This is what your timing sprocket looks like, it's got that little nub made as part of the sprocket. When the pulley goes back on, that little nub has to go into a keyway in the pulley. Then, the pulley needs to be tight enough to pinch the gear to keep it from moving. If the pulley isn't torqued, that little nub will eventually shear off. (ask me how i know) That sprocket being properly assembled to the crank pulley determines the cam timing in relation to the crank.
EDIT: it just hit me, i
think that little nub fits into a keyway in the crank, not the pulley. The sprocket faces the opposite direction from what i was thinking. Either way, if it were assembled with things out of place, it would let your cam timing be off, which would cause the low compression. I could be barking up the wrong tree though
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