1977 Mercury 850 85 hp

[Arronh]

Hi. I’m wondering if anyone has a repair manual for the 1977 mercury 850 85 hp. I bought this used boat. It turns over with compression and spark. The bottom carb leaks gas pretty bad. I’ve started taking it apart, but I want to make sure I do it as simply as possible. Thanks

 

[Chris1956]

A service manual should be avail on the web. Parts places like crowley marine will have exploded pictures of carbs and other stuff. Normally carb service is done on both carbs. Get 2 gasket sets, 2 floats and 2 inlet needles and seats.

Disconnect battery, Remove top cowling support, remove front cowling support. You may want to remove starter as well. Remove carb bowl covers as an assembly. Now remove carbs.

Sidebowl carbs right? Remove brass plug on top (exposes idle restrictor tube) and brass hex bolt(exposes main jet) on bottom of bowl. Soak in solvent, blow out all passages.

Reassemble with new gaskets, inlet needles and seats and floats. Set float height.

 

[Arronh]

Thanks. Any recommendations for when it won’t idle well? There’s a switch wired to it so I can start it on high rpms in gear and that’s the only way it starts. When I back the throttle down, it dies before it gets to neutral. It can run in gear for as long as I want it to. Thanks again!

 

[Arronh]

Actually I can get it started in idle. It’ll idle for about 20-30 seconds then always dies. I’ll check out the carb specs to see if I can adjust the idle.

 

[Texasmark]

Do the carbs like Chris said and then worry about your idle. I had one of that vintage. Super engine. Used to be the fastest 1st start for the outing at the ramp. I don't remember. Idle may be set by timing knurled knob on the throttle linkage like later models. Toss some Sea Foam per can directions to keep fuel cleaned out and carbon in combustion chambers.

If you find any little black specks in the carbs or find any fuel line leaking, redo your rubber (and fuel filter) from the fuel tank throughout the engine.

 

[Chris1956]

It is important that the motor be timed and synched correctly. A service manual is the best way to get it correct. On that ADI ignition, the carbs should be closed, but just ready to open as timing is between 0-9* ATDC. Max spark advance is 21* BTDC. All the adjustments are on the throttle arm.

After you clean the carbs, set the idle mixture screw to 1-1/2 turns open. Final adjustment is made in the water. Set the throttle cable to idle the motor at 1000 in neutral. You can adjust the carb mixture and then set the idle lower if you wish. Use fresh 50::1 fuel when making the carb adjustments.

 

[Arronh]

Thanks for the feedback. My serial number is 3619319. More digging leads me to believe that it’s a 1973 mercury 850 thunderbolt. I had bought a manual online, but the carbs looked different from what I have. I’m having a tough time finding anything that actually looks like the carbs in my engine.

Anyway, I Got it running on the water. I was mostly satisfied with how it operates. I had taken off both carbs and cleaned them. It still doesn’t start the easiest, but when it was running for a while and was hot, it idled very well. It was a bit loud but didn’t die. So, that has me confused. Any ideas?

At this point, adjusting the timing relative to atdc and btdc is beyond my skill level. However, I’m open to reading up on it if you have any links to share.


Im wondering if the idle mixture screw is just fine and whether I should adjust the throttle cable to begin with. Based on the different images I’ve seen, I’m not 100% sure which screw is the idle mixture screw and which is the power. Is there a general rule for where these are located in relation to each other on various carburetors? Would you think I should start with idle screw or throttle cable?

thanks!

 

[Chris1956]

Well, a 1973 Merc 850 is a slightly different animal, than a 1977 motor. Do you have a distributor? If so, it is pre 1976 motor. Most had sidebowl carbs, but with banjo filters in fittings on top of carb bowl cover.

A picture would help. Focus on carbs and coils

 

[Arronh]

Thanks for the feedback. My serial number is 3619319. More digging leads me to believe that it’s a 1973 mercury 850 thunderbolt. I had bought a manual online, but the carbs looked different from what I have. I’m having a tough time finding anything that actually looks like the carbs in my engine.

Anyway, I Got it running on the water. I was mostly satisfied with how it operates. I had taken off both carbs and cleaned them. It still doesn’t start the easiest, but when it was running for a while and was hot, it idled very well. It was a bit loud but didn’t die. So, that has me confused. Any ideas?

At this point, adjusting the timing relative to atdc and btdc is beyond my skill level. However, I’m open to reading up on it if you have any links to share.


Im wondering if the idle mixture screw is just fine and whether I should adjust the throttle cable to begin with. Based on the different images I’ve seen, I’m not 100% sure which screw is the idle mixture screw and which is the power. Is there a general rule for where these are located in relation to each other on various carburetors? Would you think I should start with idle screw or throttle cable?

thanks!

 

[Arronh]

 

[Chris1956]

Carbs only have one adjusting screw, for idle mixture. Turn in for lean, out for richer mixture. The top of the distributor should have three adjustment screws. They are max spark advance, Idle stop and throttle stop. the throttle cable should push the throttle closed against the idle stop screw. The idle stop screw is set to control idle RPM.

There is a brass collar about 1/2 way up on distributor. It had fingers that open the top carb. It has two bolts (5/16" wrench), that allow the collar to turn relative to the distributor to set idle timing.

So to time that motor:

Normally you remove sparkplugs 2,3,4 and ground them. Now put timing light on sparkplug 1. Crank motor and read timing. Advance throttle until ign timing is 4-6* BTDC. Now loosen bolts on distributor collar and turn collar so carbs are closed, but ready to open.

Now advance throttle to max. Crank motor and read timing. Adjust max spark advance screw (and throttle stop if necessary) until max timing is 21*BTDC. Adjust throttle stop screw so carbs open fully, but don't force them open against the stop with the throttle.

Install all spark plugs. Set carb mixture screws 1-1/2 turns open. Take the boat to the lake. Warm her up and at idle in gear, adjust carb mixtures for reasonable idle and smooth acceleration. You will likely need to set the carbs to about 2 turns open, but it is trial and error. Set idle speed to 850 or so, in neutral and adjust throttle cable to add some preload to throttle.

 

[Texasmark]

Iboats sells service manuals, paper form or you can sign up for a time domain electronic accessible account. Ebay also sells manuals, usually used but they are cheap and so it's used.......

Per your comment, ATDC is when #1 (top) piston is on the very top of it's upward position. BTDC is prior to that during the charging half cycle.....combustion from spark origination takes time to develop and the spark has to create the bang before the piston is mechanically ready for it for it to occur at the right time and produce the power stroke. Since the engine is turning faster at the higher rpms you have to precede the TDC position earlier.

Obviously you need a timing light to make this measurement and Harbor Freight or any auto parts store stocks them. A 12v clip on unit is most convenient. The units are triggered and when the engine is rotating as Chris mentioned when the spark originates it triggers the light to flash a Xenon bulb and produce a "frozen in time of your timing mark reference and your crankshaft timing mark position. Moving the knurled screw on the throttle arm (vs the shifter arm) causes the position of the pickup coils to move with reference to the magnet(s) embedded in the flywheel under side.

When he says "preload" the throttle cable he means to take the slack out of the linkage. Do that by backing the barrel (mounted on the threaded portion of the throttle outer cable inside the engine proper) up a turn or two (how ever many it takes) to the point where you can just get the barrel back into it's mount. Set right, immediate movement of the throttle remote lever will create immediate movement of the throttle linkage in timing and butterfly positioning.

You can do it. We're rooting for you. Grin

 

[Arronh]

Thanks. I’ll get to work and let you know how I did