Is the only difference between a marine alternator and automotive the screen thats inside?

Automotive alternators are open to the atmosphere and are air cooled. Most marine alternators are sealed. This is done to make them explosion proof.

mine is supposedly a marine alternator.It has screening, like a flame arrestor, in the areas that would normally be open. Are you saying a marine alternator is completely sealed in a solid casing?

No , the screen acts like a flame arrestor. The alternators still need ventilation.

I'm getting confused here. Reed say's marine alt's are sealed. Your saying they need ventilation. The fact that mine has the screen (flame arrestor) but it is not sealed make it a marine alt? Thanks and happy holidays!

Mine were the Motorola's... and they were totally sealed... I'm sure there are screened ones out there. The main thing is they have to be made explosion proof. I've seen marine alternators that have screens. They are extremely fine mesh screens. I don't see how much air can get through screens that fine.

Has anyone had a problem with alternators not charging at idle? Mine will read 10 volts @ idle but as soon as I bump up the trottle it goes to 13 volts and stays there until I come back down to idle. Is my alternator junk?

How big is your pulley? Try a smaller Pulley. You may only need to speed up things 100-200 rpm.

That's a good idea. Are different pulleys readily available?

It's hard to say since I don't know what your starting with. I know Moroso makes v-belt pulleys. Is yours serp. or v-belt? They are avaliable.