Question about spark plugs?

Question

I had taking apart an old briggs and stratton lawnmower today to rebuild. I took the cylinder head off and thought it would be cool to watch the spark plug spark. I had two spark plugs, both were brand new. The first one i used was a J19LM. When i screwed it into the threads and pulled the cord, it produced and nice crisp sounding snap noise and a nice blue spark. I took that plug out and installed a RJ19LM spark plug, and re did the same thing. This time, the spark on this plug wasn't as loud, and you had to struggle to hear it, and the spark was a mixture of blueish yellow. Thinking something was wrong, i reinstalled the J19LM and it again had a nice loud snap, and a bright blue spark. The only difference between the plugs was the R. and the plug with the R had a weaker spark. I got done installing the motor and ran both plugs, and they both ran good and had no problems running wise with either. But what gives? would running the plug with the R cause one to foul up inside from a not so good spark?

Answer 1

The R stands for resistor and does create a weaker spark. Briggs & Stratton recommends the use of the resistor plug. As long as the engine starts and runs you will be good.

Answer 2

Running the R plug shouldn't cause any problem. Running the plug with the stronger spark would be more fuel efficient and stronger ignition will allow you to open up the plug gap which can improve power. A faster combustion promotes greater pressure inside the cylinder and that equals power. Will also run hotter.

Answer 3

My guess is the R stands for resistor plug. Your magneto probably can't overcome the resistance built into the plug. The non resistor plug with the bluish spark is far better because it is hotter and will give more complete combustion. Over time the "R" plug will probably foul and give problems. The hotter plug will burn off any fouling.

Answer 4

as smog man said, R is resistor, resistance means that it "cuts back" the spark, thus making it weaker. In an old B&S with the points style ignition, you can use either non resistor or resistor. I run resistor plugs in everything, mainly because non resistor plugs for the brand i use can get hard to find. as long as the lawn mower is running good, no need to switch to a non resistor. I will point out, however, if you've got an electronic ignition, non resistance plugs can cause a lot of problems with them. I know in the manual for my weed eater it says to ONLY use a resistor plug, and that non resistor plugs can fry the coil. as a rule of thumb, if i change a plug and it's a resistor plug i pull out, a resistor plug goes back in. If i pull out a non resistor plug, i'll install a resistor plug back instead. I doubt it would hurt anything about the "self cleaning" wise. rj19lm is a VERY common used plug in lawn mowers. only one other than that i've seen on B&S is a rj17lm, and i haven't seen one of those in a while. not sure how champions heat range works, but the 17 might be hotter than a 19, if it is hotter, then it would keep the firing end cleaner, however you'd then run a higher risk of detonation and pre-igntion, which is NOT good. But to recap, the R plug will have a slightly weaker spark by nature due to it's "resistance." I recall seeing an new murray brand lawn mower the other day at walmart, and it had a rj19lm in. and if engines were being plagued by carbon build up due to weak sparks, B&S would have stopped using resistor type plugs long ago. Like i say, i just use resistor spark plugs in everything, because they are easier to find.

Answer 5

Throw the Champion plugs in the trash. Cross reference over to an NGK or Denso Plug.

Answer 6

The R means it is a resistor plug. It has a resistor in it and it limits the current, creating a weaker spark. As long as the engine is in good shape, the resistor plug will work fine.

Answer 7

yes, that is correct.