'77 Merc 850 weak spark

[freshness247]

Hello everybody...I'm new to the forum and was hoping someone would have some insight on the problem I've been having.

The motor is a 1977 Mercury 85 hp (850) it's in like new condition, the local dealer even wants to put it in the showroom for a customer appreciation week this winter. I tried to post some pics to prove it to you but the files were too big (if anyone has some pointers on that too, it would be great).

Even though this motor is 30 years old, we have had no prior issues until 2 Saturdays ago. Ran for about 5 min and just shut off. Tried to restart several times and no luck. With some help from a mechanic, we determined that it was not sparking on the #2 plug. We changed the switchbox and now spark on all cylinders. According to mechanic it is a weak spark.

First, how do we determine if it is a weak spark?

Secondly, mechanic was concerned that timing was off so we tried an automotive timing light but the light would not work. Is this a result of a weak spark?

To make a long story short...
What should we be checking? Anybody come across this problem?

I really appreciate any help

 

[ricksrster]

Re: '77 Merc 850 weak spark

http://www.homestead.com/dolphinmarineservice/Ign5.html

 

[Laddies]

Re: '77 Merc 850 weak spark

I don't know how the mechanic decided that you had weak spark with a timing light, it sounds like he has a junk light. If you test the spark with a spark gap and it proves to be weak then you will have to us a DVA on the system to see if the stator is putting out the correct voltage

 

[freshness247]

Re: '77 Merc 850 weak spark

Ricksrster: How do I determine if I have an alternator driven ignition?

I just replaced the rectifier b/c the tach wasnt working...It is now.

Where can I get a DVA?? (digital voltage adaptor, right?)

Thanks again for your help guys!

 

[freshness247]

Re: '77 Merc 850 weak spark

Ricksrster: How do I determine if I have an alternator driven ignition?

I just replaced the rectifier b/c the tach wasnt working...It is now.

If I disconnect the rectifier and tape off the ends, should it start then? And if so, would it cause any damage??

Where can I get a DVA?? (digital voltage adaptor, right?)

Thanks again for your help guys!

 

[Chris1956]

Re: '77 Merc 850 weak spark

Fresh, I believe that motor is Alternator Driven Ignition (ADI), and therefore does not have a distributor. You may disconect the rectifier from the stator and tape the ends, but what is the purpose of this? It should not affect your spark.

 

[ricksrster]

Re: '77 Merc 850 weak spark

Direct Voltage Adapter: http://www.iboats.com/Multi_Meter_D...5781995--list_time.1190417526--view_id.271177

 

[freshness247]

Re: '77 Merc 850 weak spark

So here's my belated response guys. I apologize for my delay.

First off, thanks for your quick replies and suggestions to help resolve my issue. The motor is/was all original. That includes all wiring harnesses, ignition coils, fuel lines, etc. With that said the intermitten or weak spark was due to the stator and trigger harnesses deteriorating after 32 years of continuous use. We replaced both stator and trigger harnesses and the motor runs better than new. Running around 42-43 with one person in the '76 16' SuperSport, pulling my 240lb frame out of the water on 1 ski and just plan reliable start up and go. I plan next to replace the internal control harness and external that runs to the MerControl.

However, first i have a soft spot on the starboard side in the floor now. I've been reviewing the Starcraft forum and getting alot of good info there. I suspect that Starcraft ran a full sheet of plywood from bow to stern and filled in the extra space with a cut piece. Since I only have soft spots on the starboard side approx 6" from the side.

Thanks again for all your input with the Merc 850. She's been back in action and turning heads again.

 

[ifeeltheneed4speed]

Re: '77 Merc 850 weak spark

Fresh, I believe that motor is Alternator Driven Ignition (ADI), and therefore does not have a distributor. You may disconect the rectifier from the stator and tape the ends, but what is the purpose of this? It should not affect your spark.

Again I know this is an old post but wanted to add to this. If you remove the 2 yellow wires from rectifier, if forces the engine to draw power directly from the battery. If your stator is bad, the engine will run from the battery. This is not a "permanent" fix and can cause problems later with prolong use. It is mearly a diagnostic test.

 

[Faztbullet]

Re: '77 Merc 850 weak spark

Again I know this is an old post but wanted to add to this. If you remove the 2 yellow wires from rectifier, if forces the engine to draw power directly from the battery.

Not on a ADI or BCD system, a bad rectifier can cause the winding in stator to overheat and short the high speed coil winding in the ADI system and the BCD already draws has power from battery.