Front uprights together today, thanks to a welder mate and a very accurate turner friend the Olite top hat bushes fit a treat.
I have bushed the spring hinges in nylon and hope that the friction when I nip them together damps the system slightly (who knows?)

Didn't turn out too far away from the CAD model
I had one hairy moment when I set the first one up caster forward by accident and the steering arm clashed with the hinge (wrong side! Doh!) but now I have something to tie the springs together. Yay!

This week I have been mostly...

Getting the tyres and tubes on so the rims don't get too battered with all the fitting

Making the wooden chassis and gusseting it with ply (back end still to finalise now I have my motor, TAV and wheels)

Scratching my head on how to attach the back end and flukeing a discounted internal lintel at B+Q for a fiver that seems up to the job with a bit of grinding and drilling.

Grinding the lintel and wearing my cutting disc down to a nub and blowing a battery on the grinder

Scratching my head regarding the engine mounting plate and coming up with a cunning plan

Thinking hard about the spring set up

Persuading a colleague who turns to turn up some stuff for various spacers, bearing mounts

And having the occasional beer. Worked up until I get bits now - have brakes going around my head but I have a lot to do in the garden tomorrow so time to put the Kart on the back burner until I can get my hands on the drill press and a 15mm drill at work.

I just got mine delivered from Czech Republic without incurring any import tarrif for aboutŁ60... https://m.aliexpress.com/i/32729520713.html?gatewayAdapt=Pc2Msite

... To explain my logic.
You have a 6:1 ratio from TAV to jack shaft, a ratio we see as standard on the Stevenson formula.

You require a ratio of 6.5:1

 your ratio is 0.923 of the ratio you have. (6/6.5)

0.923 of 45 is somewhere between 41 and 42 (41.53)

41 or 42

The nearest thing in a honda lawnmower engine...
https://www.engine-specs.net/honda/gcv530.html
Just a tad more power than we should use :-)

Well, the expensive bit might be on the wane now. Ordered a clone Honda Gx200 and a clone TAV. These might not be the highest quality items available "on't tinternet" but it is a start, and until proven, the whole concept should not be too powerful. More gloriously sunny grinding and fitting today.

Job one.
 Finish the front upright body ready for welding and make a foolproof jig for my friend the welder.

Job two.
 Make a chuck out of scrap wood to mount the big rear cog on my wood lathe to put a keyway in to match the key on the wheel face - An epic battle of high speed steel vs high carbon steel and willpower. all I can say is it worked. It might not have met health and safety requirements of a workplace, but I am not at work! (the alternative was to remove the key from the wheel with a grinder)

Job three.
 Emery a few thousandths off the spacers on the Gemini hub/spindles to make a polished sliding fit into the 16mm rose joints (see CAD image earlier in this thread)

Job four.
 Set out centres on carriers for drilling to accept rose joints set to a 6 degree caster.

Drink a beer in sunshine and draw spacers for turning.
Another good day.

Having just got two outwardly identical front 17" pit bike wheels and stripped the bearings out, I find that they have differing bearing spaces.
Spacer tube 1 is 53.3mm long and the wheel had no dust seals
Spacer tube 2 is 42.2mm and had dust seals.

I have the Gemini 70mm stub axles (measure just over 69mm) which would indicate that the 53.3mm one plus 2 10mm bearings would be about 4mm of tightening down.

two questions.
Q1. Is 4mm of tightening down too much (or too little)?
Q.2 The seats on the hub are deeper on the one with dust seals. I intend to get as much space between the bearings as I can for better lateral load handling, do I need to shim out the depth of the seats to stop the bearings moving or is this not an issue.
Thanks for your responses in advance,
JimR