Okay, I'm guessing it has nothing to do with the plugs but this is what happened:

I recently bought WMW pulley kit, replaced the JCW pulley with the 15% reduction one, changed the belt, and added the tensioner stop. The car run great. A couple weeks before replacing those parts, I had done the VGS mod (https://www.northamericanmotoring.co...stem-vgs.html), even though the car had no yo-yo effect. Didn't really feel much of a difference with the VGS mod except the whistle from the Alta intake was less pronounced. After changing the pulley, the whistle came back
The car pulled very nicely with the new pulley. All was good...until last week when I changed the plugs (came with the pulley kit as well, and are the correct JCW plugs).
To be safe, I changed the plugs in the morning, after the car had been sitting for 10+ hours. I noticed that some of the coil terminals were corroded and also the plug lead ends to some extent.
The car idles fine, and drives okay until you really get your foot down on the gas pedal. Lots of short hesitation hiccups (if that makes any sense) as the revs go up. I think it only happens at WOT, but may be happening at partial open throttle as well.
If it was the plug wires or the coil, wouldn't it misfire all the time? I'm definitely planning on changing them by the way.
If it is the BPV, why didn't I feel the same hesitation when the pulley was installed? Any ideas???? The car is an 04 MCS with the JCW engine kit (no JCW injectors or air filter but have an Alta cai), approx. 80k miles.

 

If this helps....I experienced the same type hesitation hiccup as well after replacing my JCW plugs, but that soon disappeared when I changed the ignition wires (Kingsborne 8mm) a couple weeks later.

MC40 '345'

 

I hope that's all it is. Thanks for the reply.

 

Following ijcwmc40's suggestion I replaced the plug wires and the hesitation is gone! Pulls hard to redline again!

 

Glad you fixed it.

You mentioned your terminals were corroded. This is what mine looked like one day last summer. You can clean them up with a rag and some STP carb spray cleaner and/or a brass brush.

 

Yeah, cleaned them well with a wire brush until there was no visible rust left and used the grease that came with the plug wires. Number 3 terminal was the worst.

 

Yeah, #3 is always the primary culprit. Glad you cleaned them up.