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Author Topic:   Spark Plug Gap: Effect on Performance
jimh posted 05-30-2005 01:40 PM ET (US)   Profile for jimh   Send Email to jimh  
What is the effect on a classic two-stroke outboard motor if the spark plug gap is out of tolerance by being greater than the recommended gap?

During normal operation, it would be reasonable to assume that the spark plug gap would tend to increase due to erosion of the electrode. On many older two-stroke engines the spark voltage is very high (and not well regulated). I also assume that higher spark voltage causes faster spark plug wear. What would be a typical interval of operating hours before spark plug wear would increase the gap to a point where there would be an effect on engine performance?

Chuck Tribolet posted 05-30-2005 10:47 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The wider the gap, the harder it is to spark across it.
You'll see problems at high revs first, at least with a
coil ignition, because the coil is a bit less saturated
so less spark at high revs. Dunno about magnetos and such.

The narrower the gap, the faster the plug wear.

Nice square corners on the electrodes helps make a reliable
spark. Gross over simplification, and not technically
correct, but you can think of the sharp corner as
concentrating the electrons. And it's the sharp corners that
wear first.

All you need is a enough spark to start the fuel burning,
every single power stroke. Once the fuel starts, nothing
electical is going to have any influence on it.


Sal DiMercurio posted 05-30-2005 11:20 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Chuck, good post.
I'll tell you right now, the spark on the Evinrude FICHT engines is some kinda hot.
I gap mine at .028 even though OMC said .030 because the gap just widens so fast .
I change my plugs every 75 hours & thier well into the .040+ range.
You will notice a slight miss once in a while on top end & yes, the engine dosen't start in the normal 1/2 or 2 turns of the fly wheel, it takes sometimes twice as many like from 1/2 second to 2 - 3 seconds to cold start & thats a long time for these engines.
jimh posted 05-31-2005 07:16 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the information on the spark plug gap. I just checked the plugs in my 225-HP Evinrude V-6. They were worn to a gap greater than 0.040 after about 100 hours of use.

I suppose the gap distance could change the timing slightly, too. If the gap is smaller, the plug may fire slightly faster and thus slightly sooner.

Speedo66 posted 05-31-2005 07:43 AM ET (US)     Profile for Speedo66  Send Email to Speedo66     
jimh, I don't believe the size of the gap has anything to do with the timing. When the spark arrives at the plug, which I think is considered timing, is strictly a function of the distributor or magneto.

The spark kernel may perhaps transverse the gap quicker if it is smaller, but that is not timing.


bsmotril posted 05-31-2005 09:10 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
If the ignition is hot enough to keep making a spark, the larger gap has the benefit of being better able to ignite a lean mixture, and is less likley to misfire under high load than a smaller gap would. BillS
LHG posted 05-31-2005 01:18 PM ET (US)     Profile for LHG    
I was't aware outboard's used gapped plugs. Even my 1985 Merc 115 sixes use a center electrode type plug.

What would be the reason Mercury went to this type of plug over 20 years ago? Which style is better, other than eliminating the need to worry about gapping?

jimh posted 05-31-2005 08:16 PM ET (US)     Profile for jimh  Send Email to jimh     
I would assume that a gapless plug would be less prone to fouling. Maybe they are used in engines where fouling is a problem.

I would have to assume that if there were some special advantage in terms of improved performance to be obtained with gapless plugs, that internal combustion engines of all kinds would use them. Instead, they seem to only be found in certain two-stroke outboard motors.

OMC recommends gapless plugs for sustained high-speed operation. There is no reason given, but I would assume that perhaps their use would avoid problems with the plug gap eroding away which might occur in engines that were run perpetually at very high speeds.

Also, the gap on surface gap plug is quite wide, so it probably takes very high voltage to fire across them. Really high spark voltages can cause more radio frequency interference (RFI). Engines with sophisticated electronic controllers may be more susceptible to problems from RFI.

RocketMan posted 06-01-2005 08:57 AM ET (US)     Profile for RocketMan  Send Email to RocketMan     
I've always thought surface-gap plugs were used in two-stroke engines to mitigate oil fouling from the fuel. My assumption has been that the circumferential gap has more usable length with a surface gap plug, which would require more buildup to completely foul out. Also, it seems that the design would be less prone to distortion or erosion with operation time, but that doesn't seem to be supported by the fact that 4-strokes typically use "gapped" plugs.
CFCAJUN posted 06-02-2005 08:51 PM ET (US)     Profile for CFCAJUN  Send Email to CFCAJUN     
Many thanks for this post. My father was confused when I told him I have to gap my plugs. My 1978 Evinrude 85HP V4 uses the .030.

At first, my question was what happens when plugs begin to wear and .030 begins to change. New plugs start out at .030, but wear, and do I need to regap them? If so how often?

Now, after reading this, I am wondering what the effect of uneven wear is. I would suspect under normal conditions that all four plugs should wear equally, and uneven wear may be signs of problems. However, what if one plug is gapped slightly different, not noticeably, or one cylinder behaves differently - how will performance be effected?

Must I continually inspect the plugs to achieve optimum performance?

Great topic.


jimh posted 06-02-2005 09:52 PM ET (US)     Profile for jimh  Send Email to jimh     
The rate of wear or erosion of the spark plug varies with different engine brands and designs. Some models are more infamous for rapid spark plug deterioration, and these models need more frequent checking and replacing of the spark plugs.

The difficulty of removing, checking, and installing spark plugs varies with different engine brands and models. Outboard motors seem to need much more frequent attention to the spark plugs than modern automobile engines. For example, my 1995 car is running on the original spark plugs, but, before you think I have neglected it, there are only 56,000-miles on the car. With an outboard motor, I think 100 hours is a reasonable interval to check the plugs.

If your engine uses surface gap plugs, which are common in many Mercury engines and some Yamaha models, you do not have a gap to erode. You can still check the plug for wear or fouling.

My engine uses a more conventional spark plug with a gap to be set, so I need to periodically check them. Fortunately, the plugs are very easy to remove and install; it takes just a few minutes to change them all. This is not always so easily done. Some engines have plugs located in positions of very difficult access, and removing or installing them takes some time and effort.

jimh posted 06-02-2005 09:54 PM ET (US)     Profile for jimh  Send Email to jimh     
To answer my own initial question, I found, after some research on spark plugs in general, that the effect of spark plug gap wear and increase to larger gap dimension is to increase the potential for misfires, particularly at higher speeds.
seahorse posted 06-03-2005 07:23 AM ET (US)     Profile for seahorse  Send Email to seahorse     


Because most CDI system's voltage output varies with the speed of the flywheel, as plug gap increases the motor may be harder to start. With more wear on the plugs, the starter would have to crank the motor over faster to generate enough voltage to "light the fire". A worn starter or marginal battery along with corroded terminals could keep the motor from firing up with worn plugs, yet be OK with brand new ones gapped correctly.

jeffs22outrage posted 06-03-2005 09:15 AM ET (US)     Profile for jeffs22outrage  Send Email to jeffs22outrage     
Jim, I use NGK center electrode plugs on my V-6 235 Evinrude and it runs much better than with the champion plugs the previous owner had in when I bought her. The NGK's have never fouled out, and my idle was much improved. These plugs are all I have ever used in our familys other whalers which all had Merc Power.

I would recommend you try them in the 225. You can pick them up at any Boat US or West Marine.

Sheila posted 06-03-2005 08:00 PM ET (US)     Profile for Sheila  Send Email to Sheila     
Today I purchased new plugs and wires for my Johnson 1986 V4 90 hp engine. My service manual specifies that the gap be set to .040. I had noted this on the paper beside my notes about the correct plug that I took with me to the store.

The man who sold me the plugs crossed out the ".040" on my note paper and wrote in .030. He told me that he always gaps everything to .030 and that I should, too. Unless the old plugs are truly, horrendously corroded, they were set to .040.

Any idea why the guy at the shop told me to set the gap to .030?

Binkie posted 06-03-2005 08:45 PM ET (US)     Profile for Binkie  Send Email to Binkie     
I know of some boat racers who "index" their plugs, (tighten them so they are all facing the same way.) I can`t figure out of what benefit this would be. Maybe just to worry the competition.
CFCAJUN posted 06-03-2005 08:50 PM ET (US)     Profile for CFCAJUN  Send Email to CFCAJUN     
Wow Sheila, that is odd. I have a manual for OMC, let me see what I find.

Jimh, thanks again. My plugs are easily reached and the plugs are cheap. I noticed they come factory gapped at .030, but I put a gap measurer on my keyring and check them often. For me, if the plugs foul, starts become harder and it wears my batter y and starter. So, I keep a fresh set and change them at the first sign of a problem, or clean them if that's all they need.

Jerry Townsend posted 06-03-2005 09:29 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
A few comments - the performance of CD systems are largely independent of flywheel speed - and typically provide around 40,000 volts (undoubtedly varies with manufacturer). The CD or electronic ignition systems with the high voltage and low current minimizes erosion of the plug contacts.

Having a clean set of plugs in the tool box as CFCAJUN suggests is a good move.

Jimh - I have seen a few words regarding timing in my brief review of the previous comments. I would guess that a large gap could change the timing - though many manufacturers are using an optical ignition technology. I don't have any experience regarding the effect of plug gap - but years ago, I found a 'pseudo' timing shift resulting from problems with spark plug wires on 4 cycle engines - and I would then guess there would be a similar affect on 2 cycle engines. --- Jerry/Idaho

Chuck Tribolet posted 06-03-2005 09:48 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The engine manufacturer spent a lot of time (= $$$) on the dyno
selecting the plug and gap. There's no reason for the
manufacturer to give you anything but the straight skinny.
Absent some really good reason to do otherwise, stick with
it, not with some "equivalent", not with something from a
shadetree mechanic.


jimh posted 06-03-2005 10:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Some older literature from OMC advises setting the gap to 0.040, however I believe that this has been revised in many cases to recommend 0.030.
I spoke with Dave Zammitt of Lockeman's Hardware and Boats, Detroit, about the plug's for my motor. He has been an OMC master mechanic and dealer for decades. His recommendation was to go with the OEM plug, the Champion QL77JC4.

"Surface gap plugs," said Dave, "are only advantageous if the engine is going to consistently run above 4,000 RPM on a long term basis. They are the absolute coldest [temperature range] plugs you can put in an engine."

I bought six new QL77JC4 plugs from Dave. After I run the Fuel System Cleaner additive through the engine with this tank of gas, I'll be installing the new plugs. Gap set at 0.030, per the specifications.

I am with Chuck on this. The engine manufacturer probably tested plenty of plugs before picking one.

There does seem to be a certain panache associated with NGK plugs. I passed on a ten-pack new NGK inductor suppresor plugs when I sold my old boat. After we get the rest of the set-up dialed in on the new boat, we may experiment with a different plug. For now, the OEM recommended will be fine.

Sheila posted 06-04-2005 01:02 AM ET (US)     Profile for Sheila  Send Email to Sheila     
The .040 gap specification is from the Clymer manual for Evinrude/Johnson motors from 1973 through 1990.

Is your suggestion, Jim, that .030 would be more appropriate? This motor is the only one of the three that we currently run for which I have not yet obtained the OEM shop manual.

We did take her out for a little spin this evening, as we're doing safety boat duty for outrigger races tomorrow morning, and wanted to make sure she ran well with the new plugs and wires. She did.

Joe Kriz posted 06-04-2005 01:23 AM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

See this prior post..

OMC changed the gap from .040 to .030 quite a few years ago as they found the older engines performed better and the plugs lasted a little longer according to my mechanic.

Sheila posted 06-04-2005 01:52 AM ET (US)     Profile for Sheila  Send Email to Sheila     
Thanks, Joe!
seahorse posted 06-04-2005 07:11 AM ET (US)     Profile for seahorse  Send Email to seahorse     

The reason OMC changed from initially recommending a .040" gap was because the RFI generated when the gap was above .045" would sometimes affect the VRO and S.L.O.W warning systems which would sound false alarms.

As a rule of thumb, the OMC 2-strokes would erode a sparkplug about .010" in a 100 hr. period, depending on the amount of time at various rpms.

LHG posted 06-04-2005 02:19 PM ET (US)     Profile for LHG    
Do Yamaha 2 strokes use center electrode plugs like the Mercs? These plugs sure do seem a lot more maintenance free.
RocketMan posted 06-04-2005 05:29 PM ET (US)     Profile for RocketMan  Send Email to RocketMan     
My 1985 Yamaha 90 uses electrode-gap (non-surface gap) plugs as specified by OEM.
hookum posted 06-04-2005 09:24 PM ET (US)     Profile for hookum  Send Email to hookum     
A couple of good sites to find what the sparkplug Mfg., recommend's.


jimh posted 06-05-2005 09:42 AM ET (US)     Profile for jimh  Send Email to jimh     
Seahorse--Thanks for the information on the radio frequency interference (RFI) problems associated with wider spark plug gap width. Modern engines have very sophisticated electronic controls, and radio frequency interference would be an area of concern.

It is also much more common now than in the 1980's for a boat powered by an outboard motor to have a radio, and reducing RFI is very important for improving radio reception. I have noticed that my Evinrude outboard (with the plugs gapped at 0.030-inches) is remarkably quiet in terms of RFI. I can copy very weak signals on my VHF Marine Band radio without interference from the ignition sparking. On many other outboard-powered boats I have noticed much higher levels of spark plug generated RFI. Perhaps this is due to the much wider gap on the surface gap spark plugs they use. These seem particularly bad for RFI.

In the Champion nomenclature, the Q in the part number denotes an inductive radio frequency suppressor which has been designed to work specifically with marine capacitor discharge ignition systems. This combination seems to work very well for me.

In summary:

Surface Gap Plugs:

--coldest temperature range to suppress pre-ignition, so less heat to clear fouling deposits;
--less erosion of plug electrode at sustained high-speed operation;
--more electrode area, so less prone to fouling;
--higher cost (almost double);
--higher RFI potential.

Here is what Champion says about the surface gap spark plug:

"Champion surface gap V plugs have a .050-inch annular configuration with no ground electrode. Engineered for extremely cold application, the firing tip is nearly impervious to pre-ignition, however, fouling deposits can be problematic. These plugs have extremely limited use in racing and require a special high energy ignition system and a very rapid rise time(CD). Constructed with a nickel shell for faster heat dissipation and copper core or double copper (when noted) for further heat removal, all racing plugs are non resistor (unless specially noted)."

jeffs22outrage--Thanks for the field data. We will have to compare notes on which plug you are using.

Adam posted 07-25-2005 03:57 PM ET (US)     Profile for Adam  Send Email to Adam     
Hi all!! I just got a used 2001 Johnson 50-HP two-stroke outboard and I have a question for everyone. When I first took the engine out it was hard to start we finally got going and the engine seemed to run fine with a slight miss here and there nothing major. We were running 30mph max. When I got back home I removed the spark plugs and checked the gaps and they were .045, they are suppose to be .030. When I install new plugs at .030 will the motor run slower than 30mph?
jimh posted 07-25-2005 08:14 PM ET (US)     Profile for jimh  Send Email to jimh     
Setting the spark plug gap to the recommended size is the size recommended for the spark plug gap. If you can follow that reasoning.
Salmon Tub posted 07-25-2005 08:30 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Interesting topic. My Nissan 90A calls for a gap of 1mm, or approx. .040. Is there any danger in gapping them to say .030 or .035 to see what the change in performance would be? What is the general result when plugs are gapped less than specified?
Chuck Tribolet posted 07-26-2005 12:42 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Salmon Tub: In the old days of Kettering ignitions (plugs,
points, condensor), we'd routinely close gap the plugs about
10-20%. It seemed to help at high revs, at the expense of
plug life -- they were spec'ed for 12,000 miles and we were
changing more like every 2000 (though they still looked
pretty good). In this day of a coil for every cylinder, it's
probably not necessary.


jimh posted 07-27-2005 09:06 AM ET (US)     Profile for jimh  Send Email to jimh     
I believe we thus have a general rule on spark plug gap and its effects:

Gap TOO WIDE: affects high speed performance.

Gap TOO NARROW: affect plug life due to increased wear.

jimh posted 11-16-2005 10:23 PM ET (US)     Profile for jimh  Send Email to jimh     
[Removed new discussion begun on different topic and created a separate thread for it.]
PeteB88 posted 11-18-2005 12:40 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
Okay - appreciate all this but two questions:

1) mechanic says NGKs that I use in Yamaha 40 come pre-gapped and went on to say that new plugs that have the cardboard cylinder around electrode are all pre-gapped.

2) I have gapped a lot of plugs but what is best gap tool? Wire, the disc thing, feeler gauge? I'll pay for the best.


PeteB88 posted 11-18-2005 12:40 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
Okay - appreciate all this but two questions:

1) mechanic says NGKs that I use in Yamaha 40 come pre-gapped and went on to say that new plugs that have the cardboard cylinder around electrode are all pre-gapped.

2) I have gapped a lot of plugs but what is best gap tool? Wire, the disc thing, feeler gauge? I'll pay for the best.


knothead posted 11-21-2005 10:33 AM ET (US)     Profile for knothead  Send Email to knothead     

I've always used the wire gauge to gap plugs. I was told long ago that this was the preferred tool to gap plugs. Sorry, can't remember where, like many things, now lost in the sands of time.


Chuck Tribolet posted 11-21-2005 12:25 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
What counts for gapping is that:

The gap is right.

The piece that comes from the side is parallel to the surface
of the tip. If it isn't, it will erode quickly where they
are the closest together, and then you will have a wide gap.

Best is to buy plugs that are pregapped correctly. This will
often be a -xx number on the end where xx is the gap (maybe
in metric).

If you actually have to regap them, there's a tool that works
like a pair of pliers. Turn a dial to position the right
feeler guage, put the plug on the gauge, squeeze gently,
done, and the side wire WILL be parallel to the tip.


TommyBoy posted 06-21-2007 09:46 PM ET (US)     Profile for TommyBoy  Send Email to TommyBoy     
Thanks for all the good info. I just checked the gap on my 1989 Johnson 200 and the gap was about [0.075-inch]. Needless to say I had a serious loss of top end RPM. The plugs were about three years old. I also noticed that I WAS getting some freaky warning beeps from my VRO at top RPM that could not be otherwise explained. I sure hope this new set of plugs fixes the problems. Also, on an older engine with somewhat lower compression [90-PSI] on all cylinders would a hotter plug be better? Maybe that is why they recommend the QL78V? or is that a colder plug. May be I am just overanalyzing and should put in a new set of the recommended plugs and RUN IT. Thanks again.
jgkmmoore posted 06-23-2007 07:15 PM ET (US)     Profile for jgkmmoore  Send Email to jgkmmoore     
Run what the manufacturer recommends.They test a lot, and destroy a lot of stuff before the right cambo comes up. They are conservative, so you may enjoy reliability. When you screw with heat ranges, you do so at your own peril. Oft times, one manufacturers plug will run better and last longer than others.Stick with that one. Many manufacturers plugs will outperform Champions consistently.Good idea to renew plugs annually (in the spring, after the first 20 minutes of running).
Chuck Tribolet posted 06-23-2007 10:36 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I doubt a plug change is going to affect VRO.

I'd run the part number AND BRAND the motor maker recommends.
That's what they had in the engine on the dyno. "Equivalents"
are only close.


jimh posted 06-27-2007 09:25 AM ET (US)     Profile for jimh  Send Email to jimh     
RFI from the wide-gap spark plugs may have triggered a false alarm by electrical interference.
Chuck Tribolet posted 06-27-2007 07:48 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jimh, I don't think so, but replacing the plugs is indicated
by the HUGE gap, so let's see what happens.

TommyBoy: what plugs were in there? How many hours on them?


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