The header makes a HUGE difference. The stock header is WAY too small to make any serious power, and is a significant restriction. It has nothing to do with backpressure, it's all about velocity. Stock is 1 1/4", the FMF is 1 1/2". 1 3/8" would probably be a good compromise, but I don't know if anybody sells that. I use my bike mostly for woods riding, and have stock gearing. The low end power is good, and the midrange power is so much better, I'd never go back to the stock header.

 

Haha! I'm glad you corrected me on the wallaby or I'd have never known the better. That song happened to be playing on a Japanese radio station the other day and it brought back memories of 1963ish...riding with my father in a '57 ford and listening to him sing-along with the main chorous. I was all of about 3 years old.

Immediately after hearing that song again, I went home, Googled some videos and got a surprising volume of material. I know I'm dating myself, but ahhhh the memories!

Perth is the first Aus city I ever visited. Never knew the artist was from there until I hit-up good ol' Wikipedia during my search yesterday.

 

http://www.motorcycle.com/products/s...mods-3510.html

'Larry tells us the stock head pipe is actually the same unit as used on Kawasaki's KLX 250. It has abrupt internal welds and its inlet face does not mate well with the exhaust port.'

https://www.kawasakiforums.com/forum...5&d=1232949717

In case the ground out weld is weakened like what happened to dan on klxforum.net
https://www.kawasakiforums.com/forum...5&d=1232949717