I have a '64 Falcon 'Vert with a 260. Had a couple of bad guides and decided to upgrade the heads instead of repairing the 260 heads. I have a 351W block I'm going to build for it eventually, so I went with '69 4V 351W heads. Also installed a new Power Plus Cyclone intake and new 1.6 roller-tips. (had 1.6 roller-tips on 260 heads with screw-in-studs but had to go to the rail type). The problem: Now I get a constant backfire through the carb. I've checked the firing order repeatedly and its correct, timing adjustments don't help. Restabbed the dist 180 with no luck. At idle its just a small pop, pop, pop. I'm wondering if it could be the intake not sealed properly? Had a similar problem with an aluminum intake on the 350 (yuk) in my boat.
Any help is greatly appreciated!