Spark plug questions

[Jing]

Hi everyone
I have a 1999 mercury 25hp Bigfoot it run beautiful, the only little issue is the spark plug in bottom cylinder get build up after 50hr runtime ( lots trolling ). Is it the water getting in ?
Any input appreciated


First picture is the bottom spark plug, second picture is top spark pug

picture is bottom spark plug

 

[Teddy69]

4 stroke correct?

 

[Teddy69]

Plugs that look like that usually mean the engine is running to hot

 

[GPS]

How long since you put a water pump impeller in? Could be over heating. Is the plug the correct plug for that year and model?

 

[Jing]

4 stroke correct?

Yes, it is 4 stroke, but the ignition simple to fire every stroke , like 2 stroke engines

 

[DaveJ]

Looks like ash fouling..running too hot, or can be from burning bad gas, additives, oil, etc. Trolling is tough on them.

 

[Teddy69]

Yes, it is 4 stroke, but the ignition simple to fire every stroke , like 2 stroke engines

yes but that build up white colour can mean 2 different things 2 stroke vs 4 stroke I’d say it’s definitely running to hot I’d also keep an eye on your oil level as it might burning a bit of oil in that cylinder but first I’d make sure it’s not to hot I’d check your water pump impeller

 

[Jing]

yes but that build up white colour can mean 2 different things 2 stroke vs 4 stroke I’d say it’s definitely running to hot I’d also keep an eye on your oil level as it might burning a bit of oil in that cylinder but first I’d make sure it’s not to hot I’d check your water pump impeller

Thank you for reply, the water pump changed 2 years ago, motor tail stream is very strong, and thermostat work fine

 

[Deleted member 7631]

Are you sure the thermostat is working? They can fail open so the tail stream looks good. I replaceed both my thermostat and water impeller last year. My Honda thermostat didn't close. Ran rough until I changed it.

Why do white build-ups form on the spark plugs?​

White build-ups on spark plugs are often a cause of overheating because of a bad ignition process caused by a suboptimal fuel-air ratio or the ignition is not present at all. Being exposed to high temperatures, the dark carbon deposits burn out, while the more persistent light-colored ones remain.

 

[Jing]

Are you sure the thermostat is working? They can fail open so the tail stream looks good. I replaceed both my thermostat and water impeller last year. My Honda thermostat didn't close. Ran rough until I changed it.

Why do white build-ups form on the spark plugs?​

White build-ups on spark plugs are often a cause of overheating because of a bad ignition process caused by a suboptimal fuel-air ratio or the ignition is not present at all. Being exposed to high temperatures, the dark carbon deposits burn out, while the more persistent light-colored ones remain.

Thank you for your information, I tested the thermostat in 60 degree water last year, I will check it again. This motor have done this build-up for 10 years since I owned it, I clean the bottom cylinder plug 3-4 times each summer, I do lots trolling

 

[GPS]

Keep doing what you’re doing if that has worked fine for ten years.

 

[Wave Runner]

@Jing... There's definitely a problem. I troll 80-100 hours each season with my Honda 90 and all 4 plugs stay almost totally clean. I change them every 2 years just to be safe but really they still look fine never any buildup of deposits. These are the ones I changed after 2 years with probably close to 200 hours of trolling and this motor is on year 22 of doing this.

 

[Outdorman]

you might wight want to do a compression test, might be oil too. Valve seals might cause the same issue.

 

[Letsfish]

Hi everyone
I have a 1999 mercury 25hp Bigfoot it run beautiful, the only little issue is the spark plug in bottom cylinder get build up after 50hr runtime ( lots trolling ). Is it the water getting in ?
Any input appreciated


First picture is the bottom spark plug, second picture is top spark pug
View attachment 60151
View attachment 60150 picture is bottom spark plug

This is not a symptom of an overheated cylinder or a hot plug (hot burn from ign or timing) This is however a symptom of either a leaky valve seal or weak compression and worn piston comp and oil rings on that cyl. If you take a close look at the plug you will see oil on the base of the plug. If It was my motor I would be doing a compression check on all cyls. If it is a bad oil seal you will see blue smoke on startup. Particularly when the engine has sat for a period of time while not in use. Try running premium fuel if you are not already. No additives either. Check compression, and watch the exhaust on a first start after sitting for a few days. Hope this helps.

 

[Jing]

This is not a symptom of an overheated cylinder or a hot plug (hot burn from ign or timing) This is however a symptom of either a leaky valve seal or weak compression and worn piston comp and oil rings on that cyl. If you take a close look at the plug you will see oil on the base of the plug. If It was my motor I would be doing a compression check on all cyls. If it is a bad oil seal you will see blue smoke on startup. Particularly when the engine has sat for a period of time while not in use. Try running premium fuel if you are not already. No additives either. Check compression, and watch the exhaust on a first start after sitting for a few days. Hope this helps.

Thank you for all your input. I did compression test 2 years age, with wide open throttle both cylinder are 180psi and 175psi. I always use premium gas.

@Jing... There's definitely a problem. I troll 80-100 hours each season with my Honda 90 and all 4 plugs stay almost totally clean. I change them every 2 years just to be safe but really they still look fine never any buildup of deposits. These are the ones I changed after 2 years with probably close to 200 hours of trolling and this motor is on year 22 of doing this.

View attachment 60184View attachment 60185

those plug are so heathy and pretty

 

[Letsfish]

Thank you for all your input. I did compression test 2 years age, with wide open throttle both cylinder are 180psi and 175psi. I always use premium gas.

those plug are so heathy and pretty

2 seasons is a while ago. Any 4 stroke can have compression but still use oil. The oil can pass oil rings that are not wiping the oil down right and a bad valve seal will do the same.

 

[Wave Runner]

Thank you for all your input. I did compression test 2 years age, with wide open throttle both cylinder are 180psi and 175psi. I always use premium gas.

those plug are so heathy and pretty

When I change the plugs every 2 years I wonder why because I notice no difference with brand new ones. Another thing I have always run E10 gas for that whole 22 years with countless hours trolling. As @Letsfish said oil must be getting by the rings on that cylinder to have that kind of deposit buildup. Wish you well and hope you get the issue sorted out soon.

 

[skamaniac]

Hi everyone
I have a 1999 mercury 25hp Bigfoot it run beautiful, the only little issue is the spark plug in bottom cylinder get build up after 50hr runtime ( lots trolling ). Is it the water getting in ?
Any input appreciated


First picture is the bottom spark plug, second picture is top spark pug
View attachment 60151
View attachment 60150 picture is bottom spark plug

You may be right re the water. I’ve seen plugs looking like that from engines with leaking head gasket.

 

[Outdorman]

starting to maybe look like seals. Accelerate to a cruising speed then back off the throttle for a bit and then give more throttle. This will create a temporary vacuum in the combustion chamber and when you get back on the throttle if it is valve seals you will see a puff of smoke that was pulled down through the valves into the combustion chamber. You can also do a pressure test, there are adapters from a compression test that you can hook an air line to and then listen for air flow from the carb or the exhaust or the water pump. From the carb usually play in the valves, from the exhaust possible rings, from the water pump that's a head gasket. Hope you sort this pain in the ass.

 

[FlyingViking]

Conduct a leak-down test as opposed to a compression test. They aren’t the same and point to different “culprits”.

Sounds like a leaky seal to me