How hard isit to convert a gas weed eater motor over to a radio controlled engine I have a j3 piper cub on float that i woud like to put this engines on.

There is whole RCU forum dedicated to engine conversions, where you can find additional info and ideas for your project:

http://www.rcuniverse.com/forum/forumid_92/tt.htm

They will work, but enginesdesigned for RC have more power and much lighter weight. They are ported for easy starting, and don't have a lot of torque or get the extra RPM.
So after tinkering, and making parts, yo umay still have to buy a commercial engine for RC.

He likely won't have to buy one, but as stated, a purpose built RC engine will give more power. Weed eater engines are designed for low RPM torque and to stay cool in the worst possible operating conditions. That means less top end and more weight.

I have converted two 23cc weed eaters to RC and they work very well. the most difficult part was to fabricate a prop adapter. Yes they are somewhat heavy but on many old style bipes, you need that extra weight up front. Unless you have a lot of time or just like to experiment, buying one of the newer commercial units will save you a lot of time and aggravation though as the cost savings of converting a weed eater is highly problematically..

ive converted acouple of engines. i liked the experience it was fun never got them on an airplane but i liked running them on test stands. on putting them on airplanes i never got time to get around to that. there are a lot of things you can do if you know more about engines to work more power out of those engines.

Harder and/or more expensive than most people are willing to do. If you just like tinkering or have a machine shop... go for it! Alot more of these get started on than ever get in the air.

Yeah, it's a little work converting a weedwacker into a usable engine for a model, but it's fun.

Most weedeaters only have a single low end needle valve carb, setting. If this is the case, you'll need to buy a new carb with both high and low end needle valve settings to get the most rpm's.

Harbor freight frequently has Weedwackers on sale, usually reconditioned, but cheap Last one I bought for conversion was around $50. Probably spent around another $100. buying the needed carb, prop shaft adaptar, and aluminum motor mount. Mine spins a 18 X 10 Dynathrust prop at 7,000 rpm.

Here's a pic of mine

http://www.rcuniverse.com/gallery/ga...6memid%3D16901

http://www.rcuniverse.com/gallery/ga...6memid%3D16901

http://www.rcuniverse.com/gallery/ga...6memid%3D16901

I've converted 6 weed whacker engines. They all worked very satisfactory for model airplanes