'75 Mercury 850 Thunderbolt 85HP

[his1911]

OK we ( being wife and myself) just bought a project boat, and I'm not really up to speed on outboard motors so I'm looking for really basic newbie advice to a lot of these questions!
1)Is this motor a 2 stroke ( seems like it would be I haven't found an oilpan so to speak or an injection reservoir)
2) The spark plugs are flat ended affairs, are these available from a parts store or do I have to look for specialty boat/marine shops???
3) Lower unit oil is what weight range oil??
4) Someone please school my on props and pitch!
5) Whats the difference between Marine grade(tinned I believe) wire and regular primary wire ( just for info)
6) Wiring for sensors....this boat has no gauges of any kind so where would you find or how would you install tilt sensors, temp sensor, tach etc?????

Thanks in advance guys, and for piece of mind I'm not a newbie to mechanical affairs, I'm qualified diesel mechanic and a Industrial Maint technician , I simply have never messed with outboards before.

 

[dub1620]

Re: '75 Mercury 850 Thunderbolt 85HP

purchase a seloc manual.

the plugs can be found at a normal suto store


It is a 2 stroke with probably a 50:1 ratio


the tach /temp/ other sensors should be relatively easy to install, given you have the right hookups.

 

[Chris1956]

Re: '75 Mercury 850 Thunderbolt 85HP

Motor is indeed 2 cycle. Mix gas/oil at 50::1 ratio (1 pt oil per 6 gal regular gas). That motor has no thermostat, and so a temp gauge is useless. A better choice is a water pressure gauge, however, the presure is very low, naturally. Rebuild the waterpump with all new rubber and gasket parts.

That motor has a distributor ignition, driven by the battery. Check the wires as a lot have insulation falling off. Replace if necessary. Motor has stator/rectifier charging system (unregulated) of about 9Amps.

Spark plugs are avail at most auto parts stores, since they are very common. Take a compression test and spark test on all 4 cylinders and post results. Carbs are likely dirty, but are pretty simple to clean.

Gearcase takes 25oz of Marine Gear Oil for mechanical shift motors. Use the marine stuff. Grease the zercs with multipurpose grease. Grease goes on the propshaft splines each year so the prop doesn't get frozen on. Props are graded by pitch. If you post the boat type, length and weight, we can guess at the right prop.

I recommend a Merc Service Manual.

 

[SnowHunter]

Re: '75 Mercury 850 Thunderbolt 85HP

Motor is indeed 2 cycle. Mix gas/oil at 50::1 ratio (1 pt oil per 6 gal regular gas). That motor has no thermostat, and so a temp gauge is useless. A better choice is a water pressure gauge, however, the presure is very low, naturally. Rebuild the waterpump with all new rubber and gasket parts.

That motor has a distributor ignition, driven by the battery. Check the wires as a lot have insulation falling off. Replace if necessary. Motor has stator/rectifier charging system (unregulated) of about 9Amps.

Spark plugs are avail at most auto parts stores, since they are very common. Take a compression test and spark test on all 4 cylinders and post results. Carbs are likely dirty, but are pretty simple to clean.

Gearcase takes 25oz of Marine Gear Oil for mechanical shift motors. Use the marine stuff. Grease the zercs with multipurpose grease. Grease goes on the propshaft splines each year so the prop doesn't get frozen on. Props are graded by pitch. If you post the boat type, length and weight, we can guess at the right prop.

I recommend a Merc Service Manual.

Thanks for the info!!! (I'm his1911's other half ) I'll be sure he see's this tomorrow.

So, there's no way to get any kind of thermostat..hmmmm...How would we tell if the motor is running hot, etc? *shrug* more my concern, since I'd probably have the boat out more then Hubs.

The boat is a 77 Bayliner Runaway 15'6" (give or take an inch) weight would be 850ish? (Can't find squat for specs on boat) I don't think this motor will be run on this boat. Boat is only rated for 70hp...so we're a bit over with the 85, but thats what it came with...so its what we have.

Hopin, really, to get the motor runnin and trade it on a lower HP OB...

 

[achris]

Re: '75 Mercury 850 Thunderbolt 85HP

Thanks for the info!!! (I'm his1911's other half ) I'll be sure he see's this tomorrow.

So, there's no way to get any kind of thermostat..hmmmm...How would we tell if the motor is running hot, etc? *shrug* more my concern, since I'd probably have the boat out more then Hubs.

At the back of the engine is a stream of water, just have a glance at that every now and again. Look at it when you start up too...

The boat is a 77 Bayliner Runaway 15'6" (give or take an inch) weight would be 850ish? (Can't find squat for specs on boat) I don't think this motor will be run on this boat. Boat is only rated for 70hp...so we're a bit over with the 85, but thats what it came with...so its what we have.

Hopin, really, to get the motor runnin and trade it on a lower HP OB...

Just because the engine is 85hp doesn't mean you have to drive all day at full noise. Having more power is better than not enough.... I've seen a few people come to some serious grief because of a lack of power in a hairy situation.

As for props... It's actually not too difficult. The pitch is the important number. That's expressed in inches, and is the number of inches the prop would move forward in one revolution without any slip, so it's a 'theoretical' number. It's also the one that has the most influence on the engine and boat performance. Too much pitch and the engine is overloaded (and that's at ALL revs, not just full throttle), too little and there is the risk of over revving the engine.

Gauges... Those engines usually came with an array of one gauge, a tacho. The only other gauge worth considering is a water PRESSURE gauge, not temperature.

The spark plugs are a little bit special. They are 'surface gap' plugs, not separate ground electrode, so don't be surprised when you see them. I would recommend using NGK BUHW plugs. Some auto shops have them, some don't. They look like this...

One last thing.... DON'T buy a seloc or clymer manual.... Complete waste of money. Spend the extra (or go to e-bay) and get a genuine Merc manual... The manual you're looking for is 90-75512.

Good luck and happy boating...

Chris.....

 

[his1911]

Re: '75 Mercury 850 Thunderbolt 85HP

Thanks guys that was the kind of information I was looking for I'll have to check into the parts store locally and Ill go cruise ebay looking for the manual from mercury!
Nathan

 

[ClassyGlassy]

Re: '75 Mercury 850 Thunderbolt 85HP

Go here for the manual and any other parts you need: http://www.marinepartsman.com/

 

[GHP]

Re: '75 Mercury 850 Thunderbolt 85HP

HI everyone, I am looking a manual about of Thunderbolt 850 motor, where can I found it? Thanks