1979 70hp no-power need wiring diagram of switch box

I am having a problem with my 1978 70 hp merc that sounds similar an earlier posting. I bought it "as is" for a 16.5' sylvan fiberglass bowrider. I checked the compression and all cylinders were ~165psi. I first had no power and very poor runability and I was afraid it was just too small for the boat. After some investigation I found my fuel pump was bad. After rebuilding the fuel pump, It appeared to run OK but the boat would come up on plane -it just plowed water at ~12mph. I suspected the prop and changed from a 14p to 12p --better but with 3 adults it still woundn't plane-just plow to ~14mph. With just my son and I on board it will acually plane and go ~26mph. I do not have a tac but the motor doesn't sound like it out of range for rpms. After some checking I found a formula for predicting boat speed and with the weight estimates I had it suggested a speed of ~31-32mph. I worked the numbers around and the formula said a 48hp motor would give me the speed I was getting. Thats when I read about an earlier post about the switch box causing problems.I looked over my motor a little better. In looking over the switch box II fould the orange wire removed from the 2nd position marked orange and connected with the blue wire on the 3rd position marked blue.. The post for the orange wire appears to be dead. I bought a repair manual on CD from E-bay but it is worthless. I t has no wire diagrams-none! If anyone could spare a copy of the wiring diagram so I can find were the orange wire goes and if it being paired up with the blue wire would have anything to do with the motors lack of power I wiould be very thankful. It idles smooth and appears to run good when it can rev up. I have power tilt with 1 cylinder leaking slightly so although it wouldn't hold position completely I was able to move it to help with bringing the nose down.

My e-mail is bsox56@verizon.net

thanks,
Barry

Re: 1979 70hp no-power need wiring diagram of switch box

If your plowong through the water, it sounds like you have to adjust your tilt. Get a real Mercury manual. For the $50 or so, you will be much better off. You might want to try a link and sync. the orange wire comes from a mercury switch which kills the engine if it is tilted up too far. If you disconnect it all together fornow your motor will run. You might not stop however if you turn the key off.

Re: 1979 70hp no-power need wiring diagram of switch box

I have the same motor on a 15' Winner boat and it has no issues of reaching 38mph.
i have a 12"x11pitch prop on there but it is underproped for most use.
it was set up for waterskiers by the previous owner.
i would expect at least 30mph

Re: 1979 70hp no-power need wiring diagram of switch box

I was able to adjust the tilt but that only helped a weak attempt at trying to bring boat up on plane. I did find the white/black wire frayed go ing to the top of the switch box. Repaired the wire but withot a wiring diagram I have no way of knowing were it goes.
I checked to see if all spark plugs were firing at idle and they were. I ran the rpm's up in neutrol and all 3 worked there also.

Please someone help me with a copy of a 1978 3 cylinder merc wiring diagram (switch box area mainly)

Re: 1979 70hp no-power need wiring diagram of switch box

The posts on the switch box whwere the wires you are questioning ( horizontal ) go like this:
1) White /black
2) Orange
3) Blue
4) Red.
Vertical From top:
Violet, Green/red, white, green/white, Brown, Green.
There should ge three blacks wires going from switch box to coils.

Re: 1979 70hp no-power need wiring diagram of switch box

Here it is:

Re: 1979 70hp no-power need wiring diagram of switch box

Thanks Rick for the sketch. The switch box has labels so I know were they belong bu what I don't know is were they go after they leave the switch box.

Up Date:
I took the boat out today 2 adult 2 kids. As usual poor speed able to get to 20 mph after 5 min of plowing. While at WOT I pulled of each spark plug wire and each one noticably dropped the rpms so each on is working. I looked in each of the 2 carbs and could see gas spraying and throttle plates were full open. Why is this motor so wimpy? What am I missing?

Re: 1979 70hp no-power need wiring diagram of switch box

Start here. http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=36;gtid=1114512 then get a tach and check RPM. You might need a lower pitch prop.

Re: 1979 70hp no-power need wiring diagram of switch box

Red and Blue go to the stator. white/black, white, brown, and violet go to the trigger. Ggeen/red, green/white, and green go to the ignition coils. The black wires are the ground for the coils.

Re: 1979 70hp no-power need wiring diagram of switch box

Thanks Rick,
Do you think connecting the orange wire to the post the blue wire is on will hurt anything. If I put the orange wire were it belongs the motor will not shut of with the switch but will when connected on the post with the blue wire. I think the orange just grounds the system to shut it down. I've gone from a 14p to a 12p with slight improvement but it still isn't good enough. The boat weighs 1100lbs and I have only carried the minimum extras. I remember as a kid boats bigger than this with 55hp moving along OK. I read about decarbing--Any thoughts on that.

Thanks,
Barry

Re: 1979 70hp no-power need wiring diagram of switch box

Can't hurt. You can do a carb rebuild or take them off and clean them. You should get a tach and check your RPM. Plus get a manual.