Never trust a spark plug

Eric in IL

Well-known Member
I spent two hours this morning chasing fuel starvation symptoms on a Honda pump motor. At one point I had the plug out and saw spark where I had it grounded to the motor instead of at the electrode. So put a new plug in and solved the issue.

Engine ran great with the old plug for about two minutes then would die. You could keep it running if you were quick enough to get to the choke and hit the sweet spot. But it ran poorly when nursing it with the choke.

Wish I would have suspected the plug first.
 
I did the same thing on a Kawasaki push mower once.

It would run fine, them start misfiring at random, sometimes completely quit, sometimes get over it and run fine for a while. It would always start back up as if nothing ever happened.

Went through the fuel system, replaced the coil, was about to replace the ignition module, which was very expensive...

Finally replaced a perfectly good looking plug, problem solved, many hours and $100 later!
 
I had the same thing happen to me decades ago.
The engine ran great with the old plug for about two minutes then would die.
 
Several years back I had a Briggs Engine back fire out the carb. when cracking it over. Acted like a stuck valve ? Tried a new spark plug and that fixed it. I still can't believe it.
 
I have a Stihl 038 that does the exact same thing, chased the problem for a while the first time, ever since then any time it starts acting up a new plug solves the problem.

The saw takes a NGK plug it never looks worn nor does it appear to be burning incorrectly and it will spark all day long hanging from the wire.

Add compression and it's a whole different ballpark.

I probably replace the plug every 2-3 years.
 
I had similar problems with several small engines. One common thread was Torch spark plugs. They are the worst of the worst. Every time I see one, I replace it without question.
 
I grew up on a river and a 3 HP Evinrude. Those 2 cycle engines ate spark plugs.We always carried spares and tools, don't forget sheer pins either.Good old days!! joe
 
Don't trust a clerk either. Swapped engines in a 86 Celebrity. A gamble at the best of times. A million reasons for running poorly and I went through half of em before checking the plugs. Duh.
 
I've got a cast iron 16 hp Briggs single on a Montgomery Ward garden tractor that does the same thing from time to time. My grandfather bought it new, and it's had the same thing happen to plugs several times. I think the heat finally gets to the plug.
 
You're smarter than me. My Honda generator would start first pull and run exactly four minutes then die with no spark. After cool down same story over and over. I changed the coil, fired it up and it ran exactly four minutes and died with no spark. I changed the plug and it has ran perfectly ever since. I'm still embarrassed by that whipping.
 
My uncle had a garden tractor with a 1 cyl. wis. engine. When it didn't start he had to switch plugs & put in the one he took out last time. He kept switching the same 2 plugs. Never did figure out why?
 
Eric on IL,

NOPE! My test plugs,

Guido.
cvphoto154333.jpg
 
(quoted from post at 22:58:50 05/10/23) I grew up on a river and a 3 HP Evinrude. Those 2 cycle engines ate spark plugs.We always carried spares and tools, don't forget sheer pins either.Good old days!! joe

I have one of those. When everything is right, they run like a swiss watch.
 
Yeah but that little alternate firing twin was smooth as silk and with the weedless lower unit models, a perfect (slow motion) fishing companion...easily carried for rental boats too. Only problem I had with them was when you were out of the swamp and ready to go......there wasn't much. If OMC had upped the HP with that LU like on a 7 1/2, would have been a perfect fishing companion for a 14' aluminum rental boat.
 
I have two machines like that. They will start up instantly when cold and run fine until you shut them off. Then they will not restart. Change the plug and it runs perfectly until the next time. I have two or three others that have the same plug for years. No Problem.
 
It's even amazing how many new ones are bad right out of the box. Our new cars used to come with either Champions or NGK. Then a service bulletin was issued advising to replace the Champions with NGK anytime spark plug service was required. Then the new cars came with NGK or Denso. Now most of them have Denso or Yura, with an occasional NGK. If you saw a Yura you'd swear that it's a Champion, just made in South Korea. I haven't noticed any certain brand with more new failures than the others, just seems to be more common than I ever remember seeing in the past.
 
I should add that we see more new cars with very low mileage having spark plug failure than ever before.
 
I have several champion compression sparkplug testers where you can test the plug by looking though the window, as you turn up the compression..... AND..It is extremely rare to find 4 plugs that pass the basic test of firing past 120 lbs. Most are good to 90 lbs, some to a 100 lbs... HOWEVER... ngk plugs always pass the test over the old american brands.. The Japanese plugs seem to test ok to 150ish lbs. Now remember this test set using a old coil and buzzer from the 1960s and is not a high voltage system like new cars have.. But is does correctly represent old tractors, lawn mowers, chain saws, weed eaters and old '60s motorcycles I have. I have 3 of these testers and they all test around the same.

So on the tractors I test two sets of autolites to get one good set for the tractor I am working on. If your working on 1930 and '40s tractors these weaker plugs work fine at 90 lbs of compression typical found. 1950 and newer, you need to watch your plugs!!
 

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