Remove one plug and check the compression, now do the other side. Is there a lot of difference between them? If "Everything" is adjusted right, the compression should not be that far off. Who did the repair? Try downloading the manual from Horizon and check the timing yourself (if you haven't already). Are the carbs clear of dirt/debris? Open both needles about 2 turns and try starting it. BTW, how are the plugs? If in doubt, install 2 new OS "F" plugs.