[kcorey]

What would cause dry black soot on the plug, specifically on the CRF70F and what can I do to avoid this?

[Asmith]

What would cause dry black soot on the plug, specifically on the CRF70F and what can I do to avoid this?

Too rich a mixture.

What is the condition of the air filter?? Once it becomes partially plugged with dirt and dust, the mixture becomes rich. Are you cleaning it every ride?

[kcorey]

I just got the bike last weekend for my son, second-hand. I cleaned the filter (it was terrible) but the bike still sputtered a bit, especially at full throttle. I adjusted the fuel/air screw but I think it's wrong because of the fouled plug. I tried adjusting the air screw from your post by first adjusting the throttle to high and then playing with the air screw until the idle sounds good. Is a sootie plug to be expected?

[Asmith]

You will probably need to readjust the fuel/air screw, and possibly move the needle up a clip and or drop the main jet a size.

Plug should be a light brown.

This will help.

http://www.ministothemax.com/hop_ups_carb_tuning.htm

The jet sizes are wrong as it's written for a 50 not a 70, but the theory is the same. It describes your problem exactly. Also check out their airbox mod. Do it or switch to a UNI air filter for more flow.

[kcorey]

Brilliant link, exactly what I needed, thanks for the help!