ATC 70 backfires through carb?

[GTO]

I rebuilt an ATC 70 engine with a new Chinese top end kit.
Engine I have in a pit bike, pit bike engine was used in my ATC 70 some years ago.
I had spark issues at first but it turned out to be faulty condenser and stator coil I had rewound.
Bike runs but when throttle is opened it back fires though the carb and dies(sometimes).
If choke is partial closed its some what better.
Carb was rebuilt with a new carb kit, fuel tap on carb was stuffed but seeing that pit has one on the tank I sealed up the tap with epoxy and drill a hole on the inside.
I thought it might be a flow issue but when i remove the bowl is just over half full.
If I just put the bowl on to the tap from the tank sometimes it struggles purge the air, if I hit a flat hand on the to of the tank opening it purges the air and there is decent flow.
Ignition timing I set with a light bulb and its slightly retarded.
Valve timing was difficult as Chinese head didn’t have a decent timing mark on it!, but cam sprocket seems in the centre.
Is there a way to check the timing with out marks?
Also the first two or so starts it seemed okay but has steadily go worse.

[ps2fixer]

Maybe the epoxy wasn't gas safe and is swelling up or restricting the flow somehow? The gas level in the float sounds to be the cause of the poor running (lean condition i guess based on the choke making it slightly better). Maybe the air vent is plugged so the carb can't take in more fuel, or the float level needs to be adjusted. Don't know a whole lot about the ATC70 carbs though.

Good luck on the diag work.

[GTO]

Float is completely plastic so it cannot be adjusted, vent in the pit bikes gas cap wasn't drilled right through!
But opening that up made no difference.
Epoxy is gas safe, also if I blow through the pipe its okay although it does maybe feel slightly restricted.
Another option is to buy a cheap carb on eBay, but I see all the Chinese ones have tap and choke on right hand side?
With the pit bike exhaust up top access is a bit restricted.

[Tri-Z 250]

Cam timing issue, go through TDC again, valves cam to piston. Sounds like something is off by a hair possibly the cam to valves. One side of the valves could be open too soon or long just a guess, not hearing it run.

[GTO]

I checked the fuel level in carb last night by attaching a clear hose to the drain.
Fuel comes up to +/- 5mm above the join between bowl and top section of carb.
This seems to high to me?
I adjusted the jet needle clip to lean it out and there was a vast improvement, its ride able and back fired only very occasionally(on fast wide opening of the throttle)
So I raised the clip to the last slot and no more back firing.
Plastic float is all plastic so no way to adjust it.
Maybe I should by a cheap carb on eBay?