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I've meaning to ask about this for a little bit now...
I was doing some clutch work a few weeks ago, and I had to take my header pipe off to get the clutch cover off (By the way, it was a full White Brothers system). When I got every thing put back together, my header pipe was leaking, I thought it wasn't tight enough...Come to find out, it cracked on the left exhaust port. So, I bought a stock 400EX header pipe, and a used FMF Powercore 4.
Here's what I've been wanting to ask, when I had the White Brothers Exhaust on it, It wasn't running very good, after it got warmed up, it would sometimes stall, it got pretty hot, it was hard to start when it was warm, it was kinda boggy on the bottom end, and it would burble and pop when I would decelerate. Now with the Stock header pipe, and the FMF, it's running A LOT better, no stalling, it isn't running as hot, it's starting a lot easier when it's hot, it's not burbling and popping as much when I decelerate (It burns a tiny bit of oil, so it's going to still burble a bit)

The guy I bought it off of said the carb was jetted specifically for the White Brothers pipe...And, I don't think that was true.

What do you guys think?
 

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Buyer beware - Obama said you could keep your doctor, PERIOD! Would you buy a used (or even new) anything from him?

Whenever I hear, "It's tuned so only a professional can ride it." it means it doesn't run right and I don't know what's wrong with it.

All else aside, reducing the restriction of the exhaust from stock is going to cause backfiring from the exhaust (regardless of the brand name on the product). It's a phenomenon caused by the air/fuel mixture being too lean when the throttle is closed and the wheels are driving the engine. Enrich-en the mixture by opening the fuel screw a bit or/and increase the idle speed to decrease or eliminate the popping during deceleration.
 
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