20 hp merc running on 1 cylinder?

[ole1kanoi]

Hi there! I'm hoping someone can help me with this. It's a '69 I believe, red stripe. Sometimes it won't even start, but when it does, it will run fine, then bog down and shake and pop, I would describe it as running on one cylinder. Then it will come back to life and run fine. It mostly does it at idle, I don't think it has ever happened up on plane. When it starts to do it at idle, if I give it some gas, it clears up, runs okay, but it kicks so bad, it almost rips the tiller out of my hand. Then sometimes, it doesn't do it at all.
New plugs, new wires, new condensers, new fuel and I have gone through the carb, although I do not suspect a fuel problem. I haven't replaced the coils or points, because they are all about $100 a piece. The points look fine. Is there a way to test the coils? That is about the only thing left that I can think of. I've had this motor apart about 20 times in the past year trying to narrow it down. I know, I should just take it in, but I'm broke, and just want to get out fishing! Thanks in advance. Ole

 

[Laddies]

Re: 20 hp merc running on 1 cylinder?

Any place with a MercoTronic or Stevens coil tester can test the coils and condensers for you.

 

[ole1kanoi]

Re: 20 hp merc running on 1 cylinder?

Laddies,

Thanks for the reply, but I was looking for a way for ME to test it. I have a lot of tools, and access to a lot of tools. I read somewhere that I might be able to use a multimeter for testing coils. The condensers are new, I know that doesn't rule them out though. So, help me out though, am I on the right track? I really want to fix this on my own. This old motor isn't worth $100 an hour, plus parts. Thanks.

 

[Laddies]

Re: 20 hp merc running on 1 cylinder?

You could check the winding with a OHM meter for a opening the primary and secondary windings resistance should perry well be the same or do as I did buy a tester as OHM testing is inconclusive.

 

[Doug Redinger]

Re: 20 hp merc running on 1 cylinder?

I am having the same problem with a 1966 6 h.p. twin Mercury. I replaced the points and condensers, and it still missed on one cylinder.I then made a new point grounding harness and installed it along with some new plug wires and it still missed. I also checked the shorting switch wiring and disconnected it, just incase it was somehow shorting out the ignition. So I switched the coil positions on the stator plate and the miss moved to the other cyllinder. I am assuming it is a bad coil and have ordered a replacement.
When I would first fire it up it would run on both cylinders, but after about 10 seconds or so it would start to miss, and after about 30 seconds it would not fire on the bad cylinder. I am assuming the heat of running it is increasing the resistance in the bad coil and causing it to start shorting.

Good luck

 

[hkeiner]

Re: 20 hp merc running on 1 cylinder?

Ole1kanoi,

Systematic troubleshooting and diagnosis is often less frustrating/expensive than fixing/replacing parts on a hunch and finding out that the part is not actually bad. You don't seem to mention if you tested the ignition with a spark tester yet. I suggest that you confirm that the spark is actually failing (weak or missing) and causing your problem before replacing any more ignition parts. If the spark is is not failing, then you can troubleshoot for other causes, such as fuel supply or mechanical. If the spark is failing, then a DVA meter and an ohmmeter will help to determine what part has failed. A shop manual for your motor will best describe how to troubleshoot your ingintion system. Just my thoughts...

 

[ole1kanoi]

Re: 20 hp merc running on 1 cylinder?

Hey guys, thanks. I just got back in from working on it. I hooked a spark tester up, and when it happens, the top cylinder has no spark. But, the throttle has to be in the same exact place for it to happen. It is just above the "START" mark on the handle. Also, when I turn it off, the rpms increase, then it diesels for a couple of seconds. Is there timing that needs to be adjusted? When this originally started, I was out on the water, and had to limp back. I just wonder if the jolting of the engine caused the timing to go out of whack. But then why would I be losing spark? Hmmm... Must be the coil, eh?

 

[hkeiner]

Re: 20 hp merc running on 1 cylinder?

The throttle has to be in the same exact place for it to happen. It is just above the "START" mark on the handle.

Something may be shorting out when the handle is just above the START position. I suggest that you do a visual inspection of the wiring and linkage to see what wire is moving when the the handle is put in that position and see if a short is occuring. If nothing is visually apparent, performing some DVA meter or ohmmeter tests while moving the handle through the START position may help find the problem.

You may have more than one ignition related problem at the same time and doing a link and sync is never a bad idea to eliminate possible timing issues.

 

[ole1kanoi]

Re: 20 hp merc running on 1 cylinder?

I replaced all wiring, and cannot see anything shorting out. It will idle all day long, until I slightly twist the throttle. I ohm tested the coil, it reads a steady 1.2 off of the 2 leads coming out of it. Is that the right way to read it? What about where the plug wire is screwed into it? How is the timing adjusted? The dieseling puzzles me. Is it possible that something internally (timing) got bumped out of place?

 

[hkeiner]

Re: 20 hp merc running on 1 cylinder?

The dieseling puzzles me.

If a cylinder is still firing with the ignition off (which is what dieseling is), it would most likely be caused by excessive compression in a cylinder. You might try to do a decarb using SeaFoam to correct this. Regarding your problem with the missing spark, I can't add any more than what has already been been suggested by myself and others in this thread. Good luck...

 

[ole1kanoi]

Re: 20 hp merc running on 1 cylinder?

I ran sea foam through it this morning. I talked with every marine tech in my city, and nobody has the equipment to test a coil that is this old. 2 years ago, on the same cylinder, the insulation on my plug wire broke, and was grounding to the block. I fixed it, and ran it for 2 more years, until now. Could that have caused damage to the coil? I'm sorry for running in circles, but all of my sources are exhausted. Nobody even wants to work on it.
Another thing I thought of was, my kill switch failed, wiring and everything. So I got rid of it, and put a regular toggle switch in. (It near impossible to find parts for this thing) However, looking at it again today, I noticed that the cluster of wires from the kill button itself contains 3 wires. I only ran 2 to my toggle from the plastic insulator on the back of the motor. The 3rd wire must be a ground, huh? Could the way I have been killing the motor caused some sort of damage? In a sense, I am touching the 2 coils together to kill it. Correct?
So maybe by doing that, I melted something in the insulator, and that is what is killing the spark. Just thinking out loud here...

 

[ole1kanoi]

Re: 20 hp merc running on 1 cylinder?

After running around trying to find someone to test my coil, I gave up and put it back together. The only thing I did different was switch the condensers around. It ran fine for over an hour, never once cut out. WTF!? I dunno. Thanks to all who replied.