Just finished a partial B18 engine rebuild and I am having a strange problem I need assistance with. I just put in new bearings, rings rebuilt head, timing gears clutch etc etc. Started the engine but it is intermitantly backfiring and lacks power. I re checked timing, valve clearance ,points, plugs, carbs...everything seems fine....most of the time the engine runs smoothly but then it will sputter and backfire, especially when I try to throttle it up....I am at a loss ?? I checked the timing marks on the new gears twice, so I can't imagine it's that, but I am getting ready to pull the timing cover off again to check. A vacuum gauge says late ign timing, but is steady...Anyone have any thoughts ???