... A mate of mine suggested, making sure the mags are off and giving the prop a pull through to check each cylinders compression ...
Originally Posted by
Tarq57
No point in pulling the prop through to check compression.
Yes there
is a point, I regularly pull through my engine (90hp) before start - always before the first start of the day when all is cold and quite often otherwise.
What are the advantages? First, if you do it regularly, it
is possible to detect a difference in compression between pots. This is not through a "calibrated wrist" but simply because each cylinder can be compared with the previous and following cylinders. You will not find a small difference but a significant loss of compression will be obvious, whereas in flight it may not be noticeable. Second, if you have a stuck valve - or exhaust valve leak - you will hear this through the exhaust as you pull through. You also are likely to hear air moving/hissing within, if you have (say) a broken piston ring. None of these may be detectable either during run-up or static checks, or in normal flight. You will also hear the "click" of the impulse mag(s) as the engine turns.
Furthermore, if you prime before you pull through, you will be "sucking in" a fuel/air charge to each cylinder which makes the work of the starter (whether it be electric or Armstrong) easier, as the start becomes almost immediate, saving cranking the engine before start, which not only wears the starter and engage assembly but also can deplete the battery. Following this procedure, my engine will routinely fire on the first turn with switches ON. If it doesn't, that is usually indicative of another problem, like oiled plug(s).
There are plenty of advantages to a routine manual "pull through" of a small piston engine before start - don't let anyone tell you otherwise - although once you get much above 100hp, the effort required will make it progressively harder work.
JD
