I reckon it's the head gasket

[terranosaurusdoug]

It doesn't run too bad with the radiator cap removed, but with it tightened it runs rough, lacks power and kind of feels like it's missing.
https://youtu.be/xIfbG_p0h5Y

 

[solarman216]

Yep think I will go head gasket too, Rick

 

[terranosaurusdoug]

I'll be taking the head off soon to have a look