What size sprockets?

I am making a motorized bike similar to the one on youtube by davidsfarm (sorry, not allowed to post link), but I am not sure what size sprockets I should use where the 2 red circles are on the picture below. I dont want to buy new sprockets, the sprockets i have now are 14, 17, 20, 24 and 28 teeth. Should I just use the smallest sprocket (14 teeth) on the engine and the biggest sprocket (28 teeth) on the bottom part?

For the other side of the bike i going to keep the 2 sets of sprockets so i will be able to change gears.

Also, i am using a 31cc Ryobi weed wacker engine with clutch.

Thanks.

EDIT: What size gears would i need on the left side of my bike without changing the 2 sets of gears on the right side?

I am using a 24inch bike.

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I dont know if this is going to change much, but I only weigh around 50kg.

Anyone?...

Hey man. I'm new around here also but I've read alot of the previous posts. This has been covered a few times. The search feature is a great tool to use to find things out. Alright lets say that you put the 14tooth on your engine and the 28 tooth on the pedal crank. that would give you a 2 to 1 gear ratio (28tooth divided by 14tooth). lets say that your engine is at 7000rpm that would make you pedal crank gears spin at 3500rpm. This is way to high of a gear ratio. I did what your wanting to do on my bike. But in order for mine to work I have a 23 to 1 gear ratio. and to be honest it could be geared lower. I removed the pedals completely. the engine has a 10tooth sprocket going to the pedal crank which has a 48tooth. on the right side of the bike(the side that has the factory gears) i have a 10tooth sprocket on the pedal crank going to a 48tooth sprocket on the rear tire in place of the factory gears. this is with a 26inch bike.

Thanks hick, that is very helpful. I will have a look at the older posts as well.

What size gears would i need on the left side of my bike without changing the 2 sets of gears on the right side?

I will have a look around my garage if i have anymore sprockets, if not i will probably buy some.

Yeah I saw them before, but I dont really get them.

I tried the Index of /ratiocalc one, and my speed turned out 156.2 MPH.

GO DAVE!!!!!
you can use the gears but the big pedal crank sproket will have to be replace with a smaller one
10t or so just leave the sprokets on the back wheel and and do like 5:1 on the left side
like this

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^ Thanks a lot man!!!

Thanks for the help guys, but I have done some thinking and I am pretty sure that im just going to make a friction drive bike. The chain drive method is much more complicated then i thought it would be.

What size drive roller?

yeah the chain drive can take more time and planing and skill but in the end i think chain is the way to go for the longest lasting most efficient bike the very word friction implies power loss, yes friction does have it's place and i'm shure people will argue it's better