engine trouble

[StarTech]

Only one problem with those inline spark testers. They indicate spark even when the plugs are shorting to ground. And plugs with cracked insulators can short when hot. Plus testing plugs out of the engine's compression zone only confirm they fire outside the compression zone. They still can misfire under compression.

I even had one plug a few years ago to cause dieseling under compression load at full throttle only which push beyond the coil's rpm limiter.

It takes experience to diagnose some problems. Yes basic problems are easy but there are unique problems sometimes.

On top of that just because a plug is new doesn't mean it is good. That is why you must a known good plug for testing.

 

[Hammermechanicman]

You are correct the neon inline testers do not test the plug itself. When troubleshooting a problem like this i would put in new plugs and use the the inline testers and run till the problem shows up and see if the testers show a problem. That is why you really need inline neon testers and spark gap testers. Of course most DIY's don't have them.

 

[bertsmobile1]

I think you are both missing the point of using the tester
IT is there to see if the problem is electrical , mechanical or fuel
A first line test to eliminate the magnetos and mower wiring
New plugs will not help you find a coil or bridge wire that is faulty .

 

[StarTech]

They only indicate if the coils and trigger circuits are working and that the kill circuit not grounded. It tell if the coils are shutting due to heat soaking due will not tell if the plugs are actually firing or not. So a problem can still be electrical in nature when plugs are failing themselves. As I said new plugs don't always mean good plugs which why you need to be testing with known good plugs.

Unless spark plugs are firing by something other electricity which btw they can but still need electrical juices to get them hot enough.

 

[Hammermechanicman]

I think you are both missing the point of using the tester
IT is there to see if the problem is electrical , mechanical or fuel
A first line test to eliminate the magnetos and mower wiring
New plugs will not help you find a coil or bridge wire that is faulty .

We are doing basically the same thing. Troubleshooting, eliminating things that are working until you find something not working properly. New plugs eliminate one variable then using the neon testers can identify or eliminate a coil problem WHILE the engine is running poorly.

I have had customers bring in equip and say it has spark by holding the plug to engine and pull rope. Neon tester shows spark. Put plug in and neon tester shows spark but plug arcs under compression and engine won't run. Bad plug. We can easily test coils but don't believe any of the clowns on utube about testing a sparkplug with a meter. Since i don't want to spend $1K on an aircraft sparkplug tester i substitute a new $3 plug and assume the plug isn't the problem. Always a first time but so far all the new plugs have been good.

 

[StarTech]

I wish that was the case with new plugs here. I have seen bad ones right out the package. Not usual if I buy the box full, just individual plug purchases.

As you know Hammer plugs can fail under compression which I proved to one customer this year. When he saw the plug outside the engine he said it was good but I went and pulled a known good and the engine fired right up. He is now a believer that plugs can be even when spark outside the engine.

I just took a couple Oregon plugs apart to find out the insulators where cracked so bad that one plug was broken in two and only held together by the crimped plug shell.

 

[Hammermechanicman]

I have seen quite a few of the chineese no name plugs in the amazon and ebay tune up kits fail within minutes of running. These always go in the trash and it gets a champion of NGK plug. Lots of folks loke to hate on champ plugs but B&S uses million or so
every year in new engines.

 

[smhardesty]

I wish that was the case with new plugs here. I have seen bad ones right out the package. Not usual if I buy the box full, just individual plug purchases.

I just experienced that today. I was working on a Briggs motor on a push mower. It was a simple tune and service. I needed a new air filter and an RJ19LM plug. Made a quick trip to pick the stuff up. Came home, checked the gap to ensure it was correct, shoved the plug in, connected the wire, and spent the next 5 - 10 minutes pulling and cussing. Wouldn't even try to fire. I eliminated things one by one and, on a whim, stuck the old plug back in. One pull and she fired and ran.

I'll be taking this "new" plug back tomorrow. I'm not really sure why I thought to try the old plug, but I'm sure glad I did. LOL! So, yeah, a new plug does NOT mean a good plug.