Hesitation and Stall of a 99 YZ400F

Hi group ! I'm the new kid on this forum, I recently bought a used 99 YZ400F and the bike starts at first kick, and idle perfectly but there's a problem, when I touch the accelerator the bike dies instantly, I have readed some other posts in this forum and I think that I need to clean the carburator and check the spark plug condition, so I need to know what parts of this bike do I need to get off to reach the carburator and the spark plug, if somebody got any advice with this problem please post ASAP or reach me at my email: humbertoben@delbravo.com

Humberto Benavides

YZ400F (Is anything faster than this?)

Hey Mr Humber!

I would make my approach two prong.

First remove the tank and seat, check your plug (special wrench should have come with the bike). You might want to post what the plug looks like. You'll want to describe color of the little round column under the electrode.

Secondly, I would get a manual and pull the carb off the bike. Want you'll want to do is clean your accelerator pump. The manual will help you, dont worry. While you've got the carb off you will want to blow out all of the passages with a compressor. Then, I would find the little hole on the bottom of the carb that contains the Fuel Screw. You'll want to count the number of turns it takes to screw the screw all the way in. At the same time you will want to check which needles and jets you currently have in the carb. Generally the size/# will be written on the jet/needle. Start a notebook for various changes you will inevitably make, and write down these numbers. Follow your manual cafefully.

'Go ahead an return the screw to it's previous number of turns out and RUN IT! Let us know what happens!

Good Luck!

Mr T

Actually I bought the bike used and I don't got the special spark plug key, is there another way to pull off the spark plug......? Also I don't got a manual of the bike with me, can you tell me where can I get one......?

Thanks again for your valuable answers !

Does the bike still have the same symptoms when warmed up?

Bill :)

You don't need the special tool. I use a standard spark plug socket (that has a little rubber grommet inside to "grip" the plug) from my craftsman socket set, a wobbler and a 3" extension. No problem.

If your spark plug socket doesn't have the grommet, get some fuel line long enough to push down over the end of the spark plug to pull it out.


Steve T

Now I'm a happy man again ! The last owner taped the top of the air box cover while crating the bike, so I contacted him and he let me know that, removed the tape and the problem went away !

What a silly I am !!!

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