Jetting and spark plug test

I did the spark plug test yesterday and I got strange results that I do not understand and will be grateful if someone clarify them for me. I had a stock ’02 WR426F and I changed the air filter to TwinAir and put an MIG complete exhaust system. At the moment I started the bike and reved it about 1/4 throttle a strange ugly metal noise came from the engine, but only for two seconds and then disappeared. I was unable to reproduce it again. It was as there is a nut in the cylinder. I drove the bike like this for one day. I noticed on my temp gauge that the bike is running a little bit (10%) hotter that before. I replaced the main jet from 165 to 170, then I made the plug test. I drove about 5 km (3ml) on 1/4 throttle and when I opened the spark. The porcelain insulator was black, very dry with aluminium/metal colour around the centre electrode. I changed the plug with a new one. I tested it at 1/8 and 1/2 and it was clear white without any indication on the porcelain that it had worked. On full throttle I got a little grey on the porcelain and black on the metal electrodes. My bike also is not starting on every kick as before and it starts ok after using the hot start. I understand that all symptoms are for lean mixture, but why hot start helps, when it make the mixture even more lean? Today I will raise the needle 2 clicks.

What do I have to change to get nice brown colour like mine friends KTM and my old XR6?


[ July 13, 2002: Message edited by: sabin ]

Post the rest of your jetting specs so that we can assist you in troubleshooting....

PJ/PAJ MAJ/MJ Needle and clip setting...

Your metal sound could be a multitude of things. I would tell you to check your oil galley bolt for flow but you have been riding without seizing so that should be good. Your plug was in tact when you pulled it so that would basically rule out FOD in the combustion chamber. If you do not have a Zip-TY racing magnetic oil drain plug ..Get one... this will give you a good indication of what is going on in the lower end when you change your oil...

Bonzai :)

Bonzai :D


My settings are:

PJ 42

PAJ 75

MJ 165

MAJ 200

Needle obdqr-4

TwinAir air filter with Airbox cover is in place.

Racing exhaust system from front to end.

I changed the MJ to 170 and raised the needle to 5th clip. The plug is now nice tan colour, but its does not rev good. I also have some trouble with the 1/4. I can feel the bike missing. It accelerates like hell, but cannot feel good on steady throttle, it's like going front-back-front... I think I have some unburned fuel as my exhaust fires blue flames from time to time at reving on neutral. How do I get the correct setting of the PS?

I'll try with MJ160 to see what will happen. Do I have to lower the PJ when raising the needle? I can order only from yamaha dealers over here, and delivery is one month.

Thank you for your time!


I put the 165 MJ back and the bike revs nicely. I can feel in my wheelies that the power is not smooth like before, in the middle, and I am still having trobble with low to mid rpms on steady trottle. What does this mean?


all the tests are made on 1600ft altitude, when I ride all in the working days. In the weekends I ride on 5000 - 6500ft. If I set the bike to work ok in 5000ft won't it be too lean on 500ft and brake the angine, or how to know when it's too lean?



The metal sound you heard was probably a wire hose clamp. Are you short one on your carburetor hoses? Also, a large part of your steady throttle problem or lack there of is the grey wire…un-hook it.


This sound is not a problem, as I did not hear it again. Maybe it was on the joint of the header pipe and muffler... About the jetting I returned on my original settings. I do not have the grey wire connected, as my bike is European model. I have now nice power all the range and no gaps, maybe this mean my jetting is fine, is that so? I do not want more power, but just to make sure I'm not lean and I'm not going to break the engine running lean...

Thanks for the reply!


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