My bike is a KTM EXC 125 2016 and my fuel mix is 40/1 using motorex cross power and high octane fuel

 

Go back to the stock head l reckon.

 

You’ve altered the compression so that’s now thrown your jetting slightly out. If it’s a high comp head it might want a bit more fuel. Try air screw adjustment first. Then next larger pilot. Might be worth going up on the main jet as well.

 

Looking at that plug, it needs less juice not more.

 

What did it run like with the stock head before the change ?

 

It ran amazing I was just hoping to get a little more out of it with the head change

 

Once it’s running it runs amazing it’s just been a pain in the arse from cold start lately since the change (I run it with a pro circuit R-304 and a Scalvini cone pipe of that helps)

 

What did the plug look like with the previous head in comparison to how it looks now? The symptoms you mention are like its weak, but plug says rich. Maybe try adjusting airscrew as Pindie said to richen low down to see if that helps starting at all. A new plug would help if that one is over 20 hrs. Also, is 40:1 std on a 125?

 

I think 40/1 is pretty standard on 125s I could be wrong though it’s what I’ve always ran, I’ll try and find a old plug tomorrow when I get some time to compare them. I’ll adjust air screw tomorrow also and get back to you to see if it’s helped

 

Remember that premix with less oil (50:1 for instance) will richen up the air fuel ratio a bit and clean the plug up a bit with regards to oiling.

 

start with a new plug and see how things progress before making jetting changes because if it runs great when warm that doesn't sound like a fuelling issue.

 

Steady on. Careful with your language. This is 2021. That is a spark plug of colour.