I have a 75 Mercury that I am having some problems with. Sometimes it starts and runs perfect then I may stop and fish for a while and then it won't start. What may be causing this? Is there anything that I can try before taking it to the shop?

These guys on here are going to need more info to help you.
what have you tried?
is the motor turning over and not fireing?
be very specific in your question.

It will turn over all day. The battery is fully charged and new. It will just now crank sometimes and sometimes it will. I usually will raise the neutral idle and that is when it starts best but now it won't even doing that.

Take a can of starter fluid and spray some in the intake, if it tries to start you got fuel issues, if it doesn't start, you got electrical issues.

DITO

Whoa there on the starting fluid. It's a quick way to smoke a two stroke. The ether really wipes the lubrication off of everything. Do a spark test. Either purchase an in line tester or be cheap and just ground the plugs against the engine and see if the spark jumps the gap. If you have spark then you know it is a fuel delivery issue. How old is the engine, is it carbed of EFI?

You need to be more specific in the motor type and the problem. There are a gadzillion reasons a motor will crank but not start. There are a lot of things you can try and spend your money on or you can just be more specific and head down the correct path or just take it the service shop.

1. Your carb(s) could need to be cleaned.
2. Choke may not be working properly.
3. Spark plugs could be bad.
4. Stator could be going out.
5. Power pack could be going out.
6. Is your starter engaging the fly wheel?
7. Timing could be off.
8. We had a boat come into my mechanic's boat shop (I help out there from time to time) and the problem was the piece of plastic connecting the 3 carbs was broken, not allowing the bottom two carbs to function properly.
9. Bad fuel. Bad fuel filter.

if you have to raise high idel to start it up, its a carb/fuel thing.

clean the fuel pump screen, filters, and try cleaning carbs first.

My older 70 did this and I rebuilt carbs......

It is a 84 Mercury 70HP. If it is a carb issue, what does it usually run to get these fixed?

The number one problem with cranky carbed Mercs usually goes back to the fuel pump. You have a mechanical fuel pump. Fuel is pumped by means of diaphragms. Once they get stretched or pin holes in them, they don't pump fuel. As one of these pumps go out what you usually see is that the motor starts fine first thing. After that once the engine gets warm, the diaphragms get stretchy and it is hard starting for the rest of the day.

If you have never rebuilt the fuel pump, they would be my first engine related suspect after fuel line, filter and primer bulb issues have been eliminated.

Have you replaced the fuel lines yet my 84 Evinrude alcohol gas made old lines crumble and fill carbs with trash

Power Pack...that is where I would look