1977 Merc 850 - Revived but not quite there

[jeff_smith_0423]

Greetings all.

With a lot of help from folks around here, I sucessfully revived my 1977 Mercury 850. Plugs, carb rebuild, trigger wire repair, switchbox, fuel pump rebuild, and under-cowl fuel lines.

I decided to have a marine mechanic handle the impeller change, control cable replacement, and the link and sync. $480 later, I've had the boat in the water twice since then. First time out, it was between 48-50 degrees. The motor would start and idle fine in at the dock, but stall out when put into gear unless i really gave it some gas while engaging fwd gear. I was easily able to plane the boat and achieved pitot indicated speeds of 32-33 mph in a pretty decent chop

Yesterday, it was between 65-68 degrees, and I could not get the motor to maintain an idle at the dock without the fast idle lever up. I was still able to manage to get it into gear, plane the boat at 34mph pitot indicated. I could probably continue using the boat like this, but it's just hell at the ramp trying to manage keeping it alive, lining up with the trailer, and loading.

When on muffs, the motor kicks over and starts with half a crank, easily engages fwd and reverse. Compression is good and I've got good spark.

I do not currently have a tachometer. I know that I need one to really diagnose this problem but I need suggestions as to what i should buy. I suspect that my "marine mechanic" didn't do the link & sync under proper load - he said he did, but at most, it was in a small test tank. I've toyed with both increasing the idle speed and fattening the idle mixture - although he told me it was pretty fat to start with.

Any suggestions on potential troubleshooting tips OR about installing a tach? I'd prefer to connect the tach to the tach plug on my MerControl box if at all possible.

 

[Yepblaze]

Re: 1977 Merc 850 - Revived but not quite there

Go back over your fuel pump and carb work.

Did you dial the carbs in?

Try pumping the primer bulb to see if the pump is weak.

Look for the carbs to be dumping gas. Or pull them and clean out any debris that has found it's way into them.

Sometimes after fuel system work, a person will hook the motor right back up to nasty dirty water laden fuel and cause the problems all over again. I hope that wasn't you.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Definitely not water in the fuel. I removed the built in tank and switched to portables. I've taken fresh dry fuel with on both trips. I have a few extra pump diaphragms, so i could swap that. Also have another new primer bulb, although it seems to be holding pressure.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Anyone have suggestions as to tach rigging? My plan here is to replace my fuel pump diaphgragm and check for crap in the fuel screen, then get some kind of tank so I can at least get the motor running with a load on it and hopefully get the idle dialed in. I'm guessing if i can get it to idle without the fast idle lever up, i'll be closer once i get it to the ramp and have to do the final adjustments. However, my ears decieve me sometimes and I don't want to do any of it without a tachometer.

 

[Yepblaze]

Re: 1977 Merc 850 - Revived but not quite there

You may want to venture deeper than the screens.

Have you tested the compression?

Many of the Mercontrol Controllers had a tach plug right in socket right in the front of them.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Replaced diaphragm, checked the check valves and all of the various nooks and crannies for junk, it was all clean. Reassembled, tested compression on the top 3 - even with my short compression tester i've never been able to get to the fourth. I'm getter 135-140 on all three. Fired right up and ran on muffs as usual.

I'm off to find some sort of suitable tank so I can try to get it idling in water.

Yes - i have a tach plug on the front of the mercontrol unit. I just didn't know if i needed a mercury manufatured tachometer for that setup, or if i could just purchase the harness and a teleflex tachometer and use it that way. I've located 121 of the harnesses on ebay, so at least I know i can get that.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

well, i picked up a 45 gallon rubbermaid tote, and it was enough to get the motor running without flush muffs. I think the mechanic set the carbs too rich. When i took it to him, it definitely had a lean pop. His exact quote was "i really fattened those things up". I guess I'll have to count them back in to see what he did.

Unfortunately, putting it into gear throws about 2/3 of the water out of the tank (when it doesn't die), so the tank won't be a solution to getting it running out of the water.

Ordered the mercury wiring harness and once i get confirmation that a standard marine tach will work, I'll get that ordered. Hopefully I can get this problem solved with a few minutes of tuning dockside. Weekend launch fees go up to $27 next week, so the fewer unsuccessful trips, the better.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Has anyone rigged a tach to one of these old 70's Merc motors? Will something like this work?

http://www.iboats.com/Euro_Tach_700....248010--**********.722861345--view_id.133001

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Ok. Swung by the shop that did the original work and talked to the mechanic that worked on it. His thoughts are that he just set the idle too low. Suggested i just open it up a bit and see how it behaves. Preferring to be more methodical, i picked up a used quicksilver tach that I think will work with my setup. Pictures below.

Should this work with my setup?

 

[Chris1956]

Re: 1977 Merc 850 - Revived but not quite there

Some older Merc Tachs worked off a terminal on the switchbox. This was on the distributor based ignitions. I do not know if the tach you have will work, It is best to ask the mechanic, if it is compatable.

However, since you have an ADI ignition, most any aftermarket tach will work, since they run off the stator signal.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Pulled out the wiring diagram and got everything hooked up. Cranked the motor over a few times and it definitely registers RPMS. Then it started to hail. Hopefully the weather will clear and i'll be able to put the motor in my tank and see what the idle speed is actually set to. Hopefully that's all my problem is!

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Looks like once it warms up the idle settles down to aroun 850-900 rpms. Who knows if the tach is correct. I bumped it up a tad by ear last night. Couldn't get much else done before it started to rain.

Question. I installed new plugs during the "revival" process, but since then, they've suffered through running on two cylinders due to a bad switchbox, too rich due to a stuck-open inlet needle (and associated gushing carb), too lean due to suspect out of water carb adjustments, and a 3 gallon/1 pint seaform mixture on our maiden voyage. They look "OK" but could all of the aforementioned abuse have aged them prematurely? My source for replacements no longer carries them (walmart).

Is there cross reference to any other plug manufacturer that I might be able to get just driving down the road? I've also got conflicting information as to which plug to use. The original operation and maintainance manual specifies L78V, and my seloc manual says l76v. Had 76's in the motor when i got it and a set of 78's in the glove box. Something in the back of my head says that i'm looking way too far into the problema and i might just need some plugs.

 

[gogono2]

Re: 1977 Merc 850 - Revived but not quite there

I recently had a similar issue with a 84 merc 150. The motor idled like a cat when in the driveway with muffs. However, when i got it in the water it wouldnt idle for anythingand stalled after 5 or so seconds. So while in the water i made quick adjustment to the idle stop and it did the trick. I know its crazy simple but it worked for me.

 

[Chris1956]

Re: 1977 Merc 850 - Revived but not quite there

OK Jeff, It sounds like your Tach is working. 850-900 RPM at idle is fine. You can go up a bit if necessary, the backpressure of the lake will slow it down a bit.

Any Champion application guide will list the correct plug, NGK and Autolight also make those plugs. My '77 Merc 1500 used the L-76V plug,while my '73 Merc 850 used the L-78V. I think the difference is small,

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Well folks, I'm at the point where I don't think it's the plugs, idle mixture, controls, coils, switchbox, or any of that. Any more ideas?

 

[bchoate]

Re: 1977 Merc 850 - Revived but not quite there

Try spraying some sort of combustion chamber cleaner through the engine like Mercury/Quicksilver Powertune or Yamaha combustion chamber cleaner, I have found that this helps bring back compression at "some" times. Follow instructions on the can. Good luck!

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Yeah, I'm definitely going to try a heavy shock of deep creep.

 

[Chris1956]

Re: 1977 Merc 850 - Revived but not quite there

Jeff, You can remove the transfer cover on #4 and inspect the rings and cylinder walls. You may find a sticking ring, as the lowest cylinder on the inlines tend to get more oil and cooling and therefore last longer than the upper cylinders. Your estimate of $200 to rebuild will cover the parts, boring the cylinder and gaskets. It is significant labor, which you will need to do yourself.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

Yep. If it doesn't behave after deep creep, I'll pull the transfer port cover. I'll have to pull pretty much all of the wiring to get to it, so I want to at least try the decarb before I tear into it.

 

[jeff_smith_0423]

Re: 1977 Merc 850 - Revived but not quite there

First treatment is in. Definitely a lot of white smoke - they weren't joking! took considerable effort to actually load up the engine and stall it out. Used about half a can in the carb throats and the remainder through the spark plug hole for each cylinder. I've decided to let it sit for 24 hours.