1978 Merc 9.8 hp - Top Cylinder not working

[Infectedtoe]

Hello Everyone,

New user to these forums.
I just purchased a used 1978 Merc 9.8 hp Model 110. When I purchased the motor, both cylinders fired perfectly. However, once I brought it home the top cylinder refuses to work. There is plenty of spark, and compression (~120psi). I switch around the spark plugs and the top cylinder still wont work.
The one 'red flag' that I see is there doesn't seem to be fuel on the sparkplug when I pull it after running it for a while.
I am not a 2-stroke guy. I am confused on how the top cylinder isn't getting fuel, but the bottom cylinder does? What do you guys think that would cause this issue? What troubleshooting techniques can I try?

Thanks for all of your help!

-Toe

 

[emckelvy]

Re: 1978 Merc 9.8 hp - Top Cylinder not working

I just worked on a similar 7.5hp Merc that showed decent spark, but would barely run. I finally traced the problem down to 2 cracked ign coils. Plus it had a bad spark plug, which made it even more difficult to isolate the problem!

I had a couple of good used coils kicking around, threw them on along with 2 new spark plugs, she runs like new!

It's very common for these coils to crack with age & heat, then they will arc to ground or just not work at all.

So, before a lot of other troubleshooting, I'd recommend undoing the (3) long screws holding both coil assemblies to their bracket, and inspecting both sides of the coils for cracks.

The crazy part was, I had to rev the motor up considerably to even get it to run at all; at that time I checked the bottom cylinder with a spark tester, and it showed spark. And it would spark a surface-gap plug, out of the cylinder. But wasn't enough fire to pop a good spark to make that cylinder run.

Since you do have one good coil, an easy trick would be to swap 'em out and see what happens. If the problem moves to the other cylinder, it's a bad coil. If it doesn't move, you've got issues with either the power pack or the stator windings under the flywheel.

To troubleshoot those items, you'll need a meter and a Direct Voltage Adapter (DVA) which will allow you to properly read the high voltages generated by the Capacitor-Discharge electronic ign system.

If it comes to that, do a keyword search of this forum on "DVA" and you'll find many postings on use of one, and how to build a very inexpensive adapter that will work as well as the expensive ones.

If you're not too handy with building that kind of test equipment, here's an inexpensive DVA that should do the trick:

ESI 640 Electronic Specialties 640 DVA Adapter

BTW, I doubt there's an issue with fueling in the upper cylinder, if you're getting fuel to the bottom it's there for the top, too. I'm assuming the compression readings are OK and you don't have nasty things like a leaking upper crankshaft seal (sucking air), or bad reed valves (spitting gas back out the carb). And it's really hard to read surface-gap plugs anyway. Most of the fuel is just going to go out the cylinder, unburnt.

In my experience, ign coils are the #1 problem on these, followed by the switchbox. Hopefully you'll get the inexpensive repair (although new coils are almost $100, you can pick up a good used on on eBay for considerably less).

HTH...........ed

 

[Infectedtoe]

Re: 1978 Merc 9.8 hp - Top Cylinder not working

So, before a lot of other troubleshooting, I'd recommend undoing the (3) long screws holding both coil assemblies to their bracket, and inspecting both sides of the coils for cracks.

Thanks for the quick reply, emckelvy.

I did what you suggested and switched the coils around. I won't be able to test it until tomorrow after work (my son had my truck tonight, so I couldn't launch the boat). I took pictures of what I found.

Funny thing is that the cylinder that is firing perfectly had the coil that had some cracks. The cylinder that is not firing had a coil with no cracks.

I'll post an update tomorrow night.

 

[emckelvy]

Re: 1978 Merc 9.8 hp - Top Cylinder not working

Well, the black part does not contain the portion of the windings which actually do any sparking, so as far as I know, a few cracks there shouldn't hurt. The important part would be the orange body of the coil, any cracks at all in the body spells trouble.

HTH & G'luck..........ed

 

[Infectedtoe]

Re: 1978 Merc 9.8 hp - Top Cylinder not working

The important part would be the orange body of the coil, any cracks at all in the body spells trouble.

Thanks again, emckelvy!

Today, after work, I emptied the old gas out of the tank supplied by the seller and put new gas and oil in, with SeaFoam. With new plugs and the coils swapped, I headed to the lake.

I started out by testing it at the boat landing. It seemed to run the same as before, so I decided to pull the top spark plug wire while it was running like I did before. Well, that was a bad idea as I had a few thousand volts surge through me several times before I could let go. I was standing in the water at the time, so I made a great pseudo-sparkplug. I left the top sparkplug wire off and attempted to start the motor - no luck. I then only had the top sparkplug plugged in and it started. I concluded that the problem (weak spark I assume) followed the coil.
Frustrated, I decided to see if it would run better if I opened up the throttle. I shoved off and started across the lake. To my surprise, the motor took off with both cylinders! It ran perfectly. When I ran it at low RPM, I could tell one of the cylinders cut out again. Medium throttle would go between 1 and 2 cylinders working.
Am I right in assuming that one coil should be replaced?

BTW - None of the orange plastics were cracked on either coil. only the black material on one coil as shown in those pictures.

- Toe

 

[emckelvy]

Re: 1978 Merc 9.8 hp - Top Cylinder not working

Sounds pretty much like what mine was doing, only yours might not have been as bad as mine was.

Since the problem followed the coil, that pretty much confirms the diagnosis. If it were something else, the top cylinder wouldn't have lit-off like it did. And the fact that the top runs great confirms that there's nothing else wrong with it.

Now for the fun part, them thar new coils ain't cheap!

Maybe you'll find a good Craigslist deal for a parts motor, or a decent used coil on eBay. Or bite the bullet for a new $100 coil!

Here's a couple of coils I found on eBay from a running motor. Maybe he'd make a package deal for 2 of 'em, doesn't hurt to have a spare:

Mercury 7 5HP Outboard Starter Parts 5288A 2 Ignition Coil | eBay

Mercury 7 5HP Outboard Starter Parts 5288A 2 Ignition Coil Longer | eBay

Cheers........ed