Mercury “thunderbolt” 850 hard, grinding shift neutral into gear

[Captainkf]

Hello everyone, I’ve been reading posts on this forum for some time (excellent resource!) but this is my first post. I live in a small community near Vancouver BC and recently acquired a used 15’ closed bow runabout (in the process of fixing it up) that came with a mercury 850 “thunderbolt” outboard. It runs well, needs the manual choke now that it’s a bit cool out though. Sadly the remote activated choke no longer works, it just makes noise. I’ve had the boat out on the water after getting it running with muffs in the driveway and it goes like stink! That motor is a screamer!

Also, I have not been able to locate a digital service manual for it, so if you have one and would be willing to share, that would be truly amazing and so very helpful.

However I have a problem: there is a grinding sound a very rough clunk when it shifts from neutral into gear. I think the idle might be a bit high which is causing the issue. I checked the throttle linkage and the timing assembly (I’m sorry if the term is incorrect) to see if there is any adjustment left in the stops, but it is as low as is allowed (from what I can gather). Once in gear, no other issues, smooth and consistent. I changed the oil in the bottom end and there was no metal shavings or evidence of damage.

Is it possible it it could be caused by the idle being too high? This can be adjusted on the two carburetors. I have been reluctant to mess with them as it runs well, just idles a bit high.

However, I’m new to 2 stroke outboards. I have a fair amount of experience wrenching on snowmobiles, atvs and vehicles though.

Thanks everyone!

 

[jimmbo]

Welcome to iboats fellow Canuk.
First the grinding, is the gearcase full of the correct oil? What is the idle speed when in gear? What is the the idle in neutral? I hope you are shifting it/out of gear very quickly, not easing it to to gear. Those engines like all outboards with gearshifts, are either in gear or out of gear, there is no clutch plate to slip/ease it into gear. Slow shifting makes for grinding/clacking sounds and basically destroys the gears and the clutch dog. The gearcsae used on the 850(I will assume it is a mid 70s(73 - 77) 4 cylinder) is strong enough for the inline 6s, with 50 % more displacement
The carbs and idle speed. There is no adjustment on the carbs to set the idle speed, it is set by how far retarded you allow the spark timing. There is an idle mixture screw on each carb, but that is not for speed adjustment. All other adjustments on the throttle linkage/distributor are for Max Spark advance, Primary pickup timing(at what timing spec that the carbs start to open ever so slightly, and the forementioned timing retardation. You may have to go on e-bay to get a Factory Service Manual. The 800 - 850, and 75, 80, and 85, are part included in the inline 6 service

Here is a link to an aftermarket manual(Seloc I think), but beware there are some typos and vague areas as it covers many motors
http://www.boatinfo.no/lib/mercury/m...40-115.html#/0

 

[Tassie 1]

G'day.... welcome...happy new year and all that...

you should be able to find a cd manual for that motor on ebay,
anywhere between $5- $10,
at least in Tasmania,

could be high revs making it hard to go into gear but more likely to be the way you are moving the throttle...has to be done quick and harsh rather than slow and carefully,

is the motor a redband or blueband?

if you are getting that motor to "scream" on muffs in the driveway,
stop...coz it will soon stop screaming and you will start shrieking as bits and pieces of it try and go through you and any one standing nearby,

don't do it...use a large tub and put it in gear

 

[Captainkf]

No driveway screaming, only low rpm cleaning it up and getting it running. By screaming I mean it’s loud and fast on the water. However I try to keep it under 3/4 throttle as I don’t need to run it wot and add more wear to an old engine on the water.

Unfortunatly i don’t have a tachometer so it’s a bit of a guess. It seems high, not letting it drop.

Yes, new, correct marine gear oil topped up. However I will check it again to be triple sure.

No, it felt bad shifting into gear so I was going slow. However I will stop that. So maybe it’s partially error on my part? It feels so wrong but I will be fast and forceful from now on.

It is a blue band. What is the difference?

i see some people add inline fuel filters but others do not. I’d like to filter the fuel but I wonder if it’s not done due to lack of pump pressure/flow rate or laziness. Thoughts?

Thanks again!

 

[Captainkf]

So how do I drop the idle using the spark retard timing?

How do I install a tachometer in this motor? I see it is wired for one on the control module near the throttle (I think) but do I need a special mercury type specific for that engine or will a generic one work?

 

[jimmbo]

Don't worry about red or blue on the cowl decal. All 850s have a cowl with blue bands.

Regarding the tach, in the 70s there was switchover from using a pulse from the ignition system, to using pulses from the Alternator. You will have to determine if the tachometer lead is connected to the Stator or to the ignition module.

Adjusting the Max Timing Advance, Pickup timing, and how to set the idle can vary depending on the year of the engine. That's why a manual is so important. That manual at the link I posted has a variety of diagrams, and specs in the appendices. You will need the Serial # to determine the year of the engine, the oldest being 1973, the newest was 77

Some stats, off the top of my head

4 cylinder 66.6 cubic inches
WOT rpm range... early ones were 4800 - 5500, later were 5000 - 5500
Gear Ratio 2.33:1
Bore and Stroke 2 7/8"X 2 9/16"
50:1 gasil mixture
Approx 235lbs

1974 brochure http://www.boatinfo.no/lib/mercury/b...ercury.html#/8

1976 brochure http://www.boatinfo.no/lib/mercury/b...ercury.html#/8

There should be a filter/screen in the fuel system. Adding additional filters can add enough restriction to create a Fuel Starvation situation, so use caution

 

[Chris1956]

The first thing you can do to lower idle speed is to adjust throttle cable at idle to push throttle cable against idle stop screw.

That assumes that it is not currently pushed tight to it, as it should be.

To do that remove throttle cable from barrel clamp and turn barrel toward the boat to push the throttle to the rear of the motor.

Depending on the year of the motor, the idle stop screw is in different places. On distributor models( till 1975), the idle stop screw is above the distributor. On 1976 and later models, the idle stop is on the throttle arm, directly behind the throttle cable attachment point.

Don't adjust anything else.

This is likely as clear as mud right now. A service manual will make it real clear, however.

 

[Tassie 1]

Don't worry about red or blue on the cowl decal. All 850s have a cowl with blue bands.

l asked the question re red/band coz l thought there was a 85 redband,
after looking through the books there clearly wasn't a 85 redband,
but there was, according to the clymers manual ( lol ) a 1964 85hp,

I was going to say that the redband was a " more purty" motor than the blueband ( in my eyes anyway )
everyone likes those nice shiny blingy covers lol
but as there wasn't a redband 85 hp l can't say that anymore...so l won't

 

[jimmbo]

The was an 850 XS for a couple of years, it did have a red band.( it is shown in the 76 brochure) They were a hopped up version of the 850. They weren't a common item, and because of the torque curve, not suitable for anything but very light hulls and racing boats.

 

[merc850]

I think you should put in a tach before you start adjusting idle and it will also tell you the the full throttle rpm which falls into a range eg. (4800-5500). The only tach you can get is the Alternator driven one and it connects to 1 of the YELLOW leads on the rectifier but before anyone can give you definite answers we need the serial Nr. located between the transom clamps.

 

[jimmbo]

I was going to say that the redband was a " more purty" motor than the blueband

I think the red bands were prettier too, with the 1969 being the best. In mid 80s I acquired a complete 1971 1150 cowl. I placed it on my 1984 115 and was very happy with the Improved appearance of the motor

 

[Faztbullet]

Also don't shift slow or it will grind ..just a fast push to forward with a small clunk is normal.