# Spark plug quandry



## Maytag (Dec 10, 2007)

I have a spark plug question.
Why are spark plugs in 2 and 4 cycle engines changed so often? I will give you the fact that it is a small price to pay for general maintenance. No argument there. Is that the only reason?

But my question goes deeper. If the plugs made for lawn machines are even HALF the quality and durability of a car spark plug we should NEVER have to change one short of physical damage (e.g. shrub breaking an insulator). The plugs in my car have over 60,000 miles on them. That's 1,000 hours at about 2400 rpm assuming 60mph. Many lawn machines run at about 3400 rpm. For the sake of simpler math lets say they run at 4800 rpm or double my car. So they would fire the same amount of times in 500 hrs (more like 700hrs at lower rpms) as my car does in 1000 hrs (assuming both also fire on the exhaust stroke - my car does). Ok - so - if I mow 25 hrs a year (1 hr every week for almost HALF A YEAR and I'm in the snow belt) my plugs should last 20 years. 

Way conservatively 10 years if the plugs are only half as good or used twice as long. Why did I need the other nine plugs? What am I missing?

And if every side of my house has a southern view what color is the bear outside?:freak:
>Maytag


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## dehrhardt (Apr 1, 2009)

Maytag said:


> I have a spark plug question.
> Why are spark plugs in 2 and 4 cycle engines changed so often? I will give you the fact that it is a small price to pay for general maintenance. No argument there. Is that the only reason?
> 
> But my question goes deeper. If the plugs made for lawn machines are even HALF the quality and durability of a car spark plug we should NEVER have to change one short of physical damage (e.g. shrub breaking an insulator). The plugs in my car have over 60,000 miles on them. That's 1,000 hours at about 2400 rpm assuming 60mph. Many lawn machines run at about 3400 rpm. For the sake of simpler math lets say they run at 4800 rpm or double my car. So they would fire the same amount of times in 500 hrs (more like 700hrs at lower rpms) as my car does in 1000 hrs (assuming both also fire on the exhaust stroke - my car does). Ok - so - if I mow 25 hrs a year (1 hr every week for almost HALF A YEAR and I'm in the snow belt) my plugs should last 20 years.
> ...


Maytag,

I,m not an engineer, but I have some answers.

First, two cycle engines have to burn a fuel/oil mix for lubrication. This has the unfortunate side effect that carbon accumulates in the combustion chamber including the spark plugs, and the exhaust ports/screens/muffler and increases the maintenance required.

With four cycle power equipment engines, there probably is more plug replacement going on than what is required. Car engines didn't always go 60,000 mi between plug changes. The removal of lead from gasoline helped a lot. One major difference between cars and small engines has always been fuel management. Even when cars had carburetors, they were more sophisticated than what you find on power equipment. Also, until small engines came with fixed jets, people would adjust the mixture on their small engines to whatever they thought right. Often it would be too rich and lead to carboning of the plug. Now that there are fixed jets on all carbs, it wouldn't surprise me to find plugs lasting longer. That being said, my brother has a 1972, 15hp Sears Suburban lawn and garden tractor my father bought new, that probably hasn't had more than five plugs in it during its life.

None-the-less, it is usually cheap to change the plug.
The bear is white!


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## Maytag (Dec 10, 2007)

Cool dehrhardt. Thanks for the response. I wasn't thinking along the line of "fouled" as much as I was damaged, worn out, or otherwise "tired" plugs. Didn't think far enough back for the lead issue either. But you point out very valid issues. Again I agree with "for $.25 more just change the plug", but I would think a soft wire brush (brass bristles?) would get most of that crud off and still not "require" a plug change. However I guess loosening up the crud only to have a missed piece fall in the engine more than justifies the $2-$3 bucks.

Just trying to generate some discussion on a topic that always befuddled me.

Thanks again.

And the answer is: It WAS white. Until I shot it. LOL 

>Maytag


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## rotti1968 (Apr 26, 2009)

One thing I have found on almost all small equipment is at some point the owner,customer,or who ever worked on it before me replaced the spark plug. This being said I would go as far as 80% of the time the plug that was installed at that time was of the wrong heat range/size, or not what is required by the manufacture of the engine. 
You go down to auto zone with your plug out of your power equipment and the kid behind the counter looks up the plug you gave him and returns with the will fit plug, he did not take the time to make sure its what is called for in the application. 
So you return and install that plug in your equipment and go about using it. Well now there maybe things taking place inside that engine that may shorten the life of the motor, or give you headaches and spend money on repairs,or chase a ghost of a problem that may have been caused by this spark plug.
The engine manufactures spend lots of money to come up up with the correct plug for the application. Also there are specs per the manufacture just like the owners manual for your car or truck on when that spark plug should be changed. I came out of golf course equipment repair and for what it cost me to replace spark plugs during winter maintenance was worth every penny, when in mid season not having a machine not working due to an ignition issue.
Just some of my thoughts on why I replace plugs on repairs for customers.

just on a side note if I get a unit in for repair and I don't find anything wrong with the spark plug I will not replace just because. there is no reason to waste a customers money on something that is not needed. the only time I may not do this is on the 2 stroke engines that are being used a lot harder,at way faster rpms 7000 to 15,000,mixed fuels , and very harsh environments. And not knowing the last time it mat have been serviced if ever.


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## 30yearTech (Sep 14, 2006)

I would say that probably the majority of spark plugs replaced on small engines are good. I change a plug whenever I service an engine, I call it cheap insurance. I don't always know the history behind an engine I am servicing, and therefore cannot predict whether the plug in it will not give the operator any problems. There are many things that can affect the life of a spark plug. A dirty air filter, a malfunctioning carburetor, stale fuel, water in fuel, short run cycles on the engine, etc...

Plugs fail for a reason and rarely just because. My 1983 Toro mower still has the original plug in it. It's mine so I will replace it when it finally fails, but if this was a customers unit, it would have been replaced a long time ago. I don't want to have to go back and redo a job, just because I didn't replace the plug.


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## Maytag (Dec 10, 2007)

Thanks for the response 30yeartech (and all). You mention several things that can affect the life of a plug (dirty filter etc.) That makes me think of the plug pictures in the Haynes auto repair manuals. I'll have to revisit that. I forgot all about them.

I concur on the change a customers plug. It's basically good will. If the customer comes back for just a plug how do you justify his down time and trip to and from the shop, and extra shop charges (or just as bad - no charges), and ..and.. and for a $3 part that is so critical and so easy to change. It's like getting a new tire on a car and they leave the old valve stem in the rim only to have it fail from dry rot a week later.

Cool.:thumbsup:
>Maytag


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## indypower (Apr 8, 2009)

Another way of looking at it is that you use your car every day for a couple hours or more. You use your mower every 2-4 weeks for a couple hours. Your mower won't start. Cheap easy fix: replace the spark plug. Mower has 1 or 2 plugs. Your car has 4-8 plugs. Mower plugs easy to get to. Car plugs not so easy.
The older mowers had adjusting screws on the carbs. Everyone had to play with them not knowing how to properly adjust them and ended up fouling the plugs. Very rarely did they try to adjust the carb on their car.
Human nature says "there isn't much to this mower. it can't be that hard to adjust it. there are only 3 screws on the carb. if the first screw doesn't work, just try the other 2."
Cars are complicated and needed to get to work every day. Mowers are simple and I can let the grass grow for a couple more weeks or borrow my neighbor's if I can't fix it.


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## Mr.fixit (Mar 25, 2010)

Another reason like said earlier is carbon. I think Briggs says to remove heads and clean them every 50 to 100 hr. An engine the stays and a set rpm accumulates carbon very easily. Just my 2 cents guys. AJ


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