1999 Yz400f Throttle Response

I Have Just Recieved A 1999 Yz 400f For Free.the Bike Only Has About 15hrs On It,i Know This Is True Because I Know The Owner Well.the Problem Was The Bike Sat Outside For About 5 Yrs Without Ever Being Used.the First Thing I Did Was Cleaned It Up Really Good,then Before Even Attempting To Start It I Took The Carb Apart And Cleaned It.i Then Installed A New Plug,and Changed The Oil And Coolant.the Problem Im Having Is When I Put The Carb Back On And The Bike Started It Idled Fine But As Soon As I Would Touch The Throttle It Would Stall So I Turned the Pilot Fuel Screw Out About 1 1/2 Turns Over Stock.now The Bike Will Rev But It Still Bogs Down Low And Shoots Flames Out Of The Exhaust And Backfires Or Pops When Revved Up.the Bike Starts Easily And Idles Fine.i Checked The Valve Clearence,the Tps Resistence,and The Volts Going To The Tps From The Cdi,everything Checks Out Good.

Check the accelerator pump...

I need your friend.

The Accelerator Pump Seems To Be Working.what Else Could It Be?

I know you already did this, but check your plug again. Try a new one and make sure the gap is correct. :applause:

The Accelerator Pump Seems To Be Working.what Else Could It Be?

Is the squirt duration set up correctly? Also if it sat outside for 5 years - did you remove the rubber boot off the accel pump - it was notorious for dirt build up as well as getting material inside the diaphram.

He's got the vacuum release plate on the slide upside down.

Merry Christmas.

The hole goes toward the bottom.

The point points up and the little relief in the flatside goes down. I know it looks wrong like that, but its not.

your accelerator pump duration, before midifying your carb, should be ~ 3 > 4 seconds.


0.5 seconds was recommended by a Team Yamaha Mechanic, Brian Kinney(sp?) (John Dowd's Mechanic back in the day).

A locking collar, installed on the AP rod, will fix you up.

I too, own a '99.

Buy an Edlebrock carb and never have a problem again. I fought with my FCR carb for a year tring to fix it. Did all the free mods tried different jets cleaned it time and time again. Bought the Edelbrock and never looked back. I know most people think the FCR carb is fine but mine was junk and the Edelbrock carb was the best thing I put on my 99 YZ400F

???Flames out the exhaust???? I would look for a air leak also. Also was the cam ever changed/adjusted?

Its the slide...Im telling ya...my 426 blew flames a foot long out the pipe when I put that sucker in there wrong...

does anyone know why the fuel is coming out of the rubber boot on the rod ?

A reasonable first response might be, "what boot on what rod?"

But, I will boldly risk that you mean the accelerator pump rod, and tell you that in that case, it's because the accelerator pump diaphragm is torn. Were that a correct assumption, you would need to replace it.

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