wow guys there is a virtual goldmine of good advice there. I'll keep all ur suggestions in mind and report on my progress soon.

BTW if the airflow through the muffler causes a low pressure around the centre forcing the exhaust gases (in my case more fuel less gases!!!) out thru the back, then the muffler nipple, which is towards the back (refer last pic posted) should be able to pressurise the tank? What say?

meanwhile its off to the field with my trusty OS max 20 (1971 vintage!!!) on QB15.
But i must say one thing, there is more to fuel powered flight than meets the eye and i am enjoying this conversion from electric to glo!! A totally diferent ball game!!