Effects of spark plug gap being too small

[PowellSixO]

Last year I changed my water pump, along with my fuel filter, my fuel bulb, and my spark plugs (IZFR5G) as part of my yearly maintenance routine. At the time when I did my spark plugs, I was told by my uncle that I should NOT gap iridium spark plugs. He said they will come perfectly gapped straight from the box. Install them and run them. So that's what I did. The boat idled great, got out of the hole great, and ran pretty much like normal. I did however lose roughly 2 mph on my top end. I didn't think too much about it, and my boat has ran for an entire year with no issues (other than I never got my 2 mph back). So anyways fast forward to yesterday, and I'm doing the exact same thing to my boat again getting it ready for the local bass tournaments that start in March. I pulled the plugs out, and they all looked as if the engine has been running very rich. I took my feeler gauges, and checked the gap on each spark plug. Every single plug was gaped WELL under the recommended spark plug gap of .032" - .033". They all ranged from .025" to .028", and that's after running them for an entire year (I probably put 175 hours on the boat in that time). So now to my question. What are the effects of gaping your spark plugs TOO small on an Optimax? Will gaping them too small have any performance issues? Can I adjust the gap on iridium spark plugs? The new spark plugs (IZFR5G) that I purchased to put in, also were gaped in the .026" - .028" range. Any info would be greatly appreciated. My serial # is in the title fyi. Also I have included pictures below of the spark plugs, and they are arranged in the same position that they were in the engine. Thanks.
Jake

Oh, and yes I run Don's cocktail in every tank of gas that I run. I run only ethanol free fuel, and my boat stays in my garage year round.













This is a .028" feeler gauge and it just barely fit.

2002 225 Optimax (0T468454)

[howieranger]

First off it is best to measure the gap with a round wire plug gap tool, a flat blade can be misleading. To change the gap most plug gages have a little hook that goes around the ground at the base to bend the ground open a bit. Be careful the ground bends very easy and it only takes a slight bit to move 15-20 thousands. If the gap is too large lever the hook up to close down the gap. Do not use the plier type gapping tools, they will damage the electrode in a heart beat. In general a slightly larger gap helps idle and a narrower gap helps high load high RPM. Don will need to comment as to how the gap will effect your motor.

[ronnieh]

Howie is correct about the flat gauge. It will not work. It will show the gap less than it is. It is my experience that NGK comes very close to the gap that is advertised. Don will elaborate but according to his commonly asked questions certain Optis require a wider gap. I think it is .044. If this is the case for your engine and you can not find the -11 plugs that are gapped wider then follow howie's advice on gapping. You will need to verify but, I think your engine uses the "5G" plug which may not be available in the -11. I think Don indicated the wider gap is for better idle. Only certain engines recommend the wider gap. Be sure to check on this. Don will most likely give you a definitive answer in the morning.

[SilverFox579]

The correct plug for a non-ProXS 225 is IZFR5G gapped .032" - .033" - http://www.bbcboards.net/mercury-3-l...ed-1-15-a.html

Never install spark plugs without verifying gap first and ALWAYS use a wire gauge to measure gap.

[allouttrimmed]

Looking at your pics, looks like there is no anti-seize on the threads at the bottom of the plugs. There appears to be some oil, but make sure to use anti-seize when installing the new ones.

[PowellSixO]

First off it is best to measure the gap with a round wire plug gap tool, a flat blade can be misleading. To change the gap most plug gages have a little hook that goes around the ground at the base to bend the ground open a bit. Be careful the ground bends very easy and it only takes a slight bit to move 15-20 thousands. If the gap is too large lever the hook up to close down the gap. Do not use the plier type gapping tools, they will damage the electrode in a heart beat. In general a slightly larger gap helps idle and a narrower gap helps high load high RPM. Don will need to comment as to how the gap will effect your motor.

Thanks for that tips. I measured both with the flat feeler gauges and the disk spark plug gap tool. The disk did read a little bit bigger than the flat feeler gauges. I'm sure I'll get an even different reading with the wire feeler gauges. I'll pick up a set after work and double check them all.

[PowellSixO]

Howie is correct about the flat gauge. It will not work. It will show the gap less than it is. It is my experience that NGK comes very close to the gap that is advertised. Don will elaborate but according to his commonly asked questions certain Optis require a wider gap. I think it is .044. If this is the case for your engine and you can not find the -11 plugs that are gapped wider then follow howie's advice on gapping. You will need to verify but, I think your engine uses the "5G" plug which may not be available in the -11. I think Don indicated the wider gap is for better idle. Only certain engines recommend the wider gap. Be sure to check on this. Don will most likely give you a definitive answer in the morning.

I'll be sure to be careful when gaping these new plugs. Thanks.

[PowellSixO]

Looking at your pics, looks like there is no anti-seize on the threads at the bottom of the plugs. There appears to be some oil, but make sure to use anti-seize when installing the new ones.

Thanks, I'll be sure to add some anti-seize to the new plugs.

[ronnieh]

BTW that next to last picture appears to have a broken insulator. You have to very careful with a feeler gauge.

[PowellSixO]

BTW that next to last picture appears to have a broken insulator. You have to very careful with a feeler gauge.

It's just a bad picture. The second picture and that picture are of the same spark plug. No broken insulators. I was very gentle with all of the spark plugs. I realize how fragile they.

What I'm really after here is, the affects a spark plug can have on my Optimax if it's under gaped. I ran these old plugs for 175 hours, and the boat still ran good. It started great, got on plain great, accelerated great, and performed overall how I would expect it too run. My only issue was that at the exact same time that I replaced these plugs, I lost 1.5 - 2 mph on the top end. I'm trying to figure out if this could have been the cause. Mercury came up with the spark plug gap specs for a reason I would assume. .032-.033" didn't just come out of thin air. So again my real question is, what affects will I see by running spark plugs that are gaped too small in my Optimax? Thanks.
Jake

[ronnieh]

I am not a spark plug expert and I have not been near a Holiday Inn in over a year. Things like available voltage, compression, fuel mixture and so on can affect gap requirements. In VERY general terms all other variables being the same a smaller gap is conducive to max power. A wider gap more for starting and idle. I seriously doubt that your plugs caused your loss of speed. Your mileage may differ but a 10 deg change in air temperature can cause the change you are talking about. I suspect it would be difficult to measure a top end difference of 1-2 mph even over a range of .030 - .040. Air temp, baro pressure will have a much more dramatic effect on power output. Chasing 1-2 mph over a wide range of operating conditions is going to be difficult with out a dyno. You might find this link interesting.

Ignition System | Spark Plug Gap | CCUS

[EuropeanAM]

Too narrow a plug gap can cause a RICH running condition, incomplete burn of fuel, and potentially carbon buildup. It can also cause a loss of top-end power.

Spark plugs MUST be gapped before installing in the engine. Even iridium plugs! Most of the IZFR.. plugs will be found at .027" right out of the box.

Use ONLY, repeat- USE ONLY A WIRE GAUGE WHEN GAPPING IRIDIUM PLUGS!

Plug gap should be checked via the "Go-and-No-Go Method". NEVER exert even the SLIGHTEST amount of force between the firing pin and the arm, as this will almost certainly destroy/damage the plug.

ANY gap adjustment must be made via a tool that securely grasps the ARM, and manipulates it without ANY CONTACT with the electrode or firing pin (and it's surrounding porcelain).

Very important- GRASP PLUG BY IT"S METAL HEX CENTER when gapping. Do not hold the PORCELAIN PORTION, or you will likely fracture the plug while gapping.

Example of fractured plug:

SparkPlugCrack.jpg

When installing plugs, a LIGHT coating of Anti-Seize on the plug threads is wise. Emphasis on LIGHT COAT.

Spark plugs should be installed with a piece of fuel line (to prevent CROSS-THREADING), and should be TORQUED WITH A TORQUE WRENCH.

Proper torque is 20 lbs/ft.

[PowellSixO]

Thanks for the response Don. I got the wire feeler gauge set last night and I gaped the plugs to exactly .032" and installed them. I probably won't get to run the boat for another week or so due to my work schedule, but I'll update my results when I finally get to run it. I'll also be sending in my fuel rails, injectors, and what not to have you service them once I get some free time. The overhaul I'm working right now doesn't get over until March 21st. I can't wait. I've only had 3 days off in the last 37 days.
Jake

Too narrow a plug gap can cause a RICH running condition, incomplete burn of fuel, and potentially carbon buildup. It can also cause a loss of top-end power.

Spark plugs MUST be gapped before installing in the engine. Even iridium plugs! Most of the IZFR.. plugs will be found at .027" right out of the box.

Use ONLY, repeat- USE ONLY A WIRE GAUGE WHEN GAPPING IRIDIUM PLUGS!

Plug gap should be checked via the "Go-and-No-Go Method". NEVER exert even the SLIGHTEST amount of force between the firing pin and the arm, as this will almost certainly destroy/damage the plug.

ANY gap adjustment must be made via a tool that securely grasps the ARM, and manipulates it without ANY CONTACT with the electrode or firing pin (and it's surrounding porcelain).

Very important- GRASP PLUG BY IT"S METAL HEX CENTER when gapping. Do not hold the PORCELAIN PORTION, or you will likely fracture the plug while gapping.

Example of fractured plug:

SparkPlugCrack.jpg

When installing plugs, a LIGHT coating of Anti-Seize on the plug threads is wise. Emphasis on LIGHT COAT.

Spark plugs should be installed with a piece of fuel line (to prevent CROSS-THREADING), and should be TORQUED WITH A TORQUE WRENCH.

Proper torque is 20 lbs/ft.

[ronnieh]

I think you are much more likely to get the 1-2 mph back with fuel rail/injector service done by Don that anything to do with plug gap. JMO

[PowellSixO]

I think you are much more likely to get the 1-2 mph back with fuel rail/injector service done by Don that anything to do with plug gap. JMO

You are more than likely right. We'll see where the spark plugs, and annual maintenance gets me. I'll be sure to report back. I know that I need to have my fuel rails and what not serviced for sure, even if it's just for peace of mind. I would like to squeeze as much time out of this motor that I can. I just haven't had the money to do it, or the time. Now I finally have the $$$, and not the time. Lol. Soon though, very soon. Thanks everyone for the responses.

[SilverFox579]

Too narrow a plug gap can cause a RICH running condition, incomplete burn of fuel, and potentially carbon buildup. It can also cause a loss of top-end power.

+1.

[tmmfont]

Last year I brought my boat in for service and had my plugs changed. Next time out it idled up fast when cranked up, then idled down so low it would die. When I got home, I pulled all the plugs and they were gapped at .036 when they should have been .042-.044. I regapped all plugs and everything is good. Gap them right and the motor will respond like it supposed to. Good luck

[Savage]

Too narrow a plug gap can cause a RICH running condition, incomplete burn of fuel, and potentially carbon buildup. It can also cause a loss of top-end power.

Spark plugs MUST be gapped before installing in the engine. Even iridium plugs! Most of the IZFR.. plugs will be found at .027" right out of the box.

Use ONLY, repeat- USE ONLY A WIRE GAUGE WHEN GAPPING IRIDIUM PLUGS!

Plug gap should be checked via the "Go-and-No-Go Method". NEVER exert even the SLIGHTEST amount of force between the firing pin and the arm, as this will almost certainly destroy/damage the plug.

ANY gap adjustment must be made via a tool that securely grasps the ARM, and manipulates it without ANY CONTACT with the electrode or firing pin (and it's surrounding porcelain).

Very important- GRASP PLUG BY IT"S METAL HEX CENTER when gapping. Do not hold the PORCELAIN PORTION, or you will likely fracture the plug while gapping.

Example of fractured plug:

SparkPlugCrack.jpg

When installing plugs, a LIGHT coating of Anti-Seize on the plug threads is wise. Emphasis on LIGHT COAT.

Spark plugs should be installed with a piece of fuel line (to prevent CROSS-THREADING), and should be TORQUED WITH A TORQUE WRENCH.

Proper torque is 20 lbs/ft.

Might want to add this ^^^^^^^ procedure to the Commonly asked questions Don.

[OKDennis]

Yes, this should be added to the Commonly asked questions. I was told that the iridium spark plugs couldn't be gapped and were good straight out of the package. In fact I was told not to touch the firing pin period. I would bet this is why my motor has not been idling well. Thanks for the help Don!

I never saw a response on if this scenrio (too small of a plug gap) would cause any long term damage. Potential carbon build up was a problem. Would additional Quickleen be recommended for some time?

[EuropeanAM]

Thread added to the "Commonly Asked- Commonly Used" Sticky post (in 3 pertinent locations).