odd spark plug

[russ2400]

I have a 00yz125 that runs good but the plug ends up with a black spot on one side . The plug is tan except for the black spot on about 25%. The side that is black is the part of the plug that faces the intake ports. Anyone ever had this situation and or maybe found a cure. thanks

 

[reelrazor]

Yeah, that is fairly normal.

You can get shims that go under the plug sealing washer that allow you to 'index' the plug so that the ground electrode faces in the direction of the dark spot. This will sometimes allow a bike's engine to be more foul resistant.

Mark the plug's nut and install it, notice which direction the ground will be facing, spacer it to get the ground aligned. Sometimes it just takes a leeetle more torque to get a plug there(be careful doing this!)

Ever notice how once in a while you get a 'good' spark plug, seems to last forever and doesn't foul? Chances are that that plug just naturally indexed itself.

 

[russ2400]

Hey, thanks for the tip. I've tried indexing the plug and I've tried switching to the next hotter plug (br8eix) and the results are still this same black spot. I do have (which i forgot to mention) a boysen rad valve on the bike, maybe that? Or maybe a porting issue? I'm stumped! Maybe its nothing to worry about but i've had alot of bikes and seen alot of plugs and have never seen a plug do this. I use the plug color for jetting the bike and a plug like this is a little confusing. Thanks again for any help.

 

[reelrazor]

Yeah, it could be a slight imbalance in the ports, but likely it is that last little 'whoosh' that the intake port applies way up in the stroke. That engine likely has a 'boost' port slotted above the intake that is the last port open(besides the exhaust). Those ports are usually steeper angled and tend to shoot the charge right into the head(thus plug)

RZ's do that commonly, especially if the boost port has been widened any.

I like to use the ground electrode to 'block' that so I point the open side of the plug right at the center of the exhaust port.

Indexing the plug this way also keeps the ground electrode cooled some which will help with consistent ignition in long hard pulls or top end runs.

(EGT gauges are getting fairly cheap, so are infrared thermometers.)