70's Mercury thunderbolt wiring upgrades?

ShoestringMariner

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Apr 18, 2015
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So I'm debating on trying to keep my 1976 Merc 650 going for a few more years. I replaced the lower unit and water pump last year and it ran great by the end of the season. It's got 130psi on all three cyls. My mechanic told me it is on borrowed time due to the wiring issues that plague these motors and that the trigger (distributor?) cannot be rewired leaving me with the option of scrapping it or looking for one on Ebay when it goes.

I'm considering a repower but funds are tight this year. I'd like to save enough so I can get a motor in the 2000's complete with PTT as opposed to a gap-stop motor.

Because I'm concerned with being stranded and worse, having a fire, I'm wondering what it would take to freshen up the wiring. My mechanic claims that the wiring on these is very fussy in regards to gauge and has to be as close to the original as possible. I dont really understand this to be honest. So as a stop-gap freshen up, could I not simply clad all the existing wiring in heat shrink tubing? or is the problem with brittle wires or corrosion within the factory wire insulation?
 

Chris1956

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Mar 25, 2004
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usually the insulation is the bad part of the wiring harness on older Mercs. The conductors are usually marine-grade, tinned copper, which could still be good on your motor. So shrink tube could work, but will be tedious and you will need to cut off the ring terminals and then solder on some new ones.

I would recommend you do an internet search and see what kind of price you can get on a new wiring harness. I expect you to have several choices of source. I bought a brand new wiring harness for in inline 6 Merc for less than $100, several years ago.
 

GA_Boater

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Give a serial number and the exact CDI harness can be ID'ed for the search. I think they go for about $130 today.
 

merc850

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Jul 7, 2010
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I'm surprized he didn't mention the reed valves! Take the cowling off and assess the wiring: I bought a CDI harness for my 850 - the wires are of a smaller gage and it has worked for 7 yrs.
 

ShoestringMariner

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I'm surprized he didn't mention the reed valves! Take the cowling off and assess the wiring: I bought a CDI harness for my 850 - the wires are of a smaller gage and it has worked for 7 yrs.

Heres where I’m wondering if I can get a similar extended lifespan out of mine.

What is is it about the reed valves, should I change them?
 

racerone

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Dec 28, 2013
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Not sure why a mechanic would say that wire size has to be close to original.-----Bigger is usually better when it comes to wires.
 
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