1973 Mercury 850 Won’t Idle

[Screamineaglet]

I’ve a 1973 850 Mercury that I purchased in 2021 for my 1973 Peterborough boat project . The floor and transom were rotten so I rebuilt with Coosa . The plan was to hang the 850 on her so out went the rotten wood and in came the Coosa . Transom , stringers and floor . That went well and is solid once again . The 850 I went through as much as I could pre summer . The information is extremely valuable here and that’s what’s guided me along on this adventure . New wiring harness , stator rewound , carb kits , fuel pump rebuilt , water impeller too . I examined the reed valves when the carbs were off and they looked really good . All visible pedals were laying flat . Did a link and sync .. brought the timing from 27 deg to 25 deg ….compression is 130 ish +/- 5 lbs across the 4 cylinders . I’ve run it in a barrel instead of muffs to simulate the lake levels for exhaust pressure too . This old girl will run all day long at 1000 rpm …..adjust the idle down to 850 ….she tries but just can’t hold it for any length of time ….of coarse that is frustrating and dangerous trying to dock a boat that can’t stay running . I put a piece of cardboard in front of the carbs while running looking for splash back ….nothing there either .I’ve been chasing this elusive idle up till now ( Jan 2023 )There isn’t any apparent excess fuel/oil leakage under the stator where the top crank seal is ….soooo…to look at the bottom seal the power head had to come off . nothing obvious there either . From reading here in some post it has mentioned excessive clearance between the reed block inside diameter and crankshaft outside diameter could allow seepage between cylinders . This makes sense as it would pull air/fuel from the other cylinder instead of just fresh air and fuel from the carb at low idle rpm .. I must say I love these old Mercs and this is a labour of love and its worth it to me to try to save … No choice but to go in and have a look see ….I’m glad I did as it does need some attention….been 50 years after all The crankshaft has wear grooves that match the ones on the reed blocks . I’m assuming that’s not like that from new . on the other hand it’s hard to imagine aluminum wearing into the crankshaft like that . Does anyone know the tolerance between the reed blocks and crankshaft ? I’m going to measure the depth of the grooves on the crank and the inside diameter of the reed blocks accurately . If this is the issue my thought is to grind the grooves out of the crankshaft to maybe .010 undersize then chrome that diameter and grind to a larger than factory diameter to suit the reed blocks . Thanks 8n advance for any help

 

[racerone]

??----Maybe it just needed proper carburetor adjustment.----I would look for used parts on -e-bay.----Lots of cranks and reed blocks around I think.

 

[Chris1956]

Set the idle mixtures using fresh-mixed 50::1 fuel. Make sure the idle pickup timing is 4-6* BTDC. The brass collar on the midsection of the distributor rotates to allow the idle pickup adjustment. I like to use 21*BTDC for max spark advance, with today's lousy fuel.

Initial carb setting is 1-1/2 turns open from lightly seated. After adjustment in the water in gear with motor warm, you will likely need to open the top idle screw about 1/2 turn and the bottom screw about 3/8" turn, as they are usually too lean to allow acceleration. As you richen the mixture, the idle quality will suffer. So the proper setting is as lean as possible that still allows reasonable acceleration.
Those 4 cylinder Merc do not idle like champs, but you should should be able to get her to idle well enough, with some careful adjustment.

 

[Screamineaglet]

Here is a pic of grooved crank and one of a reed block …I’ve a 1975 thru
1978 Mercury Fond Du Lake service manual coming in the mail … hoping it will also give some specs on those tolerances …
Chris 1956 … thank you …will definitely go through those procedures again after the rebuild ..
racerone thank you too… I’m still searching e bay …would be nice to get good used parts as that would be easier way to go .

 

[racerone]

The pistons show lots of wear.----You need to think this project over very carefully !

 

[merc850]

Some of the pistons look like they have damaged or stuck rings

 

[racerone]

This motor needs more work than what the owner is aware of at this time.-----In some areas there are better motors than that thrown out for scrap.----Sorry to be blunt here.

 

[Screamineaglet]

The plan is bore and hone +.015 ….new pistons and rings , gaskets and crank seals …it’s a project for sure …no dispute there …it’s pretty much a total rebuild ….this old girl ran surprisingly well last summer…other than the idle…my labour is free as I don’t mind doing this ….it’s time spent doing something I enjoy … I’ve access to a machine shop which is helpful too . Ignition is healthy ,carbs were done last ( winter / spring ) ,stator rewound , fuel pump rebuilt …..just a little internal work … can’t shy away from that
Does anyone have any input on reed block clearance ? Trying to get to the bottom of that issue before I proceed ….

 

[Screamineaglet]

Look … just found some water impeller debris under the exhaust water jacket cover …..previous owner didn’t change that out often enough …was the 2nd thing I did after getting this ol girl ….first was the compression test . The impeller was pooched by the way ….now I see where those little pcs ended up

 

[Screamineaglet]

Update … new Wiseco pistons +.030 , block bored and honed , transfer port bleed holes tapped 8-32 …installed 1/4 long stainless set screw with .040 drilled hole ( permanent loctite ) new needle pins for rods both ends and main bearing too . Acquired 6 merc piston ring compressors and installed crank assy this afternoon … so smoooth with the chain fall …very controlled installing pistons in the block this way . Very happy with the progress so far .

 

[racerone]

Make sure that the reed block are located on the dowels.

 

[Faztbullet]

Reed blocks need replaced a labyrinth seal edges are hammered