Plug issues

[evo550]

I have been having some issues with plugs failing after as little as an hours use. It starts with a little cough and splutter, then within a minute or so starts running real rough with backfires...then dead.
The plug looks fine, almost brand new, jetting is spot on, oil 50:1 motul 800.
I have tried all different types and heat ranges, makes no difference.
I have heard somewhere of a small film of carbon build up on the insulator that allows the charge to run down and earth out instead of jumping the gap. Anyone heard of this.
F#@kin sick of buying plugs by the 6 pac, I'm sure I didn't have this problem 20 years ago.

[JohnnyO]

What bike is it in? Usually a weak spark or rich jetting causes fowling.

[evo550]

What bike is it in? Usually a weak spark or rich jetting causes fowling.

..mmmmm, sons yz85.
Jetting is pretty much spot on, already 1 size leaner on pilot, plug comes out looking new, no oily deposits or carbon build up in the gap.
When plugs used to foul, the bike would start to miss and eventually get worse over the course of the day till it eventually stopped.
This thing misses about twice, then coughs, wheezes and backfires like mad then stops dead in about 30 sec. Fit a new plug and it's off again like a rabbid dog.
I will post a pic of the plug I just pulled out and see if I'm missing something.

[Montynut]

HT Coil breakdown or stator on the way out

I assumed you checked the plug cap? make sure you have not fitted a resistor plug plus a resistor cap

[Lozza]

I assumed you checked the plug cap? make sure you have not fitted a resistor plug plus a resistor cap

My thought also but fitting a non-resistor plug and cap.

Need to have a 'R' in the plug number or a 5K Ohm cap one or the other but not both or niether. 

[evo550]

Plug and cap compatibility, well I never
Just checked, plug is a br10eix (irridium), but have also tried many others.
Cap has TD T-150 stamped on it. Not sure if it has been replaced in the past.
How do I check cap Ohm rating?

[TM BILL]

Put your Ohm meter across the too contacts lead end and plug end and measure the resistance . or get the kid to stick his finger in the plug cap while you crank it over , if he jumps above the seat of the bike its a non resistor cap and below the seat its a resistor cap 

[evo550]

Pics of plug,

[PCMAX]

Need to have a 'R' in the plug number or a 5K Ohm cap one or the other but not both or niether.

Lozza,
I'm running a non resistor cap AND non resistor plug, wasn't aware that this was a bad thing. Why?

Evo555,
I was having similar plug problems with the standard NGK B8ES I though I had just been unlucky and bought a bad batch. I have had better luck with B8EGV or Iridium BR8EIX

[John Orchard]

Pics of plug,

Looks too rich/wet to me, I run a dry chocolate brown at the richest, I like a tan colour if pos.

[OverTheHill]

was starting to suspect stator charge coil issues [which still could be the case] but one pic shows the plug enough to say it's at the cold end of the range & looks well oil'd up [shorted out]. At 50/1 mix i wonder if it's something like --is it sucking gearbox oil in through crank seal on the clutch side [unlikely but!!], or--clutching at straws here--have seen the tail pipe on [old] trail & mx bikes blocked enough to effect top end power, consequently the rider has the throttle wide open but not getting much performance as such. Or is the carb over-rich for some undiscovered fault like[i dunno] main jet fallen out, choke accidentally left on or idle mixture screw turned all the way in, ummm thinking--had a road 200 twin once that had a blocked air bleed into the needle jet & ran rich midrange. Sorry if i'm woffling on but does basically look too rich & oily. Goodluck. ps, muffler falling to bits & semi-blocking outlet.

[evo550]

but one pic shows the plug enough to say it's at the cold end of the range & looks well oil'd up [shorted out].

Shorted out ? That has me intreiged.
The exhaust system is almost new, and the bike revs out nice and crisp, carb has no issues and I've even taken it back to Yamaha twice to check float level. Needle is on leanest clip and I have dropped the pilot (slow) jet one size.
Have tried plugs from 8 through to 10, makes no difference.
Still curious on the resistor cap thing as well, although Yamaha has a "R" plug listed as standard, so it should be right(unless someone has changed the cap...

[Lozza]

Plug and cap compatibility, well I never
Just checked, plug is a br10eix (irridium), but have also tried many others.
Cap has TD T-150 stamped on it. Not sure if it has been replaced in the past.
How do I check cap Ohm rating?

This doesn't cross refernce with any NGK cap number. Sure it's not TB --EM/A ?

Need to have a 'R' in the plug number or a 5K Ohm cap one or the other but not both or niether.

Lozza,
I'm running a non resistor cap AND non resistor plug, wasn't aware that this was a bad thing. Why?

From NGK website

Q: When should I use a resistor spark plug?

A: NGK "R" or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking.

NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.

They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer.

In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer undesirable side effects such as an erratic idle, high-rpm misfire, engine run-on, power drop off at certain rpm levels and abnormal combustion.

[evo550]

The cap most definately says TD...but T-130 not 150, the entire ignition system is brand free except for the CDI which has a "W" in a box moulded into it.
I am struggling to come to terms with the rich jetting theory purely because we have two other bikes that both are jetted rich, one to the extent of the dreaded black splodge covering the swingarm each time we go out, but neither of the other two bikes has ever had a plug problem.
Would like to know what overthehill meant by the plug "shorting out"

[Montynut]

That looks like an EIX plug so is a resistor plug. They come with a very large gap which may need reducing even though NGK says not to adjust it.

You have said that all the ignition system has been replaced? Why?

Have you rrefurbished the bike such as painting the frame and engine cases? If so did you removed paint from the engine mounts both chassis and motor and also under the HT boil mount? Could be bad earthing causing weak spark. People seem to forget that the earth system is more important than the wiring itself.