Yes meaning the 15x6, You were typing at the same time last night and I see you have gone to a lighter load.

If its possible can you post a pic of the outlet on the cowl bottom, it may be inadaquate if you have not opened it enough. Something that is almost always neccessary and made worse if there is a pitts type muffler present. An in cowl muffer is just another heating element in the oven and something else to block airflow.

John