Bit confused here. First you say it is a 1GM, then a YSE12. although both are Yanmars they are totally different engines (and pumps.Pal of mine has 1GM ... he had poor starting and quite often it would 'rebound' and fire ... when it ran - it was 'lumpy' - but as far as I know it didn't run backwards.
He is a 'tinkerer' and decided to have a go at it ... he found that the strange little pump gadget was only really working partly ... it seems to be a particular design only on Yanmar .... he tried getting it to pump ... but failed.
I posted about this part on here quite a while back ....
Anyone have an injection pump - YSE12
In the end after he'd dismantled the engine ... and then had no solution - he found a second hand engine for LESS than the cost of a new pump !!
Fitted it and he's now very happy.