Your engine is not too tight. Are you trying to start it with jumper cables because that just doesn't cut it. You need good cables in place because you can't afford much of a voltage drop in a six volt system.
I wouldn't be putting my hand on top of the carb when trying to start it. If it backfires you could get badly burned. Normally you can open the throttle some, close the choke and let the throttle return and it will stay closed until you work the throttle. Another approach is to leave the choke for the moment, dump a few ounces of gas in the carb and then try it.
You can also give it a better spark by disconnecting the coil from the supply and powering it with a battery charger. This way the supply to the coil doesn't get affected by the heavy demand on the voltage when the starter is cranking.
