Spark "blow by" on spark plug

[dtrojcak]

I'm having trouble with my 1976 Johnson 35hp.
It's only running on the top cylinder. Other than at idle, it runs very smooth, especially at full throttle on the water, but obviously it won't run very fast and has minimal power.
I've swapped spark plugs and coils around, and the problem stays on the bottom cylinder.
I've noticed that there is black soot on the white insulator part of the spark plug. It would appear that the spark is escaping from the wire boot and grounding itself to the metal nut part of the plug.
Is this normal on these motors or what do I need to look at/for?

The plugs and coils were replaced just before I bought the boat back in March of this year.
I have new points and condensors on order. The existing points look OK. I thought about swapping condensors locations, then points, but decided just to wait and install the new ones and be done with that.

 

[Tim Frank]

1) Get an OEM manual
2) Check spark with a proper tester. Should jump 5/16" gap.
3) Check compression

 

[F_R]

Arcing out from the boot is not normal. The boot is probably compromised with age, cracking or whatever, and should be replaced. Meanwhile, you may be able to help it by smearing some dielectric grease inside it.

But the first question begging answer here is do you have a good spark on both wires? Will it jump a 1/4" open air gap? And of course it won't run on both cylinders if there is another problem besides ignition. Possibilities are compression, reeds, crankcase compression (seals), water ingestion, etc. Overheating was a particular problem with that year motor. Has yours ever been hot?

 

[pro-crastinator]

My first impulse is to replace wires and plugs.
Internal break in wire a possibility, small money.
Yes, spark plugs can fail. More small money.

 

[Tim Frank]

My first impulse is to replace wires and plugs.
Internal break in wire a possibility, small money.
Yes, spark plugs can fail. More small money.

Money is money.
If you troubleshoot sequentially and sensibly, you can replace only what needs to be replaced, and save a lot of time...often MORE valuable than $$$$..

 

[dtrojcak]

I haven't tested for spark on either one, but looking at the spark plugs, I would guess that the bottom one isn't getting a consistent, if any, spark.
The top plug is a dark tan color and relatively dry. The bottom plug is black and oily.
I reversed the plugs, same results. I reversed the coils, same results. The top plug looks normal, the bottom looks black.
The plugs, coils, and plug wires were replaced just before I bought the boat back in March.

When running it at full throttle on the water, it runs very smooth, just not very fast. Every once in a while, maybe 3-6 times per minute, it will act like the other cylinder starts to fire. It revs up, with noticeable acceleration, but it only lasts for about a second or two each time.

It acted similarly a couple of outings back. We were camping at a nearby lake over a weekend. At first, it ran fine. Then it started missing out. I removed both plugs and noticed the black marks on one of them. I switched plug locations, and it ran smooth, but no power, just like now. I then noticed one of the plug wires was loose, almost falling out of the coil. I pushed it back in, and it ran fine the rest of the weekend.

The next trip out it ran fine, no problems.

Now on this last trip, it started acting the same as the weekend camping trip. I unplugged and reconnected the plug wires to the plugs on the water with no change.
I didn't bring my socket set with me on the water, so I couldn't/didn't do much other troubleshooting on the water.

When I got home, I did all of the swapping around, etc. that leads me to believe the bottom cylinder is not getting spark.
The only thing that has not been replaced/swapped is the points and condensors. For $18, it's at least good insurance to replace them, IMO. If the installed ones turn out to be good, then I at least have a set of spares for when they do go out.

 

[Tim Frank]

I noticed the words "guess" and "that leads me to believe".
The fact that you are guessing leads me to believe that you haven't got an OEM manual for that motor. I'd suggest that as next step in troubleshooting.

Most telling, you said

I then noticed one of the plug wires was loose, almost falling out of the coil. I pushed it back in, and it ran fine the rest of the weekend.

I don't have a manual for that year and model so it might be an unfamiliar configuration to me, but all OMC O/Bs that I have ever messed with plug wires, have either threaded connectors on the end that screw into the coil OR just screw onto a threaded post in the coil receptacle....about 6 turns minimum.
I have NEVER seen a push-in connection of the plain end.

This sounds like an intermittent connection between the coil and plug.
Check and fix that, and you might be home and dry.

 

[dtrojcak]

I noticed the words "guess" and "that leads me to believe".
The fact that you are guessing leads me to believe that you haven't got an OEM manual for that motor. I'd suggest that as next step in troubleshooting.

Most telling, you said
I don't have a manual for that year and model so it might be an unfamiliar configuration to me, but all OMC O/Bs that I have ever messed with plug wires, have either threaded connectors on the end that screw into the coil OR just screw onto a threaded post in the coil receptacle....about 6 turns minimum.
I have NEVER seen a push-in connection of the plain end.

This sounds like an intermittent connection between the coil and plug.
Check and fix that, and you might be home and dry.

I've got the OEM manual and a service manual. I'm still relatively new to small engine repairs. I learn better hands on than out of a book.
For me, taking advice/directions from someone who has actually done it is a lot easier than reading about it from the manual.

I'll take a picture of the coils later, but the wire simply pushes into a hole in the coil. There is a prong inside the hole that wedges itself into the center wires of the plug wire.

 

[Tim Frank]

All the coils that I have seen have a threaded centre prong. You have to twist/thread the HT wire into place to make proper contact/connection.
Someone else may weigh in, but I still think that's part of youer problem.

 

[dtrojcak]

All the coils that I have seen have a threaded centre prong. You have to twist/thread the HT wire into place to make proper contact/connection.
Someone else may weigh in, but I still think that's part of youer problem.

That may be a very good possibility.
I will look closer at the prong in the coil tomorrow.
One coil wire would come out easily. Every time I unhooked it from the spark plug, it would also pull out of coil.
The other one held better, but this is the one that is not firing. I had to pull on it quite a bit to get the wire to come out of the coil. When I got this one out, the end of the wires were burnt looking. I snipped off about a 1/4" and reinserted it hoping that would fix the problem, but that didn't work either.

If the wires are supposed to be threaded/screwed into the coil, then they are definitely not making good contact by being just shoved in.

Thanks for all the helpful replies so far.

 

[Tim Frank]

Since we are focusing on plug wires, don't overlook the other end...the plug boot.

Make sure that the metal spring clip thing's pointy "barb" (how's that for tech jargon? ) makes great contact with the plug wire metal core.

Which is a good segue to making sure that the wires are marine, metal core....not the automotive type.
And ....there should be absolutely zero ohms resistance from end to end of those wires.

 

[GA_Boater]

You have swapped plugs/coils and it stays with the bottom cylinder. Have you swapped plug wires while you're at it? If you did, never mind.

 

[dtrojcak]

No Title

Ok, sorry for the long delay. I got busy with other stuff and didn't have time to work/troubleshoot on the boat.
I took a picture of the coil with the spark plug wire removed. There is simply a prong inside that makes contact with the wires in the plug wire.
I fired up the engine after dark so I could see any "leaking" sparks and I could clearly see the spark jumping from under the boot on the bottom cylinder. I pulled the boot off of the spark plug about 1/4" or so, and the jumping spark stopped.
Using a timing light as a sort of spark tester, I discovered I have intermittent spark on the top cylinder.
I tried swapping parts around, but I could not get the problem to "move" to the bottom cylinder.
I swapped coils, then condensors, and then points. The problem stays with the top cylinder.
When running it at half or more throttle, the top cylinder will intermittently fire about a second or so, then it will not fire for 5-10 seconds.
When the top cylinder is not firing, the engine is running smoothly on just the bottom cylinder, so I'm thinking timing, magneto, etc. are all good.

What would cause intermittent spark on only one cylinder?

 

[Tim Frank]

There have been a number of suggestions that you have ignored....or at least not acknowledged.

2) Check spark with a proper tester. Should jump 5/16" gap.
3) Check compression

Arcing out from the boot is not normal. The boot is probably compromised with age, cracking or whatever, and should be replaced. Meanwhile, you may be able to help it by smearing some dielectric grease inside it.

Since we are focusing on plug wires, don't overlook the other end...the plug boot.

Make sure that the metal spring clip thing's pointy "barb" (how's that for tech jargon? ) makes great contact with the plug wire metal core.

Which is a good segue to making sure that the wires are marine, metal core....not the automotive type.
And ....there should be absolutely zero ohms resistance from end to end of those wires.

You don't say whether you swap HT leads with the coils or not. If the HT leads were not swapped as well, I'd suggest that you remove the boot from the bad cylinder's wire and cut 1/2" off the end and reinstall boot.

 

[dtrojcak]

There have been a number of suggestions that you have ignored....or at least not acknowledged.


2) Check spark with a proper tester. Should jump 5/16" gap.
3) Check compression
Arcing out from the boot is not normal. The boot is probably compromised with age, cracking or whatever, and should be replaced. Meanwhile, you may be able to help it by smearing some dielectric grease inside it.



Since we are focusing on plug wires, don't overlook the other end...the plug boot.

Make sure that the metal spring clip thing's pointy "barb" (how's that for tech jargon? ) makes great contact with the plug wire metal core.

Which is a good segue to making sure that the wires are marine, metal core....not the automotive type.
And ....there should be absolutely zero ohms resistance from end to end of those wires.





You don't say whether you swap HT leads with the coils or not. If the HT leads were not swapped as well, I'd suggest that you remove the boot from the bad cylinder's wire and cut 1/2" off the end and reinstall boot.

I don't have a spark tester, but I one time the plug wire came out of the coil while the engine was running. The spark was able to jump about 1/2" from the coil to the wire. Spark tester is on the "to get" list, but using the timing light, I have no spark on the top coil.

I don't have a compression tester. It is also on the "to get" list.
However, I don't see how low compression could possibly cause no spark.

I've already stated that the coils and wires were replaced sometime last year by the previous owner. The boat was not run until I bought it this past March.

However, replacing the coils and wires are next on my list.


The metal spring clip pointy barb is in good shape and is making good contact with the wire core.

The plug wires come with the coils, so I would think they are the proper type. They will be replaced when I order new coils though.

I have swapped just the HT wires and the problem stays on the top cylinder.

 

[dtrojcak]

After re-reading my response, I realize it could be read as being defensive/aggressive.
I apologize if it is interpreted that way.
That is definitely not my intention.

I am becoming frustrated with this motor and I am determined to fix it myself rather than take it to a mechanic.

I am trying to slowly aquire the necessary tools and testers, but it is taking time.

Hunting season is right around the corner, so the boat won't be getting used much until spring, but I definitely want it working by then.
Thank you to all who have already given advice/tips.

 

[Tim Frank]

With the chronology of replaced parts, I do not think that you need any new parts....I think the problem is with the installation.....or some related component.
I would zero in on the points for number 1 cylinder and make sure that there is not some systematic problem....mounting plate worn allowing them to move....stripped lock screw....etc.
And double-clean them!!

 

[Chinewalker]

What spark plugs are you running? The plug listed in the book is Champion UL81J.