Small Weed Whacker or 7.0HP Lawn Mower?

Well you could go for the middle of the road and use a Harbor Freight motor,

2-1/2 HP Horizontal OHV Gas Engine

Being that you're an engineering student you could build your own gearbox and drive system.

Come up with a lefthand pullstart and a righthand drive system that would do away with a jackshaft for a shift kit.

Welcome to the forum. The weedeater motor can work if you don't mind pedaling along with the motor. If you don't like to pedal, I guess you have to use the mower motor.
However, the weedeater motor would probably be street legal, if you plan to ride on campus.

Bryc3 said:

Right now I have a 7.0HP lawn mower and a smaller weed whacker. I'm definitely a larger guy (~220lbs) and I was thinking that I should go for the lawn mower engine. Does anyone have any objections or any tips?
[...]
My main concern is that the lawn mower is overpowered and the weed whacker is underpowered.

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A seven horsepower gas engine is not overpowered in any meaningful sense of the word. But it could be excessive in all the other things that go along with it-- size, noise, vibration, weight distribution, etc. It also might move the speed of your bike into a range where either you or the bike do not operate safely.

It seems to me like a better engineering exercise to make effective use of the limited power of the smaller motor. If you have the motor drive through the bike's gears, you can do a lot with a small amount of power-- which is what bikes are for, anyway. You could end up with a bike that's road-legal in your area, and more fun to ride too, because of its light weight and small displacement.

You'd need to carefully match the motor's output speed to the bike's drive chain speed for a range of real world conditions, which is a more complex and satisfying problem to solve than just changing the orientation of the output shaft.

Chalo

There is another thing to watch out for with a vertical mower engine- mfgs put a light flywheel on most engines because the blade becomes the flywheel.
Remove the blade and you may not even be able to start the engine. It will rip the rope right out of your hands as it backfires. You need a heavy flywheel engine or a horisontal engine.
Take the blade off and see if you can get it running before you do anything.