Originally Posted by Big G
...My next project is a four stroke. Do you know of one larger than 50 cc that I can adapt?
The Harbor Freight Predator 99cc and 212cc are "fairly popular" builds, however they are also prime examples of the fallacy behind the saying "there's no replacement for displacement" as they've remarkably poor preformance for their size & weight, the $99 price tag is also somewhat deceptive as this ofc doesn't include the aforementioned preformance mods, nor the redrive system needed.
Simple & straightforward - utility engines can be fun, practical and reliable motors... but if economical preformance is desired they're not all they seem at first. For example the HF 99cc is 2.5hp at a max 3600rpm, whereas your typical moped engine either two or four stroke make that same hp with a mere 49cc, such as this example: http://tboltusa.com/store/50cc-moped...ls-p-1301.html
Granted, that example is $200 more then the HF99 - but consider it makes the same power stock as the HF twice it's size, comes included with it's own actual, integral transmission (other models offer 3, 4 & 5sp w/auto/semiauto or manual clutches) & even has electric start & full charging system... should you modify a HF to emulate this, it will most likely cost significantly more. Best yet if hopup mods & overall preformance is your game, as it's a Honda CRF50 clone has an extremely vast selection of common, even inexpensive aftermarket parts & upgrades from innumerable suppliers - opposed to the somewhat limited selection of parts & vendors for our motorized bicycles with utility engines.
Admittedly, another problem is fitting such a horizontal in your typical bicycle frame... yet I suspect if you've the wherewithal to make a utility motor work, then this would also be achievable, if you like choppers - it may not be a problem at all: http://motorbicycling.com/showthread.php?t=18970
or, for inspiration the motoped;
The utility engines are indeed fine motors and they can be an inexpensive way to get started in custom building motorized bicycles, it's just there's many other engines to choose from as well - which one suits your overall needs best only you can decide, but as with anything it's somewhat more complicated then it seems, it's more then price & displacement alone