I have a YS 120 and an OS 91 4-stroke that can be a bit hard to turn over with an electric starter, but here are some tips that help get em going with no problems.

first of all, prime your engine. WITHOUT the glow drive on the motor, open your throttle wide open. Cover the carb intake with your thumb, and hand flip the prop until you see fuel being pulled into the carb. then return your throttle to idle. doing this will mean that you have to turn over your motor few times with the electric before it starts. I find that once my OS motors are broken in, I usually don't have to use the electric starter on them. once they are primed I can flip start them with a chicken stick.

As stated above. back off (clockwise) your prop to just before the compression. this gives you a bit of a "running start" before the starter hits the compression.

You might want to practice this next one a bit, but it makes all the difference in the world when getting a bigger motor (or a new motor that is still a bit stiff) started. hold the starter about an inch away from the nose cone, and then turn on the starter. once it's up to full speed firmly place it on the nose cone (this is where the practice comes into play). you'll get a lot more power to turn over the motor this way.

the last tip came from my YS 120 motor. My starter could barely turn it over when my field box battery was any lower than a full charge. solution?? get a bigger battery. I replaced the battery in my field box with a lawn mower battery. You can get a 12v lawn mower batter at wal-mart for about $19. It's a sealed battery, so you don't have to worry about leaks. You will have to by a new battery charger, one designed for charging auto/marine 12v batteries. I don't remember the cost because I already had one for my boat. Once I put the mower battery in my box the starter will turn over and crank my YS quite nicely.

hope some of this helps