Help needed, not sure if Speedfight 4 is standard

Hi All

Recently bought a Speedfight 4 aircooled 50

Previuos owner has messed about with it and now it seems to be heat seizing when I test it.

main jet was 60, carb is 17.5. - I have tried main jet at 64, then 70 and engine still heat seizes

Standard exhaust with welded up holes

Not sure what to look at next

 

Do a plug chop? How does it run at just under full throttle? Like at 7/8ths throttle?

 

I am concerned that something is not quite right, as I should only be going up main jet sizes around 8% - 10% with no other mods, and at a 70 now that is about 17%, is a 17.5 carb standard?

I have been looking at it again and there are some air bubbles in the clear tube that supplies oil to the carb, could this be the problem?

I am getting a mini sieze as the oil feed hits a bubble?

I have taken pipe off at carb end and pointed up higher than pump, but the bubbles are not moving, so still trapped in the clear pipe.

I cannot find anything online on how to bleed this pipe- anyone know how?

 

I think the oil pump is electric controlled from engine rpm. Run some extra oil in with the petrol and see how it goes.
Could be an airleak around the carb running it lean?