All engine timing is off top dead center of the number 1 piston. If you are at TDC the keyway for the crank gear should point straight up. When the gear is put on, if a dot exists, it should be straight up. If it's a multi keyed gear there will be a number above the slot to indicate how much advance/retard that position will render when used. When the cam is in the right position in reference to the crank, #1 intake and exhaust should be closed and the lifters on the base circle of the cam lobes for those particular valves. Then, and only then, can the chain and sprockets be installed. Slots in either gears allow for slight advance or retarding of the timing between the crank and cam and I would suspect that either a gear is put on backwards or the timing marks are not at 0 position. This would be because you've used a key slot in the gear that is for more advance or retarding of cam position. You must leave the crank at TDC and not move it when you apply the timing chain. Only the cam is to be moved. Examine your gears closely for their markings and positions and redo the operation again. If it's not right you'll run out of distributor adjustment, be subject to farts out the carb and generally have a poor running engine, if it fires at all. Use your original gear to verify the tooth position on the new gear for TDC