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1974 Mercury 850 bogs down intermittently when warm

myls29

New member
Hey everyone
Having an ongoing battle with my ol mercury, I'll start describing and what I have done so far.

Engine will always start without too much trouble, will always idle happily, always cruise pretty much any desired rpm, changes gear acceptably.
At the beginning of the day it will operate perfectly, shift to forward and accelerate how ever you want it to but after a bit of a run (I keep forgetting to get an approximate time when I go out but probably 15 mins or so with a mixture of idle, slow and fast speed) the problems begin. Engine will occasionally die when pulling up to neutral from higher rpm's then the biggest problem starts. Engine will start back up no worries shift into gear okay but then any attempt at accelerating it will bog down and die. I have tried accelerating slower, faster, very fast and doesn't really make a difference, every maybe 5 - 10 attempts it will get up and go and then run great until trying to pull up to neutral again.

What I have done so far
Fresh petrol
Cleaned and rebuilt carbs (including float which was bad in 1 carb)
New plugs
Adjusted carb mixture screws
Cleaned dist cap

Important Notes
The throttle arm has a crazy amount of play, after shifting about 40degrees of motion results in no movement in throttle cable and last about 10 degrees is the full rpm range.
When problems start it feels like the throttle arm reaches max without actually moving the throttle cable but that may be my imagination.

I'm not sure it could be the throttle cable as it operation shouldn't change after a short run ??
I feel like it may be electrical in nature but I am horrible which electrics and don't know where to start.

Any help would be greatly appreciated, sorry about the essay !!
Thanks in advance
Michael
 
Me thinks fuel problem.
First you need a new throttle cable,that's way too much play.
Check the carb link and sync.Does the carb butterflies open fully?
Did you rebuild the fuel pump? May be a diaphram on it's way out.
Was she under load when you adjusted the idle screws? PIA these old Mercs but don't you just love them:D
 
Indeed they are aha, but you're right, when she gets up and going you gotta love it :p
roger that on the throttle cable I thought so. Carb butterflies open fully, I think link and sync is okay, may redo it all when I get new throttle cable as per guide on here. Haven't rebuilt fuel pump, I foolishly ruled it out as a problem because of the intermittence. I'll give these things a go over next few weeks and see what happens.
thanks mate
 
You're welcome.Hope you get her up and running,do a compression check as well if you haven't already.110psi+ would be nice
 
Throttle cable installed and fuel pump rebuilt, ran great on the muffs but then again it always did haha. Can't wait to test her out on water. Will update when I manage to get there.
 
Still haven't had a chance to take her out on the water :(. Been really busy with studies and work, helpfully next month !
 
So I finally got a chance to get up to the river and test her out. In a nutshell problems still remains but very much improved. After initially good period it only took about 2 restarts to get going. Not ideal sure, but I can deal with it and I still had a great day on the water :) any ideas as to what else it might be are very welcome though :)
 
I still think it is fuel related.There is little in the electrical line that would cause hard start except a poor earth/ground contact or a cracked wire somewhere.And as you say she starts first time easy.
The fact that she bogs makes me think idle screws backed out a 1/4 turn might help.I have to ask,did you set the idle screws at operating temperature,in water and in gear?
 
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Do a compression test.
Low comp on the cylinder that runs the pump could be a problem.
But if it starts right up and runs good the comp is probably good.
The readings for compression will vary from gauge to gauge.
The results should all be real close within a couple of pounds of each other.

I was given a Harbor Freight gauge and it read 15# less than my Mac Tools one.

Now the fuel problem. It could be a bad hose?
Fill out the profile. Location?
In the US you can have Ethanol in the fuel.
Ethanol's been melting the hoses or delaminating the inside of the hose and it can collapse.
Also it can cause the hose to crumble and fill up the pump or carbs.
Old hoses can do this without the Ethanol.
IF?? you don't have one. Install a filter between the carb and the fuel pump.
Wal-Mart Has Fram filters G-2 or G-3 or G-12 depending on hose size.
The low speed air screws(not idle screws) set at 1 and 1/8th turn out from lightly seated and then adjust when warm and in gear.
They set the air/fuel for low speed.
The squeezie has a one way valve and it can go bad casing the problem.

Get a spark tester and next time it acts up check the spark.
 
I did set everything in water and in gear but I am new to carby's, everything else I work on is fuel injected so very possible its not quite right, I will give it another go asap. I'm thinking its still fuel related also :)

I compression tested it when I bought it and was all good but cant hurt to do a recheck anyways ! The fuel hoses are all new with new fitting done professionally before I bought it so they should be okay. I'm in Adelaide, Australia and no ethanol fuel has been used so all good there. I don't have an external one, there is a small ones in each carb itself, is this good enough ?? I'm getting the feeling it may well be my idle and mixtures settings, I will definitely redo them. Good thinking, I'll grab another one and chuck it on.
 
Update ! I took the beast up to my missus' shack last week and good news, got the sucker running great. I backed up the mixture screws a 1/4 turn as suggested and re adjusted my idle screw (was nowhere near fat enough in, must have set it at idle after all, whoops!) which I think was the last of the problems sorted. Had loads of fun on the water and went through about 30L of fuel without so much as a hiccup, at least until the key snapped in the ignition haha (I had to pull the controls apart and chick the appropriate wires out where the ignition used to be and Hotwire it every start for the next 2 days haha !!). The old gal operates like its brand new !! In the key fiasco I lost choke and still started easy as which is even more impressive ! I'm extremely happy with it, thanks very much to all that helped !!
 
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Yet another case where simply opening the idle mixture screws a bit cured the problem.

Have a fun time boating with your Sheila!

Jeff
 
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