Finally got to ride my (fairly) new YZ400

Just took my '99 YZ400 out for the first time today after 1 week of having it. It was a long week! The famous "bog" is there, but nothing that was bad enough to make the ride not fun. I was riding at Stoddard Valley (SoCal) and the bike seemed to have some jetting problems...or so I believe. The bike is very very fast and makes power quickly. The bike pops a lot upon deceleration. There is an FMF Power Core IV2 on it. Could this be an exhaust valve issue or just a signature of that hot aftermarket pipe?

Anways, the bike was TON of fun and I SMOKED my buddy on his brand new 2002 CR250. He said he crapped his pants every time the thunder would roll in behind him...then I would fly past him like he was looking for parking. He said it threw rocks like rain from the back tire. Another buddy on an XR600 ate a lot of my dust, too. All smiles here. :)


Ye i have a '98 YZ 400F and i love this bike more than anything. I cant get over the power and the bike surprises me by letting me go bigger on jumps and hill climbs everytime i go out for a ride. The fun doesnt stop on a YZF and the power isnt something that gets boring or weak.

Dont worry about the exaust back fire They all do it i have a stock exaust and my friends '00 426 has a FMF pipe and it still back fires.


Popping on decel is a lean condition. Basically, you need to turn out the fuel screw (richen) until the backfiring stops. Do a seach on jetting and you'll get more posts than you can read at one sit down on tips for adjusting.

Something you might think of is making a fuel screw adjustment tool which makes it real easy to fine tune the jetting between temperature changes. Look up Garetts post from last week and how to make a tool (I made one from his post and have used it every time I've ridden in the past couple of weeks, ITS GREAT).

Enjoy the YZF, I've had mine for about a month and a half and love how much fun it is to ride.

When I rode it today, I would say Stoddard Valley is between 3800-4100 feet above sea level. It was cold (for SoCal!) in the morning...maybe 45F and then it warmed up to about 55-58F for the rest of the ride all day.

I will mess with the fuel screw to richen it up and test it out next week.

3 cheers for no more pre-mix!

...almost forgot...thanks for the tip, RichB!

Congrats on the 426. RichB was right about the decel popping but you might even need to change the pilot jet to a larger one if the mixture screw doesnt help. Do the BK mod if you think your bike rips now! It is very inexpensive and easy. Cya


My bike is a 99 YZ400, so the BK mod won't work on mine.


My bike is a 99 YZ400, so the BK mod won't work on mine.

hey thumpsalad, i too have a 99 yz400f, im pretty much in ur area(vegas), i had that pop on decel too, all i did was went to a 48 pilot, i believe 45 is stock, cured it fine, no pop at all now, u might want to try a 48 or a 50 pilot, if it bothers you that much,



Before you make any changes, turn the fuel screw in and count the number of turns it takes to bottom (you know this works) then try going out (from the original position) 1/4 turn at a time until you get to 2.75 turns. If you get that far out on the fuel screw, you need to go up one on the pilot jet.

After installing the new pilot, go back to 1.0-1.25 turns out and start over if needed.

Bill :)


Thanks for the info. I will monkey with the carb tomorrow night and let you know how it turns out. Will this also cure the "missing" that happens at mid-throttle? It misses very very slightly at that point.



do you ride at the track here in vegas? I'm getting a couple of friends together and heading to Victorville MX complex this Sunday to ride with YZernie and any other TTers he can round up. You should think about going too.


Might be a little lean in the mid, try moving the clip down and the needle up, one. You may want to pull the carb and give it a good cleaning and see what all the jets are and post your findings here.

Might be able to take some of the testing out of the equation.


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