I have installed a GM SP357 crate engine in my 1980 Z. Part # 19367080 I have an Edelbrock Performer intake w/ spread bore opening. I have a Summit billet dist, remote coil and CDI ign box. Timing set at 12 BTDC. No vacuum adv. I cannot get this engine to make more than 14-15 in of vac. I tried a couple 'used carbs' I thought were good. ( Q jet, holley 4150, holley 4165, Edel Performer) All had idle and off idle issues. Works good at 2500 rpm & above. Always erractic idle w/ run on if idle screw is in too far. Mix screws always seem to change, but can't find smooth & crisp response. Performer seemed to idle best, but had not very good top ond. Just bolted on a new QF 600 Brawler mechanical sec. ( Also I have the adapter plate for under the Holleys. ) The new QF still has same idle conditions. 4 corner idle screws all respond. Plugs stay clean. At idle it is hard to tell if it is rich or lean. Introduce a vac leak, idle changes better a little. Choke the air horn, hardly any change. Installed a 4.5 Power valve to help a low speed hiccup. (better) Just jetted to 67 from 70. Drivability is worse. Just going to jet up to 72s. (not done yet) My question is has any body used this engine and had low vac issues? Am I missing some simple? I am wondering if valve timing is off. This engine has less than 2000 miles on it. I checked 1 valve adj. (seems OK) 3/4 of turn tight after 0 lash. I have never seen this engine run smooth since install. Pulls good at RPM. Sorry for long winded story. Any input would be appreciated.