Junior's first Briggs bike

The gearing down, seems to be approximately what I have going on my early builds. Don’t think it will patch out, but will go quite a good clip maybe 25-30mph. I used to run one of these it in wooded area up a 100ft run up to the base of a hill. Up the hill about 10% grade to maybe 20ft up and the angle of the lip made for the front wheel off the ground by about 2ft. The rear wheel about ½ ft with forward fly about 2 to 3 ft. What a rive.
I have a Briggs 3 or 3.5hp not sure exactly. I have an old frame that barely allowed me to fit the engine in a position just inside the 3 tube sweet spot of the frame (i.e. seat post to crank tube/crank to fork tube/fork to seat post tube). I welded a platform that has the engine more centered by weight than by volume in its position.
I have in the past made two motor bikes before with a 2hp and this current size engine 3hp and had it off to the side to avoid the need for the jack shaft. Only at slow speeds is it a bit trying to fall to the side, but once moving at over maybe 5-10mph you really don’t notice it. Wheel gyro effect maybe takes care of that.
I used on this current build and as before, a washing machine pulley mounted to the rear wheel spokes is what is used. I am a little more particular this time for the way it is attached to the spokes not bending tabs that are part of the washing machine pulley to make it happen. This has me now needing to make shims between the clamping to the spokes.
If I had known earlier that clamping near the hub rather than half way out from the hub to the rim would essentially remove the need to shim at all and I would have done that. Even more, I would have probably for about $12 and shipping get the ones used on the motor bike kits and welded some metal to make the washing machine pulley compatible with the replacement part they sell.
I am guessing that since before I used the washing machine pulley that is the large one, maybe 12 inch diameter and had no extra gearing down back then when the wheels were 24inch, currently now on a 26inch I guestamate the same ratio.
I have now a little more gearing down since from the engine to a mini bike clutch sprocket about 2 inch, then the jack shaft is used with maybe a 3inch sprocket and 2 inch pulley on to the 12 inch washing machine pulley at the hub, but the catch is now using 26inch wheel. So I am expecting it to be a wash.
Counting teeth on the sprockets and exact measures for the pulleys with math number crunching, is the correct way I know, but I am using what parts I have from the trash an as little as possible new. I can get a larger sprocket on the jack shaft if I need more gearing down.
I though of centering if you think maybe you would try to change to centered weighted engine placement, but I can see that the frame does not have the space. I still want you to know that I have used motor bikes like this and they do work. Aside from slow speed having to be careful of lean of motor weight, my right leg was out to the side since the engine keeps foot pegs from being symmetrical.
Just for thought I’d mention, I as others too thought of to allow the tight fit of engine on the frame, is to use of an elbow from the intake and maybe also the exhaust ports to avoid conflict routing around seat post tube. I even am using a non original part from another brand with my adaption plates to make the elbow to a bowl card. Now with that I am using an actual lawn mower gas tank up above on the frame tube. It may looks a little strange since a nice design tank looking like a motor cycle would be better, but it should work.
One other adaption plate I had to make since this bowl carb has a very small hole that is at the mating gasket surfaces between carb air intake and the air filter on the original filter. I don’t have that filter and am using another. And what the heck is that for anyway you might ask. It is for where it routes the primer bulb air pressure which is built into the original air filter design. I took another old carb that was not useful since fuel inlet leaked; I am using the still good primer bulb to run a hose from to my specially made adapter plate that make the connection to prime the new actual working carb. Call it Rube Goldberg, but if it works I’ll chalk one up to having said I told you so, it works?
My current build, I am thinking that the banana seat I put on will allow me to sit further back and not have to straddle the engine awkwardly as much. The seat height is another matter that for clearance of the rear wheel, the foot pegs I have already added may mean I could have trouble touching the ground at a stop. If I can slide forward when stopping and visa versa to get to be stable, I will see. Otherwise, this will have to rethink the seating.
A side car is something I have in mind as an add on. I must see this motor bike first without that as I am looking to get State sticker of OHV for motor cycle parks where I could ride legally. I have already welded covers over where the pedal crank is in the frame tubing to keep it dry.

Measure Twice