I installed a new 302 engine (bought from craigslist) it was already rebuilt with a flat tappet cam. I went with carburetor and installed a GM distributor. It ran sluggish so I thought it was the holley carb. I installed a new shelf elderbrock and it still run sluggish. It also backfires through carb. I do not know the timing for the engine and this is driving me nuts. Any help?