it runs fine with just the required change in needle setting, then you know you might have been heating the tank with heat from the cylinder and head. It's worth a tank of fuel or two to know exactly what was happening and you really don't have to change anything other than add a piece of fuel line and adjust the needle.
  Type at you later,
   Dan McEntee

Hi Dan, The fuselage was not exactly open between engine and tank, but there was a gap of 2 mm between the engine cover with cool air duct and the tank area. Now I sealed it with a rubber strip. For what it is worth. I will enjoy the new setup for some time and then go back to the muffler pressure for "science" sake :-). I understand that the amount of muffler pressure was too high to be comfortable on the needle. The pressure was also higher than average because the muffler had an extension pipe to direct the exhaust away from the fuselage. This must have added to the pressure. Why the setup had this richening effect needs more investigation. Thanks!

   OK, I see now. If you want to do one simple experiment, fly a flight as you have it, then fly a flight with the rubber deflector off and you will see an definite difference. They restrict the exhaust more than some realize . I some one must use them, I advise putting the biggest one they make on for the largest opening they have and do what you have to do to get it to mount to the muffler stinger.  I like my engines to breathe as best as possible. Good exhaust out makes for goon intake!!
   Type at you later,
   Dan McEntee