What should 273 compression be

Spark plugs are deffenitly fuel fouled I went from a 495 CFM Holley Mech sec. to a 600cfm with vacum, thinking it would be better, I'm going to take the Jets down to 59's or 60's to see if that helps

I'm sure now the Back firing is from the bad plugs at first I didn't because I kept pulling the the No 1 plug out to look at but that was the only plug firng properly so I kept think it was somthing else

I'm still looking for that comprssion number I keep hearing 140-150 is where it should be but with this low compression motor and big cam I'm think it should be somewhere around 110-120