Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

[SeaKaye12]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

Check the compression first. If it's OK then I would advise that you spend the bucks for the CDI internal Harness. Sure; you can make up your own wire by wire....but you would end up with a lot of splices down by the plug, and there's not much room down there.

The other issue that you will have is the wiring going to the trigger and the stator. Those are not included in the basic harness and they have a way of breaking down way up at the top; under the flywheel. Can be a nasty issue; on mine I ended up replacing the trigger and stator mainly because the wiring molded into them was shot with no way to deal with it.

Good luck; be sure that your carbs are clean. Lean mixtures can ruin your engine due to inadequate lubrication.

 

[Merc-Moron]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

Hey SeaKaye12 - thanks very much for the advice! One question which may seem silly. The manual states I should warm up the engine for a while before i do a compression test. But if I can't get a spark, I can't really warm it up?

Any thoughts on that? Should I just test the compression anyway?

 

[eclipseturbors]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

I'm in the same boat( no pun intended) you are with an older 850 but with the upside down distributer type engine with a whole bunch of corroded wires. Yes, do the compression test. I used an older rip cord laying around and was able to generate roughly 110 psi per cylinder. You are looking for about a 10% difference cylinder to cylinder. With the engine warm, the numbers will be higher and will more likely match desired numbers in any manual. As long as they are even across all cylinders its most likely OK. If you have a cylinder that is much lower than the others, add a capful of oil or fog the cylinder liberally and try again. If the number jumps up significantly, most likely you have a ring problem and should start to consider other plans. Hope this helps. There are people here who are much more knowledgable about these engines than myself and probably have a much better answer.

 

[chum1]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

Egads! another corroded 850 on the deferred maint. program!!!, the people at the shop should have given you fair warning on the cost of labor and parts for this, there available but spendy. Be prepared for one heck of a learning curve if you've never worked on an outboard and plan on getting this back to a reliable state, good luck and have fun, do a compression if you haven't already.

 

[JohnUtah]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

Same year Engine, Same problem. I know my motor has never been dunked, it's just an issue with older Mercs, and if they have sat outside a lot. It is a great motor, I have never had any other real problems with mine, so I ordered the cdi harness. I am waiting for it to arrive. i just rebuilt the water pump, case gaskets the whole pump. changed my gear lube(always use marine, so if water finds its way in it will not milkshake as fast). carbs adjusted a few years back. I am in this motor for less than 1k, I have had it 5 years , and she does a great job. If you compare the prices on a newer motor, and the long life the old mercs will give you with proper maintenance, i think you will be happy spending a few hundred to get running good. Like everyone else pointed out just make sure she has good compression before you start handing out the credit card...lol

 

[turtles11756]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

corroded wires is common . i have a nearly brand new 1974' 115 hp and the rectifier wires from stator have cracked. start with a compression gauge and check the cylinders to make sure you have a sound motor .all that work and they never checked the compression ? if it has strong even compression it's worth the effort fixing the wires. get a merc service manual (ebay) take your time you won't do this overnight

 

[ivanator]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

hi i have a 1969 80hp thunderbolt with the same problem but mine is all wires at the engine.. i need a new whire harness for the outbord..
does any one know were i can get one?????
thnak to all

 

[ricohman]

Re: Mercury Thunderbolt 850 Blue Stripe 1976 85hp Green Corroded Wires (Photos)

My merc suffers from the same thing. I've replaced a few wires but really I should buy the harness and be done with it.