planning a build

that come later after i have the kit actualy running
Gru🐝Bee is the only current 4 stroke shift kit I know of right now; it isn't cheap either. You'll also need to remove the stock axle from your rear wheel and replace it with a cro-molly axle. The stock axle is prone to bending.
 
Is the bicycle gears a 3x7? Here's the cheapest 4 stroke kit I could find.

49cc 4-Stroke Gas Motorized Bike Engine DIY Bicycle Petrol Motor Kit Chain Drive Single Cylinder 1.4kw https://a.co/d/6islMXQ
this is looking like my best bet, 80 for shipping since it doesnt do prime but its the best ive seen yet
 
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I have been heavily considering a 4 stroke project for my next bike. As a long time 2 stroke person I have had to do a bit of research myself.

If there is any flexibility in your budget and you have the aptitude to learn, you can piece together your own setup to your needs for not much more than most quality 4 stroke kits. This will allow you to bypass some of the issues you are likely to run into with your project.

Much like the cheap 2 stroke kits, the cheap 4 stroke kits have their own brand of "tweaking" that needs to be done to really make them good. It's nothing too difficult if you have some basic mechanical skills and can use tools with some level of competence.

My personal choice for a transmission would be the dual chain drive sold by Grubee. Not just because it's more affordable, but I trust the durability of a double row chain drive, and I like the design of it too. The driven sprocket has a freewheel on it just like the belt drive kits and both the drive and driven shafts are supported by ball bearings on each side of said sprockets. It will be a it louder due to the chains and sprockets, but I can live with the cost of noise for a more durable design.
Link https://www.grubee.net/collections/4-stoke-bike-engine-items/products/dual-chain-gear-box

Your speed goals are nice and realistic, and consider this, most legal e-bikes have a throttle only top speed of 20mph, so even with a 5:1 reduction transmission you will likely at least top that, all while being able to climb hills easier. Unlike the 2 stroke kits, the forced air cooling of the 4 stroke engines also means you don't have to worry too much about overheating anywhere near as much as the 2 stroke guys.
 
I have been heavily considering a 4 stroke project for my next bike. As a long time 2 stroke person I have had to do a bit of research myself.

If there is any flexibility in your budget and you have the aptitude to learn, you can piece together your own setup to your needs for not much more than most quality 4 stroke kits. This will allow you to bypass some of the issues you are likely to run into with your project.

Much like the cheap 2 stroke kits, the cheap 4 stroke kits have their own brand of "tweaking" that needs to be done to really make them good. It's nothing too difficult if you have some basic mechanical skills and can use tools with some level of competence.

My personal choice for a transmission would be the dual chain drive sold by Grubee. Not just because it's more affordable, but I trust the durability of a double row chain drive, and I like the design of it too. The driven sprocket has a freewheel on it just like the belt drive kits and both the drive and driven shafts are supported by ball bearings on each side of said sprockets. It will be a it louder due to the chains and sprockets, but I can live with the cost of noise for a more durable design.
Link https://www.grubee.net/collections/4-stoke-bike-engine-items/products/dual-chain-gear-box

Your speed goals are nice and realistic, and consider this, most legal e-bikes have a throttle only top speed of 20mph, so even with a 5:1 reduction transmission you will likely at least top that, all while being able to climb hills easier. Unlike the 2 stroke kits, the forced air cooling of the 4 stroke engines also means you don't have to worry too much about overheating anywhere near as much as the 2 stroke guys.
its been brought to my attention that the mounting methods are different and i dont have the means to make a mounting plate for a 4 stroke if i were to start with a two stroke kit. i just need something to get me to work and back, because ive been out of work for a while and cant get a better vehicle. so i may have to suck it up and do a two stroke and mix the fuel
 
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