check for cracks in the inlet manifolds too, it might be letting air in somewhere. do you have a compressoin checker? when I did my xj motor, I inadvertantly skipped a tooth on the exhaust, it started but would rev over 5 k. luckily no damage was done. I would double check timing and everything to get a base line, then have another look at those carbs as mikunis have some real small holes and needs a ultrasonic bath to get rid of detrious.