1970 Merc 50hp Thunderbolt

I have a Mercury 500 Thunderbolt that I am trying to rewire. It is a remote and I am swapping it to a tiller... I have a chunk of the wiring harness, however, it is of zero help. The wires are old, so it is hard to make out the colors, especially yellow vs orange.

My tiller conversion only has a "RUN" switch...no momentary switch for the electric starter or anything.

I can get it to crank, but I can't get it to start.

My question for the external harness:
1.) Which wires need to be constant 12V?
2.) Which wires need to be grounded?

Gray is for the choke.
Yellow is for the starter motor + solenoid.

Past that, I'm at a loss...

Re: 1970 Merc 50hp Thunderbolt


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Pictures of what I'm dealing with. I already ordered a tiller conversion.

This is the boat it is going on... a 1990 Zodiac FC470, USMC surplus:

Re: 1970 Merc 50hp Thunderbolt

You will probably find that your motor is to much for the Zodiac as there is not enough bracing for the transom for the weight and toque of the motor. It might be best to find another motor in the 25 to 30 HP range for the boat. As for wiring up the 50hp just cut off or coil up the extra wire and make a angle plate to mount the key switch and a toggle switch for the electric choke to the transom to front of the motor till you find out if the motor will work on the boat. JMO
Oldman570

Re: 1970 Merc 50hp Thunderbolt

oldman570 said:

You will probably find that your motor is to much for the Zodiac as there is not enough bracing for the transom for the weight and toque of the motor. It might be best to find another motor in the 25 to 30 HP range for the boat. As for wiring up the 50hp just cut off or coil up the extra wire and make a angle plate to mount the key switch and a toggle switch for the electric choke to the transom to front of the motor till you find out if the motor will work on the boat. JMO
Oldman570

Click to expand...

Transom is rated for a 65 hp, although since I have the rollup floor, technically I should not exceed 40 hp.

The problem being though, is I can't find out what wires I need and what ones I don't. Yellow activates the solenoid and causes the starter to crank, but I'm getting no spark... I was thinking maybe there is an IGN-CONSTANT wire I need to hook up, but I'm not sure which is? I'm coming from working on 90s automobiles & motorcycles, so I'm not exactly sure where to begin.

Motor is solid otherwise though..compression is all 145 psi +/- 5 psi.

The mercury switch on the side has a crack in it... Can I just ground that wire out to bypass the potentially broken switch?

Re: 1970 Merc 50hp Thunderbolt

The mercury switch is designed to put a ground on the ignition system if the motor is tipped/tilted up to much for the motor to pump water. If you think it is bad, then just disconnect it and see if the motor gets spark.
JMO
Oldman570

Re: 1970 Merc 50hp Thunderbolt

oldman570 said:

The mercury switch is designed to put a ground on the ignition system if the motor is tipped/tilted up to much for the motor to pump water. If you think it is bad, then just disconnect it and see if the motor gets spark.
JMO
Oldman570

Click to expand...

Still no spark.

I'm going to replace the plugs... I doubt it'll do anything, because it really sounds like I'm missing a wire somewhere...like the coil or distributor isn't getting 12V+ constant.

EDIT:
I replaced the plugs. Still no spark.

Re: 1970 Merc 50hp Thunderbolt




Here are some better pictures I took during the day...

Re: 1970 Merc 50hp Thunderbolt

The best way to find out just where all the wires go is to buy a Seloc manual Volume II for the Mercury 3 &4 cyclender motors built from 1965 to1992. On page A20 of the appendix you will find a complete wiring diagram for your motor from each component to the control. It is a cheap investment when working on any motor one wants to repair/rebuild in the long run. You should be able to get one here on Iboats or at a parts dealer near you. JMO
Oldman570