ever cupped your hand over the 4 trumpets? i did just after the rebuild and heres what happened..
T1. sucks palm in, engine labours
T2. sucks palm in, engine labours
T3. sucks palm in, engine labours
T4. gentle suction on palm, pulsing effect.
As you will know, this is piston 4 that has taken whacks on both blow-ups. You can imagine my heads screaming "low compression...low compression...low compression" I know its not strictly compression but there should be a certain amount of force even on the induction cylce.
Anyway, i thought i'd leave it for a couple of weeks, see what the plugs are like, expecting oil plug on pot 4 if something is amiss. Checked them today and its a nice browny grey buff colour, cool. Then while i was standing there it hit me, the vacuum pipe for the brake servo is into the side of throttle body 4. So i presume that its sucking air down there when i cup the trumpet. I also presume that the non return valve must be shafted, or that the induction pulse is stronger than the valve.
any thoughts ?