effect of larger spark plug gap

[Gizmobreaker]

I have 1985 Johnson Seahorse 15 long shaft J15RLCOB. After all the help (carb rebuild, ahem, twice) that I was generously given, here, last year it is running very nicely. Thank you.

Last weekend, I started it up for the first time since it was professionally winterized and stored. Unlike previous years, I thought that I would burn off all the fogging using the old plugs, then switch to new ones. It started on the first pull and ran like a champ. After about 15 minutes, I let it cool off a bit, then changed the plugs to new Champion L77JC4 plugs, gapped .030. It started right up but did not seem to be making as much power. I checked the old plugs and the gap was .035. Yes, brand new gas, 89 octane, 50:1. Cleaned the old gas out of the fuel hose, first. Put it in the car.

What happens when you gap the plugs wider? My power pack and my coils are new, last season. Am I getting "more spark"?

I should say that the old plugs were pretty ugly - fouled and the metal of the electrodes is worn and dull. Why would they run better? Just lucky plugs? Did I buy them where Jack bought his magic beans?

Thank you for any advice you can provide.

Marty

 

[steelespike]

Re: effect of larger spark plug gap

First of all I'm pretty sure the motyor is designed to run on 87 octane. the 89 is actually harder for the motor to ignite.
Won't hurt to use up the 89.
I don't think you would notice any difference in power at .030 The old plugs were probably just worn to .035.
I do wonder if you may have lost a cylinder for some reason.
You could warm her up and pull the plugs and compare them. If one cylinder is dead that plug may seem cooler
and will be wet.

 

[Rick.]

Re: effect of larger spark plug gap

If your brave enough you could (while running) pull one plug wire at a time. It should run fine on one cylinder. If you pull a wire and it quits, that is the cylinder that is firing. Best of luck. Rick.

 

[oldcatamount]

Re: effect of larger spark plug gap

I had an '86 Johnson 15hp and the recommended plug gap was .040. I noticed unburned fuel drooling out the exhaust with plugs gapped at .030.

 

[Gizmobreaker]

Re: effect of larger spark plug gap

I'll check the service manual about the gas. The service tech always says 87 will run but 89 is better.

You are probably correct about the wear.

Both cylinders are firing. (No need to be brave if you have spark plug pliers!) I swapped the old plugs back in and out and the difference is clear.

The service manual calls for .030. I haven't noticed any drooling except by me.

The fundamental question is, "What is the result of too much gap?" It seems likely that the coil and ignition would see more load and have to work harder (not sure that "harder" is the right word). But would there be a "bigger" spark? Would the larger gap have the effect of slightly retarding the timing?

OK, I did some research online, mostly at auto racing sites. Apparently, when you increase the gap, it shortens the life of the plug slightly and increases the ability of the motor to run on a lean mixture. Effectively, what happens is that the larger gap increases the amount of air/fuel mixture that is in the path of the spark. Since the carburetor does a pretty poor job of evening out the mix, the wider gap actually helps. Go figure. You don't want to increase it too much as the higher current will start to break down your ignition wires but that's not likely with .035. Some of the racers are opening their plugs up to .070 but in an outboard that might cause too many misfires.

I'm going to get a new set of plugs and gap them .035 and try them against the .030 plugs.

Thanks, guys.

 

[gm280]

Re: effect of larger spark plug gap

I'll check the service manual about the gas. The service tech always says 87 will run but 89 is better.

You are probably correct about the wear.

Both cylinders are firing. (No need to be brave if you have spark plug pliers!) I swapped the old plugs back in and out and the difference is clear.

The service manual calls for .030. I haven't noticed any drooling except by me.

The fundamental question is, "What is the result of too much gap?" It seems likely that the coil and ignition would see more load and have to work harder (not sure that "harder" is the right word). But would there be a "bigger" spark? Would the larger gap have the effect of slightly retarding the timing?

OK, I did some research online, mostly at auto racing sites. Apparently, when you increase the gap, it shortens the life of the plug slightly and increases the ability of the motor to run on a lean mixture. Effectively, what happens is that the larger gap increases the amount of air/fuel mixture that is in the path of the spark. Since the carburetor does a pretty poor job of evening out the mix, the wider gap actually helps. Go figure. You don't want to increase it too much as the higher current will start to break down your ignition wires but that's not likely with .035. Some of the racers are opening their plugs up to .070 but in an outboard that might cause too many misfires.

I'm going to get a new set of plugs and gap them .035 and try them against the .030 plugs.

Thanks, guys.

Looking at this situation the larger the gap the higher the break over voltage has to get before jumping the gap. They actually use to make spark plugs (maybe they still do) with internal gaps in them to help with fouling out. But like anything, you have to give and take with anything you do. When you force the ignition system to build up to a higher break over voltage you also force the system to, in essence, work hard to produce that break over voltage. The consequences can be break down of the coils and such because they too have only so much insulation to the higher voltage and heat produced as well. So just be prepared to replace shorted out coils and such sooner than expected... Like always you don?t get anything for free... Car ignition systems that did require .070? plug gaps also had the appropriate coils and such to handle the increased higher voltages too. And even with those high output coils, it did wreak havoc on the ignition systems. That is why they went back to something a little smaller in plug gaps... How long would a .035? plug gap last as compared to .030? plug gap is anybody?s guess. Maybe you (I mean your OB) can be the Ginny Pig for that test and let us know... :high5: