1. There is no replacement for displacement, NEVER FORGET THAT.
2. The 30lb diff is nothing compaired to the size and power potenial between a 70 and a 90.
3. 70 has 2.33 gearing, 90 has 2.0, 2.33s are for low end power, 2.0 is for speed. You can swap the 90 gear into a 70 housing (and vice - versa) but even modded the 70 will struggle to turn the 2.0 gear. And it has been proven many times that tall gear and low pitch is better for speed than low gear and high pitch.
4. The 70 and the 90 share reed cages, but with the 90 you get bigger pistons bigger carbs.
5. As far as reliability goes, I have s held my 90 at 7400rpm for over 7 miles at one time. I do it regulary. The yammi will take the abuse. Also on a 90 you have detatchable connecting rods, easyier to balance, work on, swap out in case of a failure.
6. The are ways of making a 90 into a 15" motor, takes some work and thought, but has been done many times.
7. Right now I am running a completely stock 90, with just a lightened flywheel. I can turn my big 27 cleaver right at 6000 and run 72mph on gps, STOCK. Do that with those small motors! If I run a smaller pitch it will sometimes go to 6400. When I have a modded powerhead I have spun 7400 and with proping right have been to 78mph, BUT modded gets there a whole lot quicker. I was suppose to make the attemp to break 80mph this year but I have been too busy to even start. I make 2 rides this year thats it.
Go with the 90 you will love it. http://youtu.be/KQyBnDBagdc
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