1975 Mercury 850 Thunderbolt 85hp

[ShepC719]

I have noticed that when I try starting my engine it takes several attempts. And once it is running, I have to let it warm up for a few minutes otherwise when I throttle up the motor dies. I know that the person that had the motor before me used the wrong fuel/oil mixture, and I have corrected that problem. Since I have done this the motor is running 100% better, but I'm still having the same issues as far as the motor starting and throttling up. I have also noticed that when I put the motor into reverse it revs up really high but doesn't seem to go backwards like it should. I don't know if these issues are all connected or if they are different. Does anyone have any ideas as to what the issues may be? Thanks!!!!

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

I forgot to mention that once the motor is "warmed up" it runs like a champ, but that is only after it is running for a few minutes.

 

[stormmnv]

Re: 1975 Mercury 850 Thunderbolt 85hp

75 was the last year that Merc. used the Thunderboilt dist. and old style switch box.... I have upgraded many engines with the CDI brand 114-2986 switch box, it has a hotter spark and the engine fires and warms up faster and produces better proformance.
Now, with that being said, all the older Mercs. are "COLD BLODED" so unless you want to spend $300 plus for a replacement switch box, live with it as most owners do. letting the engine warm up for three to five minutes at fast idle is not a bad thing.
As for the reverse problem, check the linkage to make sure it is engaging, also remember the prop is designed to go forward, so in reverse it is not going to go fast.

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

Thank you very much! I kind of figured it was just "Cold Blooded" but I wanted to ask around and see. I know that when I put the boat into reverse the prop starts going the other direction, so does that mean the linkage is working properly or should I check it just to be sure? Also, the prop it kind of beat up and the owner before me attempted to fix it, so could that be part of the problem? I'm going to be getting a new one soon, so hopefully that will take care of part of the problem.

 

[merc850]

Re: 1975 Mercury 850 Thunderbolt 85hp

I have that motor and it will start and go right away without a long warmup in neutral (something that's not recommended in the manual)
so if you want to do some checking:
1 What was the incorrect oil/gas mix - 50:1 is the norm and the brand of oil makes a difference.
2 Check the compession on all cylinders, this will give you a picture of its piston action. Should be 140 lbs. +- 10% between cylinders.
3 Check the low speed adj. should be 1 1/4 turns from seated and set it while engine is running in gear and under load.
4 Check the plugs and the distributor for fouling and corrosion, DO NOT PULL ON ROTOR it's molded on.
There are timing checks you can do but I can't attach them due to non-functioning attach page.
I don't know how cold the water is in IN but I boat in Canada and we get excited when the water temp. hits 70*.

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

Thank you for you information. I am new to the whole outboard motor stuff, so I need a lot of help... I'm not sure what the previous owner was running as a gas/oil mixture, but I know it wasn't right. When I bought the boat it had a full tank of gas, which I have since gotten rid of and I have replaced that mixture with new gas and Pennzoil Marine Full Synthetic for 2 cycle motors at a 50:1 mix. The motor is running a lot better, but it still has to warm up before it will go. I'm going to check the plugs for fouling, since the mix was wrong before. I think I'm just going to replace them. Any recommendations as to what type of plugs to use? Thanks again for all of the help...

 

[Chris1956]

Re: 1975 Mercury 850 Thunderbolt 85hp

Shep, The advice Merc850 gave is correct. Advice from another poster is not accurate. That motor will take Champion L76 or L78 (I forget exactly which) plugs, or the equivalent NGK plugs, depending on the application listing.

 

[merc850]

Re: 1975 Mercury 850 Thunderbolt 85hp

Go with the L76V and I would run some PowerTune from Mercury into the motor to clean it up.

 

[allan1966]

Re: 1975 Mercury 850 Thunderbolt 85hp

Hi,
Does anyone know where to get new plug wires and distributor cap for this same motor? Thanks, Al

 

[Chris1956]

Re: 1975 Mercury 850 Thunderbolt 85hp

The wired can be had at the vintageoutboards web site. Look up the part number of the dist cap at the crowleymarine website and google it.

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

I just finished restoring my 1973 Starcraft Capri, and I also completely rewired the whole boat since who ever wired it before I owned it had the wires in a huge mess. Well here is my major issue now. My 850 doesn't have the internal or external wiring harnesses, and it was hooked up with out one before. Now that I'm trying to rewire the "external" wiring I am having trouble figuring out why I can't get the motor started. I have tried to figure out the issue, but I'm lost. The starting Solenoid is hooked up to the starter and to the battery, but I'm not getting power to the ignition switch. Do I need to run a wire from the battery directly to the ignition switch? Also the starting Solenoid has the 4 wires connected to it, Starter wire, Battery wire, and I can't seem to figure out which wire needs to be grounded and which of the other two wires need to be run to the ignition switch. And last I bought a new ignition switch with 5 terminals with the letters M, M, B, S, A on it to replace the old ignition switch which only had 3 terminals I, B, and S. does anyone know what wires need to be hooked up to what on the back of the new plug. All of the wires are run and in place from the ignition switch to the motor, I just need some help figuring out these last few issues. If any one can help that would be great. Thanks ALL!!!!

 

[merc850]

Re: 1975 Mercury 850 Thunderbolt 85hp

Here's a wiring diagram to help you.

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

Thank you very much! I was able to figure out the issue, and now everything is working just fine.

 

[ShepC719]

Re: 1975 Mercury 850 Thunderbolt 85hp

Well I have the wiring figured out, Thanks to all for the help! Now I have another issue, and I have asked about it before but I'm looking for a little more guidance in the matter. When I was restoring the boat, I also changed the shift and throttle cables since the other ones were shot. That is not the problem however. When I shift into forward or reverse it sounds like the gears are grinding, and then when the motor finally goes into gear there is a big jerk. I have also noticed what appears to be gear lube leaking out of the starboard weep hole on the lower unit. It only leaks after the motor has been running, and it is a milky white. When I shift into forward like I said there is a little grinding noise and a jerk, but then everything seems to work fine except at higher speed or in turns when it seems to slip out of gear for a second, but it then goes back into gear. Now when I shift into reverse the grinding is much louder but not as much of a jerk. However when in reverse the propeller does not spin at all. I have bought a repair manual, and I have read through it a little. I know the manual will given me some guidance, but I'm hoping to get a little more help from those of you who have actually worked on these motors. I know that the guy I bought it from said that he rebuilt the lower unit a couple of years ago, but I'm guessing that it was not done correctly. Do you think this is going to be a gearing issue, or a linkage issue and where should I start? Thanks again for all of your help!

 

[Chris1956]

Re: 1975 Mercury 850 Thunderbolt 85hp

Sounds like you need to adjust the shift cable. Remove the lower (shift) cable from the cable clamp. You will likely need to remove the throttle cable from the clamp as well. Make sure the throttle handle is in neutral. Move the shift slider to the center of its travel and the gearcase should go into neutral. Center it in neutral. Now adjust the shift cable barrell nut to fit the clamp, w/o moving the shift slider. Adjust the throttle cable ballell nut to push the throttle closed and place in clamp. Now drop lower unit and replace driveshaft seal in water pump base. Replace all gaskets and oring. If you are not sure when the waterpump was last serviced, replace the impeller, wearplate and two gaskets. Reinstall the gearcase and drain and replace gear oil. New gaskets on gearcase fill and drain. Recheck for water after a few hours running.