Rough running 4 cyl Merc?

[WizeOne]

I have a neighbor friend with a late 80's Searay with a 4 cyl Merc. Not sure of the exact model.

It starts just fine but has a variable rough running condition that seems to get more pronounced once the motor has warmed up. It's using stock points, properly gapped and the timing is right on. It about acts like bad gas, but it isn't.

Here is the kicker. You turn on the depth finder and it goes from mild/variable to significant/constant. Turn off the depth finder and it returns to a mild/variable raggedness.

The depth finder is wired direct to the battery (+ and -) and cannot create much of an electrical load anyway. But this condition is undoubtedly caused by an electical issue.

I am not familiar with this Merc setup so I was wondering if there is some component or ground condition that are/is the likely culprit.

 

[tupperj]

Re: Rough running 4 cyl Merc?

Until the wiser denizens speak up, I'll take a stab.
I'd get a volt meter and check the voltage to your coil, both with the finder running, and without. See if you're running on the ragged edge of proper voltage. Does the poor running condition occur when any other electrical load is applied, like trim or blower? I'd also look at the wiring for the finder, is it possibly chafing somewhere and causing issues?

Also, good time to go through the electrical connections and clean/tighten them, particularly the grounds. A little emery cloth with do wonders.

Chasing my own demons with a merc 140, hope you get your friends straightened out.

Paul

 

[Bondo]

Re: Rough running 4 cyl Merc?

Ayuh,... Donno Boom,... The fishfinder throws Me off...
What's the voltage at the (+) side of the coil, before, 'n after,..??

 

[WizeOne]

Re: Rough running 4 cyl Merc?

..... Does the poor running condition occur when any other electrical load is applied, like trim or blower? ......

...eh, something I thought to ask after the fact. T'would be interesting to know. The trim or blower should use more juice than the depthfinder.

In checking the voltage at the coil, just place the meter leads on the coil positive and a ground? That should show the voltage thru the ballast wire? and what should that be? Do ballast wires go bad? It is a pretty clean boat with very few hours on it.

 

[WizeOne]

Re: Rough running 4 cyl Merc?

Ayuh,... Donno Boom,... The fishfinder throws Me off...What's the voltage at the (+) side of the coil, before, 'n after,..??

Yeah, that alone kinda pointed to an electrical issue.

What should the voltage be, at the coil, with the engine running? and what would be a permissable drop, if any, when other accessories are turned on? If the drop is excessive would that point to bad grounds, poor alternator output, bad ballast wire, ad nauseum?

 

[tupperj]

Re: Rough running 4 cyl Merc?

There have been a number of threads concerning coils, and replacing points with electronic version lately.

Seems the voltage across the coils should be about 9 volts or so with the engine running, and a full 12 while cranking.

Best of luck!
Paul

 

[WizeOne]

Re: Rough running 4 cyl Merc?

There have been a number of threads concerning coils, and replacing points with electronic version lately.

Seems the voltage across the coils should be about 9 volts or so with the engine running, and a full 12 while cranking.

Best of luck!
Paul

Is that 9ish volts measured across the coil when it is running or from coil positive to any ground?

 

[Bondo]

Re: Rough running 4 cyl Merc?

Is that 9ish volts measured across the coil when it is running or from coil positive to any ground?

Positive to any ground...

 

[Don S]

Re: Rough running 4 cyl Merc?

See if the condenser in the distributor is loose. That would cause rough running and interferrence. Could also be a bad condenser.

 

[WizeOne]

Re: Rough running 4 cyl Merc?

Thanks Don. Always good to have a refresher in these basic issues, but......

Latest update: This is a Merc 140. The owner has been experiencing this problem, intermittantly, for some time. This is the first time that he was able to see a connection between actions and performance, ie, tuning on and off the depth finder.

Today, on close examination he found 3 wires going into the back of the alternator. I think, red, black and purple. They were all pigtail wires coming out of a circuit board at the rear of the alternator. The purple wire was frazzled and just hanging on by a couple of threads. He always kept the battery on a trickle charger and never went out for numerous or extended outings. So if this wire had anything to do with charging, dead batteries never showed themselves.

He removed the alternator and dropped it off at a local rebuilder. This was likely the issue, for what ever reason, that has been plaguing him for some time. The rebuilder will check it out and replace the offending parts if that is all that is necessary.

Thanks all!

 

[Silvertip]

Re: Rough running 4 cyl Merc?

Ballast resistors and ballast wires do not generally go to zero ohms. The generally open but then the engine would not run unless the key was held in the START position which is when the ballast resistor is bypassed. The very first check is to measure battery voltage static, cranking, and running. If it is up to snuff, then go to the coil. Then begin turning on accessories one at a time and noting results. If additional loads makes the problem gradually worse, the issue is one of load and connections and alternator output/battery condition become suspect.

 

[WizeOne]

Re: Rough running 4 cyl Merc?

.....and connections and alternator output/battery condition become suspect.

Silvertip, you stumbled onto the root of the issue. See my last post above yours!