Help Please!

Hello all. I have a 99' 400F and I am very new to the 4 stroke scene. My problem is this. Last year when I would ride, it would cough and choke through the really heavy ridding but while cruising it seemed to run fine. Not sure if this is a jetting problem or a valve problem. When exiting a turn it doesn't want to run unless I keep the throttle up in the midrange (bogs at low RPM). The spitting and rough running previously mentioned would mainly happen while running through a long section of whoops or over rough terrain. Since purchasing I have installed a Boyseen Quickshot accl. pump cover, and I have put on an FMF titanium High Flo header paired with a Powercore IV exhaust. Could the problem be as simple as too much fuel caused by a sticky float in the carb? Any help would be greatly appreciated. Thank-you


I would start by removing and cleaning the carb.

yea i agree start by cleaning ure carb and adjust ure fuel screw if that dont help by a jd jet kit

I had a '99 400 and a had the same problems. I started by cleaning the carb. This helped but did not cure the problem. I then went richer with the jetting, I have sold the bike and gave my notes to the guy who bought it, and do not remember the exact jetting I used. I would suggest using a JD jet kit. If your turn up the idle to keep it running, you are only covering up a jetting problem.

I also used an 8 oz heavier than stock flywheel. This gave it more tractible power when trail riding, or on hard packed MX tracks.

If you are new to working on four strokes, maybe take it to a shop. But if you feel you can do it, go for it. If you take your time and pay attention to detail it is not that hard. DO NOT use wires or anything to poke through the jets. Use carb cleaner and compressed air.

Some other mods I did to my 99 include:

02 YZ426F subframe. Bolts on and is about 3 pounds lighter than stock.

03 YZ450F radiatiors. Bolted on and are beefier and keep it cooler running

than stock (had to use 01 or 02 426 shrouds)

Braided front brake line.

power now


Thanks for the info, I have no time so I guess I will be paying someones car payment on this one. One other thing guys, Are there any tell tale signs that your valves need Adjustment (shims)? I would really hate to have one come through the top end.

Hard to start is the tell tale sign that your valves need adjusting. And it's a real B$#CH when a valve lets go $$$$$$$$$$.

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