merc 850 w/thunderbolt ignition

[BaDucK]

im fixing to get started on getting a merc 850 ready for the season....

i traced the numbers back and found out its a '73 model, but cant really find any good info on the engine itself....

just wondering if anyone (or some of you great mechanics) have any experience with this motor....know of any quirks/good points ? about all ive been able to really put together is that its supposed to be a b**ch to cold start, and its referred to as a "blue band" motor(?)....i traded an old tired evinrude 50hp('74) for this motor even steven....motor, controls, and fuel fittings.....there has been no testing done to this motor (lol, i know, im gonna here it over that one) compression/etc..

i thought i was getting a '76 model, the guy actually started out saying it was an '81 model, but i did know enough to know it was at best a mid to late '70's, but this was an in the parking lot, spur of the moment, we both had what the other wanted so we jumped on it deal....

ive tried to find what im looking for by searching on here, but havent found alot of "good decent general info" on what i have and how good/bad a motor it actually is...

thanks for any help or point in the right direction,
Bruce

 

[Faztbullet]

Re: merc 850 w/thunderbolt ignition

Its a very good motor if it was properly maintained, you need to do some basic checks.
1) Compression...... should be 120psi or higher as this is a high dome motor and all cylinders should be within 10psi of each other.
2) Gear oil.......check for water intrusion or large metallic flakes
3) Internal wiring.....this years are bad about internal wiring harness being rotten...replace it,no patching.
4) Remote control....See #3
5) Starting problem... usually choke shutter adjustment or weak solenoid
6) Impeller.......automatic replacement every 2 years
7) Decarbon.....After you get engine checked out, when in usage I tell customers every 100 gallons or run Quickleen in every tank.
If all the above checks out, you will have a motor that will give several years of trouble free usage.

 

[BaDucK]

Re: merc 850 w/thunderbolt ignition

thanks fatzbullet...

i think im going to replace the wiring harness, i actually priced that out already...

one question....as far as stators and coils go....are they a if it works its working parts....or can they work but in a weak state....just wondering if i need to go ahead and bite the bullet and replace everything in the ignition system...

i think for now im going to send her off to the shop and have a pro get her ready for the season this first time, and then i will take over from there....

another question.....i havent really looked closely over the motor yet, but whats the rod that goes down into the hole on the lower unit, is that a shift linkage or something of that nature ?

thanks again,
Bruce

 

[Faztbullet]

Re: merc 850 w/thunderbolt ignition

The stator on that engine only charges the battery, what you have is the battery CD ignition which is a switchbox and distributer. I only seen 3 or 4 of the switchboxes bad in 30+ years but lots of distributers and red coils. These fail due to shorts/breaks in wiring and battery disconnection while running. Check the coil and if cracked replace it with the newer blue coil. If you do not have power trim, the rod you are talking about if its about 1/4 inch dia. is the reverse lock push rod.

 

[vanionbb]

Re: merc 850 w/thunderbolt ignition

Hi,

I just started working on my own 850 w Thunderbolt Ignition and I can confirm that the wires are rotten on mine as well. I need to find the ID number on the motor to determine the year. I know the lower unit leaks because when I changed the lower unit oil, I got water out.

The engine started right up when we applied the manual choke. I put the water on it with that water clamp thing, and let it run for about 20 seconds but the "telltale" did not spout water, so I am not sure that the impeller was working, or maybe there could be blockage somewhere in the cooling system.

My questions are:

1. How much should I pay for replacement wires, or is it ok to rebuild the wiring harness myself (shrink tube, solder splice, etc.). I know I can build a better wire harness than the stock one with wire that will withstand the fuel that is bound to splat out of the carbs. The only thing shorted so far is the choke solenoid.

2. How do I go about finding the model number of my motor?

3. How do I find information about rebuilding the lower unit or replacing the impeller?

Thanks,

Marc

 

[Faztbullet]

Re: merc 850 w/thunderbolt ignition

I will try to answer these for you:

1) No.. Order the aftermarket CDI harness part # 414-2770 sug. list $165
2) Look where engine mounts on boat , there is a metal plate above and between the thumb screws with the serial number,the steering cable passes under the plate and you may need to wipe it off as they can get dirty...
3) Factory manual on Ebay or download it from link below
http://www.tradebit.com/filedetail.php/9057729-mercury-repair-manual-1965-1989-40
4) Look up parts at:
http://www.mercruiserparts.com/selectSerailRange.asp?doc_nbr=850+(4+CYL.)
5) Impeller repair kit is part # 18-3217
6) Google your part numbers for best pricing or order here at Iboats in links at top of header

 

[vanionbb]

Re: merc 850 w/thunderbolt ignition

Wow you guys are fast to reply, I just found this site and I am very glad that I did.

From the SN 4093427, I gather that this is a 1975 outboard.

I watched some videos and I am somewhat confident that I can change the Impeller and I am convinced that it is a good idea to change.

About the water I found in the gear oil. How do I find out where that leak is? When I drained the gear oil, a good amount of water came out with it. Also this water may have frozen in there for 2 seasons. How can I tell if I have freeze damage? Where would the leak that let water into the gear oil be?

I bought this boat, motor, and trailer for $300.00. The motor did start right up and the boat is sound, so I think I did ok. It is an older Larson 16 foot fiberglass. I would like to take the boat out in March/April to see if it floats, but I need the motor working beforehand.

Thanks!

Marc

 

[lindy46]

Re: merc 850 w/thunderbolt ignition

About the water I found in the gear oil. How do I find out where that leak is? When I drained the gear oil, a good amount of water came out with it. Also this water may have frozen in there for 2 seasons. How can I tell if I have freeze damage? Where would the leak that let water into the gear oil be?

You can pressure test the lower unit to see if it is leaking/cracked. Get a pressure gauge on ebay for about $30. When you drained the lower unit, were the washer/seals on the drain and vent screws? Lack of these seals is the most common cause of water intrusion.