How fussy are spark plugs?

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#1
I did some searching in the forums and didn't find any concise info on spark plugs and why or when they fail. I was recently doing some jetting work; bike would start and run until about 1/3 to ½ throttle and then would die. I refuse to admit how long it took me to replace the plug, then the bike was off to the races. Can lean and heat ruin a plug or is fouling more a problem?

Thanks


2001 YZ125
1999 KX100
2001 YZ 80
2001 TTR 225 (Big Pig)
 

Rich Rohrich

Moderator / BioHazard
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#2
Even small deposits can build a short circuit path and cause trouble, high heat and the subsequent glazing of the ceramic can cause issues as well. Fouling or deposit formation is the more common problem though.

Plugs are cheap and it's always worth tossing them when you aren't sure. In my opinion, a fresh plug is mandatory for any type of jetting work just to eliminate it as a source of variance.
If you are trying to read the jetting on the plug ceramic then you'll need a handful of fresh plugs to do it right.
 
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#3
Thanks Rich, I will save you reply along with many previous ones. I can't tell you what a great resource these forums are and in particular your expertise. It's like hanging around with some fellows that have seen most of the pitfalls in keeping these high powered beasts going. Saves me a lot of grief.
 
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#4
HAVE SOME COMMENTS ON SPARK PLUGS THAT I'VE LEARNT THROUGH BITTER AND EXPENSIVE EXPERIENCE, THOUGH THIS MAY ONLY COUNT IN THE UK 'COS WE DON'T HAVE THE ELEVATION EXTREMES YOU GUYS HAVE.
ALSO THIS MAY ONLY APPLY TO BIG BORE 2 STROKES 'COS LITTLE ONES DON'T PULL SIDECAR OUTFITS TO WELL. IF ANYTHING IS GOING TO WORK PLUGS HARD IT'S SIDECAR ENDURO OUTFITS THEY HAVE TO KEEP GOING IN CONDITIONS RANGING FROM TAIL PIPE DEEP IN WATER OR WORSE, IDLING WHILST YOU CARRY IT THROUGH A GAP IN THE TREES THAT ISN'T QUITE BIG ENOUGH TO GET IT THOUGH, TO ABSOLUTLY FLAT STICK FOR MILES AT A TIME ALONG FIRE ROADS.

1 GET THE JETTING RIGHT, ESPECIALLY THE BOTTOM END, IF IT'S NICE AND CRISP AND CLEAN IT'LL START AND RUN BETTER AT LOWER REVS, MAKE THE MOTOR MORE RESPONSIVE OVER A WIDER REV RANGE AND WON'T LOAD UP AND FOUL PLUGS. AND IT DOESN'T LOAD THE POOR OLD IGNITION SYSTEM AS MUCH HOPEFULLY EXTENDING IT'S LIFE SPAN.

2 DON'T WASTE YOUR MONEY ON EXPENSIVE RACE PLUGS UNLESS YOUR MOTOR IS SO HIGHLY TUNED IT'S SEVICE LIFE IS THAT OF BUTTERFLY IN A HURRICANE, GO DOWN TO YOU LOCAL CAR FACTORS AND BUY THE APPROPRIATE NGK WITH RESPECT TO HEAT RANGE AND THREAD LENGTH (THEY COME IN PACKS OF 4 IN THE UK). I HAVE NEVER SEEN A DIFFERENCE BETWEEN AN ORDINARY 'ES' TYPE COMPARED TO 'EG' AND 'EV' S EVEN ON A DYNO, IF YOU WANT TO BE DOUBLY SURE YOU CAN FILE BACK THE EARTH ELCTRODE SO IT ONLY COVERS HALF THE FIRING NOSE, IT'S THE CLEAN SHARP EDGE THAT DOES IT. ALSO YOU DON'T HAVE TO BOTHER WITH CLEANING THEN, JUST HOY IT IN THE NEAREST BIN AND FIT ONE OF THE OTHER 3 YOU'VE GOT LEFT. PLUGS MEANT FOR CARS MUST BE SOME OF THE HARDEST WEARING, THEY ARE DESIGNED TO WORK RELIABLY IN 200,000 MILE OLD SMOKERS DOING SHORT RUNS ROUND THE BLOCK, SO THEY WORK JUST FINE IN YOUR CAREFULLY AND LOVINGLY FETTLED RACE BIKE. I PERSONALLY HAVE NEVER HAD ONE GO DOWN ON ME THAT WASN'T DIRECTLY RELATED TO SOME OTHER PROBLEM (I.E. IGNITION GOING DOWN,CARBS FALLING APART, ETC- I'VE EVEN HAD A YZ 490 ENGINE SO FULL OF WATER IT LIFTED THE CYLINDER ENOUGH TO BREAK THE SEAL OF THE BASE GASKET AND BEND THE ROD, THAT STARTED AT THE FIRST ATTEMPT, AFTER DRAINING THE WATER BY TURNING IT UPSIDE DOWN, STRANGLY IT DIDN'T RUN TO WELL (OR LONG) AFTERWARDS), CAN'T SPEAK FOR 4 STROKES BUT ORDINARY COOKING ROAD BIKE PLUGS WILL DO JUST AS WELL.

3 IF YOU RUN A BIG BORE 2 STROKE CR/KX ETC MUCH SWEARING CAN BE SAVED BY USING NGK 'BCM' PREFIX PLUGS THEY ARE DESIGNED FOR SUZUKI RGV 250 ROAD BIKES AND HAVE A SHORT UPPER CERAMIC SO YOU CAN PUT DUCT TAPE OVER THE HOLE IN THE FAIRING TO STOP THE WATER RUNNING INTO THE DOWNWARD POINTING PLUG CAP. YOU CAN CHANGE THEM WITHOUT RESORTING TO REMOVING THE MOTOR FROM THE FRAME, OR TAKING THE TANK OFF.

4 THAT PLUG CAP IS NEW (ISH), WATERPROOF AND THE RIGHT TYPE ISN'T IT?

5 SAME GOES FOR THE IGNITION SYSTEM, WHEN DID YOU LAST ENQUIRE AFTER IT'S HEALTH, OR EVEN LOOK AT IT 'TIL IT WENT WRONG.

ABOVE ALL UNLESS YOU ARE REALLY UNLUCKY, PLUGS FAILLING IS USUALLY A SYMPTOM OF SOMETHING WRONG ELSEWHERE. A NEW PLUG EVERY NOW AND THEN TENDS TO MAKE RACE BIKES FASTER THAN THE LATEST KIT OR LOUD PIPE.