Re: 156 starting and idling fault
its obviously not sealing properly mate.
Low comp on all four is a huge leak somewhere - is the gasket deformed or slightly different to your old one? since the cylinders are sleeved and seperate from each other it would have to be a mating fault between the head &/or gasket and the top of the block.
Did you torque the head down in the correct rotation and stages? If you dont follow the staggered pattern of which bolts to tighten, you can sometimes fail to achieve a seal between head & block.
If in doubt, start with the middle most top left-of-centre then bottom right-of-centre, then top right, bottom left working outwards from the centre to the outer bolts in a diagonal criss cross way.
This is so the head gets strapped down in the middle first, so no warping. Ever tried to hang wallpaper? If you put the ends on first the middle is always saggy....lol
Recheck and tighten the head bolts once more, especially if you warm it up first then do the bolts when its warm, youll get a better seal. Also when changing a head, or refitting it, new bolts should be used as the old ones will have stretched over time and are weaker.
Me & Her...
I love my tractor so much, If it wasn't an abomination o' god's law, I'd make it my wife...