Bad Running

I run a 99 WR 400 with a FMF power core IV2 coupled with a FMF header pie (not Poer Bomb). I have removed the air box snorkle and the engine is YZ timed. Jetting is 48 pilot and 180 main. I am having a great deal of problems at the moment of which neither myself or 3 visits to 3 seperate bike shops can cure. This is my first four stroke so after 22 years on 2 strokes bear with me and my ignorance. My problems are listed below and any help/advice would be more than appreciated. The problems are as follows.

1) when reving the engine out of gear and holding the throttle at say 1/2 the engine pops and bacfires and will not hold a steady rev.

2) whilst riding when decelerating into a corner from a fast section, the engine back fires seriously (flames from exhaust and loud bang from air box).

3)The whole note of the engine is not sweet.

These problems started whilst running totally stock bike, after a very very wet enduro (UK weather) and several good off's in rivers. The bike starts first time every time. HELP !!!


The FCR carb has a few things that 2-strokes don't. Check each of these if the bike shops haven't already.

-Pilot air jet

-Main air jet

-Bypass valve on side of carb and all the hoses(only on WR's)

-Accelerator pump, make sure the check valve is working properly and the rod has no crud buildup around it

-Hot start air bypass, make sure it's closing

-TPS sensor may need to be checked

While you're looking at the carb check the choke and pilot jet for crud. Also, water can get in and around the spark plug and cap thru the hole on the side of the cylinder. Check the cap is clean and on tight.

Good Luck,

James Dean

What is your setting on the pilot screw.

Somewhere around two turns out may solve the backfiring problem.

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