Why am I upjetting now?

mike Thomas /

Seriously, I tune my sebring in the beginning to a 74 jet. I break it in during a 54 mile ride out to the rally campsite two months ago. I commute 25 miles to a job for like 10 days and ride her for fun in between. I finally deal with a head leak in the last day and take up the excess case volume with what I did here:


I did soft sieze her after the filler block so up-jetted to a 78 (what I thought to be a drastic upjet and I figure ive changed the mix somehow with lower case volume) but the bike ate it up and got faster?!? Now I know the process of tuning and plug reading but this is new. Ive gotten curious and threw in a 86 to four stroke her and start down jetting again, but wait, she eats it and no speed loss?!? I just emptied a new can of carb clean and for once she has NO air leaks. I did the crank seals back in january so they are good. An ideas? Once the rain stops im going to upjet again and see what happens. How could I have run a 74 jet for 300 miles, blasting without a care with a slight air leak in the head and now be 14 jets bigger with improvement. I figure I would have siezed by now. specs:

Morini MO1 with a polini kit on it, case filled and blocked, 15 dell, Motomatic pipe, 13-30 gearing, points.

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