I have a 1984 Johnson Sea-Horse 70 HP Outboard. It idles too high. The throttle cable at the engine end is tightend all of the way down and its the lowest I can get the idle. Is there a way to adjust it at the other end (OMC). What should I do?

Random question: I know that while my engine is running it charges my cranking battery. Does my engine put out enough charge to link my trolling battery to it so it charges both? Or should I only keep my cranking battery on?

What is the idle speed and why do you think it's too fast?

As far as your batteries are concerned  be careful because the charging system can get overheated very easily especially if you were to have a battery short a cell.

I have seen people do it and get away with it but me personally I wouldn't recommend it.

The throttle cable should not be used to adjust the idle speed. I would recommend disconnecting the throttle cable before adjusting idle speed.

Before going any further with this topic, the idle speed can only be properly adjusted with the boat and motor off the trailer, sitting in the water, and the motor trimed so it's level. You CAN NOT adjust the idle with the engine on muffs, the lack of back pressure can have it idleing at 1,200 - 1,400 rpm. It should be set for and idle rpm of approx 700 rpm while in the water and in gear.

Before adjusting the idle, you need to do a full linc and sinc, starting with the carbs. They have to be adjusting so all three butterflys are fully closed and start to open at the same time. Then the cam has to be set so it's strikeing the roller at the index mark. There are screws on the throttle arm that adjust the stops for the trottle arm for the proper WOT stop and closed throttle stop to set the cam/roller contact and a screw holding the roller that you loosen to adjust the roller. Also, make sure you have a whole roller, should be 3/8" or more in diameter with a clear looking shell over a black center. If it only has the black center (approx 1/4" dia) you need to get a new roller first.

Only after the linc and sinc is right do you adjust the idle rpm. This is done by adjusting the the timing linkage going from the throttle arm to the timer base under the fly wheel. Only adjust this linkage to set idle rpm, DO NOT touch any of the other screws you have adjust set to get the throttle arm stops adjusted.

Timing is the only thing that adjust idle, if you adjust anything else, it will also throw your linc and sinc off you just work so hard to get right. Also, don't not mess with the stop under the other side of the flywheel, this is the max timing adjustment and can only be properly adjusted with the motor above 5,000 rpm and a timing light.

If you are doing it on a hose, do all your linck/sinc and then try setting the idle at 1,200 rpm. Sometimes that can get you pretty close when you take it to the lake to make final adjustmet.

How you can understand all this, GET A MANUAL.

Only after doing all this do you connect the throttle cable. Then you adjust it so it's pulling the the throttle arm WOT stop to the block, but not pressing it extremely hard against it. The throttle cable should have no affect on the idle speed you have just adjusted.

As for trying to use the motors charging system to charge your trolling motor batteries. There are several systems out there that will let you do that but your motor only has a 16 amp charging system. That's not enough to keep the cranking battery fully charged in a bass that's running it's live well pumps and other electronics, unless you're doing a lot more riding than fishing. I would not waste the time of money trying to use it to charge my TM batteries also, it's not going to have enough juice to do that.

what kind of manual? i have the manual for my mercury but it does not mention all of this.

That because his is an OMC, different system.  Also, some of that info was from more than just a few years experience and may not be in the manual.  Many times manuals assume you know the basics of what you're doing and only provide the technical aspect of the specific job.