A couple months ago , I had a guy with a box stock 225 efi ( late model Motorola) trade in a flywheel , inner can , 6 pedal rubber reed blocks and a set of 73 cc heads .
He took home a 13 lb flywheel , cut and cleaned out can like the one above , striped , flattened and tumble polished reed blocks with shortened stops and Carson reeds. He wanted to run 89 to 91 octane fuel , so I did a set of 65 cc heads for him .
He put just that stuff on his open fisherman boat . He called me about a week later and said it was 7 mph faster than it was before.
The whole deal with the 300 ign is it has a coupe more degree's of WOT timing which helps it accelerate harder and a little higher secondary rev limiter.
The idea of a 250 fuel box is these things run blubbery fat , so the lower HP fuel curve actually clean's some of that up . It also has the soft limiter which starts dumping extra fuel back in (nice right ) at high RPM . I never really saw much gain from the computer thing , the porting is so mild , that they quit pulling way before the "benefits" show up ...
Reed blocks look like this ...
http://i217.photobucket.com/albums/c...psxhpyiw4z.jpg
The 73 and 63cc heads have a pretty wide chamber ...
http://i217.photobucket.com/albums/c...pssoelsxfj.jpg
Anything less than 65 cc I like to just weld up the chambers and mill them back out to whatever size we're lookin for .
http://i217.photobucket.com/albums/c...ps0utzeeqc.jpg
This set is pretty tight but works well with a big exhaust port ..
http://i217.photobucket.com/albums/c...pslio52zx2.jpg