Fresh gas is the key here. Unleaded gas loses its "bang" much quicker than old leaded gas. Also, unleaded gas leaves greater residue on all those small bits. If the fuel flows just fine in the lower gears as mentioned then I would think that fuel flow/octane is not at issue. All gas is made up of the same stuff except higher test fuel.. It's higher octane rating is to control the "burn" of the fuel, used mostly in higher compression engines. I get better fuel economy with normal unleaded gas than hi-test and regular actually is fresher than hi-test. If you do a bit of reading of fuel grades you'll discover a few interseting tidbits of info. I run the normal unleaded in my motorcycle without any pings or detonation so there is no need to raise the fuel grade or increase octane. The engines in 90% of the cars running the roads only require reg. unleaded gasoline.
"The octane rating is basically an anti-knock rating. Engine "knock" is also sometimes referred to as "ping". It is basically pre-detonation of the fuel before the piston is in the correct position (either near top of it's stroke or on its way down on the power stroke). Ping occurs while the piston is on the upstroke. Essentially, it is the elevated combustion cylinder temperature mixed with the now elevated cylinder pressure (compression) that will cause fuel it ignite without the spark."
I suspected your ignition due to the sitting around for 5 mos. part of your comments. I would apply a little dab of dielectric grease to all connection points of the ign. system.
Octane cannot increase power, but an engine designed to run hi-test will already be a greater power producer and will require hi-test .
" Although octane itself does not improve power it does allow greater cylinder pressure to be tolerated in the design or modification of an engine, thus allowing it to produce more power while staying in one piece."
You want more power from you gas, start adding oxyagen.
