I will weigh the airplane and report. Dont have a weighing scale myself, I will need to borrow someone's.

When I was flying inverted, I had to give a touch oof elevator to hold level. But it was no so much that I noticed while flying, it was more of a bit of reflex, if at all. I know because I was rolling, bunting, split-S-ing etc from inverted.

With regards to the weight of the airplane with the 91, it will gain 100 odd grams in the nose 5.68 oz (161g) in the nose. Moving the battery back was making the airplane tail heavy with the 55ax but it should help with the 91. I am also considering lightweight faom wheels (2 3/4" Dubros) and a CF wing tube to replace the aluminium one if I can find it in the require dia 15mm x 47.5mm size

Using a #8 plug, already tried a Ftype, there is a lot of preignition at WOT

Airraptor,

you have covered all the bases

I put in the 14oz fuel tank to resolve the engine issues earlier. Now with a 91 I will need the additional capacity

With a 13x4, the engine sound was much harsher than a 13x6. Some at the field remarked that it may be over-revving and the soft compression may have confirmed that. After my final flight with a 12x6 when I was cleaning up, I noticed a lot of black residue on the ground and on the tail of the airplane, if that is some indicator. Though I didnt get it again when I was finishing of the fuel at home

I am not running it lean, I set the needle with someone holding the thing vertical. Looked for max rpm, then turned it back quarter turn or so towards rich side.

Ameyam