No Power When Cold - 1985 50HP

[DaDaDaDon]

I have a new to me 1985 50HP Mercury 4 Cylinder, serial # 6589020, that recently has developed a lack of power situation when cold. Hoping for some help in starting the trouble shooting process.

I purchased the boat motor and trailer last spring and only changed the spark plugs in the powerhead. (Gearcase was a different story) The engine ran great all summer long and we used the boat 3 to 4 times a week without issue. I live in Wisconsin and lately it has been cold. The engine will start up just fine and idle at the dock OK, but as soon as it is put in gear, it has very limited power. If I give it throttle, it will not go on plane and plow at about 8mph. If I leave the throttle near wide open, eventually you will feel a pulse likes its trying to kick into high gear. Shortly after, the engine will come alive and run like its supposed to. It will continue to run excellent throughout the rest of the trip even if shut off for an hour or so for fishing.

I do not believe this to be fuel related as I do have a new tank, lines and only run ethanal free high octane. I could be wrong however.

Please let me know any thoughts on where to start diagnosing the problem. I have parked the boat for the winter and hope to have all upgrades and repairs completed by spring for early walleyes. I do have the manual for this motor and am fairly mechanically inclined, just don't know my way around this motor yet.

Thanks in advance for any assistance offered.

 

[Texasmark]

I don't know if those Classics of later design used Capacitor Discharge Ignition (modules) or stayed with the conventional points and coil. At idle you can be missing out on half your cylinders and not realize it. The surge you mention tends to lead me to ignition....a plug is trying to fire but isn't being hit with enough spark energy, or fouled but you installed new plugs to no avail.

I've had CDIs with thermal problems at both extremes. Electronic circuits will have components that develop a temperature sensitivity. When you had a raw circuit board, not a potted module, you could take a can of "Freeze Mist" and individually walk through one component at a time and find the guilty party.

I suggest you take the cowl off for access and get a spark plug wrench. Do what you said you do and when it doesn't do what it is supposed to do, shut off the engine, pull the plugs and see which one(s) is/are wet and look into the ignition source feeding such.

Additionally, a battery operated "automotive timing light" connected to each cylinder individually, with your comparing the flashes between cylinders could lead you to the cylinder with guilty ignition.

Other slight possibility is that is is fuel related and the high speed jet(s) in one or more cylinder's carburetor bowl has something like a black piece of rotten fuel line that is floating around and got wedged in the cavity thus blocking proper fuel flow for high speed operation. Those engines used to run dual carbs and one bad carb could kill 2 cylinders......but the plugs would be dry, not wet as fuel is lacking, not like ignition where the fuel is presented but nothing to explode it.

 

[DaDaDaDon]

Thanks for the input. I think I’ll start with a spark test and see if I can find a cylinder or two not firing. This motor has CDI ignition which I personally have not dealt with. Probably start on one end (spark plug) and just work my way back to the other end (stator) and see what I find. It will be a process and take some time and learning.

I forgot to mention that I did try a few things already such as new plugs which showed no change. Also squeezing the primer bulb on the gas line did nothing either. Lastly removing the cowl to allow more air did nothing as well.

Under the hood everything looks clean and see no signs of stray arcing.

I’ve got until March to figure this out so I’ll take it one step at a time.
Thanks

 

[racerone]

A 1985 model has ADI ( alternator driven ) ignition , no points anywhere near that motor._ I would look at opening the low speed mixture screws 1/4 turn each and see if that gets rid of the bogging in cold weather.-----And you absolutely need to verify that both carburetors are clean.-----A blocked restricted carburetor can starve 2 cylinder of gasoline and OIL !----Post compression values.----And hopefully you installed a new water pump impeller when you got this motor.

 

[DaDaDaDon]

When I got the motor I had to reseal the lower unit and install a new impeller. Tell tail has been strong through the boating season but I did plan to change it before winter along with changing out the lower unit lube.

When I changed the plugs earlier this week I found all four were kinda wet but none looked fouled.

I’ll run a compression check soon and post numbers. Probably not today but sometime this week. While I’m down for the winter I may as well rebuild the carbs too. I haven’t ruled this out but I’m suspicious that this is ignition related.

Thanks for the input!

 

[DaDaDaDon]

Ran a compression test and found 125 psi on all four cylinders. Forgot how much a pain the lower cylinder is to get to on this motor with the cowl in the way.

I also removed the coils and did some resistance testing on my work bench. I could not get a good read on the primary, spec is .2 to 1K ohms and all my meter would show is 0.000 with the exception of the #3 cylinder coil which did read per spec. I am not convinced I was getting a good measurement. The secondary output gave readings well within the 800-1100 spec. I also CAREFULLY pealed back the rubber shell and found the ferrite cracked on 3 of four coils. And yes, the #3 cylinder coil was the only one not cracked.

I believe that these coils are original to this 1985 motor and are due for retirement. I plan to replace them but let me know if you think the cracked ferrite would cause some of the issues that I am seeing.

I still plan to keep checking the entire ignition system as there could be thing up the chain causing the trouble. Not ruling out the carbs either. I did find that the stator looks to have been recently changed.

Thanks again for the input and any other suggestions are greatly appreciated.

 

[Texasmark]

A Ferrite around a high speed circuit (wire), like a CDI that dumps the capacitors in microseconds (0.00000X seconds), is placed there to "choke" the harmonics and radiated energy from the wire helping to prevent radiation to other parts that don't like electrical interference caused by high intensity fast moving pulses. If the Ferrite is cracked it can't function properly as the encircled path around the conductor is broken and it acts as an iron core transformer with a gap, lowering the permeability and thus the value of L....V=Ldi/dt. Now, whether or not the Ferrite alone is your only problem, I wouldn't count on it. Magnetization curves vs temp for Ferrite components are long and slow to change over temp vs solid state components in particular at the extremes of the temp range.

Metal oxide resistors are common place today (over carbon) and hold values for long periods. Ceramic capacitors are hardy but electrolytics used in a lot of low voltage circuits can be suspect due to leakage over time...not temp related. Solid state components get damaged by high voltage spikes and act funny as a result over temp and is where I would look if I were troubleshooting to the component level.

 

[DaDaDaDon]

Thank you for the in depth explanation of the purpose of the ferrite. I have read it three of four times and probably will read it a bunch more until I can fully digest it. (Electronics are not my strong area. Machinist by trade) I don't think the coils are my issue, but they are suspect and to be safe, I will replace them. I'd rather invest in coils instead of a new oar!

Couple of things I forgot to mention...I did have spark at all four cylinders but was unable to test for the strength of spark yet. I did test the rectifier to find out is was OK. The Trigger bench tested to spec but there was some janky wire splicing in place. Looks like a cobble job to repair damaged wire insulation. The Stator Failed resistance tests. All measurements were considerably below the specs in my manual. Funny thing is that the stator is the only thing in the ignition system that wasn't original.

The only thing that I haven't tested is the switch box. I may be wrong, but I think I need a DVA adapter to do this? Haven't researched this one too much yet.

Bottom line, I think I need to replace the Stator and maybe coils. These Mercury parts are expensive! My last two motors were Older Evinrude's and OEM ignition parts were cheap. Easier to replace them than to test them.

Again, Thank you for your help and input. I'll follow up with results once parts are in and installed.

 

[racerone]

You need 3 new coils.---Then test run.

 

[Texasmark]

Agree on 3 new coils. On the resistance, copper is very temp sensitive as to resistance per unit length. If it was a winter day, I wouldn't worry about it. At least you have continuity and the Stator is made up of laminated iron, copper, and epoxy....not much to go bad there.

 

[DaDaDaDon]

I'll get four new coils on order and see what happens. Definitely less expensive than changing the stator. Hopefully the boat landing isn't iced over yet since running on muffs won't demonstrate my concern.

Thanks again for all the help and advice.

 

[DaDaDaDon]

(4) New CDI coils arrived today, and installed. Air temp 34, water temp 35 so these are the conditions that made the motor run very poorly.

After installing the new coils, checked for spark and was able to get the spark to jump a 7/16" gap on all four cylinders. Went to the boat ramp and the motor popped right off and idled very smoothly. (Great Sign) Left the dock and hit the throttle and it went right on plane and ran better than it has in months. To quote Johnny Cash; "We had that engine runnin' just like a song!"

I can confidently say, Problem Solved.

Thanks for the help Texasmark and Racerone!