Popping YZ 400...

Hello all,

Just bought a 1999 YZ 400 for Supermoto usage...Not much time on it...Still has the original tires and chain / sprockets...Doesn't look like the guy knew what maintenance was though.

I changed ALL the fluids, cleaned the filter, new plug, etc...

Starts right up. Will not idle. Does a bit better with the choke ON. If I hold constant throttle, it will stay running but POPS like hell. Single backfires out of the exhaust. If I turn the idle screw ALL the way up, it will idle...But still pops and that can't be right (to turn the idle up all the way)...When holding constant throttle, the idle varies on it's own. The bike feels STRONG when I run it up the road...

I THOROUGHLY cleaned the carb. It is BONE stock with the idle screw 1-1/2 turns out, and a 175 main jet. I also checked for vacuum leaks and replaced the vacuum hose to the hot start knob.

Could it be the valve adjustment? It has probably never been serviced.

Any one else experience this?


Mark Z.

AMA #196

FUSA #96

CCS #196

WERA #196

Sounds like a timing issue to me...

Is it possible the guy changed the cams or does it look like he screwed with them?

TPS hooked up?

"Supposedly" nothing messed with ever...

Doesn't look like the valve cover has ever been touched...No wear on the bolts...

How often do the valves need to be checked?

I would check the valves just as a precaution! I still suspect the carb and especially the pilot jet. Have you adjusted thefuel screw after you cleaned the carb? When you say you cleaned it, you actually took it apart not just sprayed carb cleaner right? Is it a wandering idle as you claimed or a hanging idle? That is where the rpms dont come down for a moment or two after letting go of the throttle.

Sounds to me like its running extremely lean. From you saying that it does better with the choke on, that is my diagnosis. try turning your fuel screw out and see if it helps and if not, rebuild the carburetor and re-jet. :thumbsup:

1) Check the valve clearance, more than likely they are tight.

2) Be absolutely sure the pilot jet is clean. If there is any garbage in it at all, the bike will not idle.

3) If the bike still runs bad from 0 to 1/8 throttle, your pilot jet may need to be increased (assuming it is absolutely clean!).

4) Check to make sure the crush washer used between the exhaust and the cylinder doesn't need to be replaced.

Thanks guys...

I am a long time roadracer (AMA Pro)...Just getting into a (hopefully) cheaper sport of SM and hare scrambles...Not retiring the 03' R6 yet...Just more time on the YZ...Upgrade later (450?) I'm sure...

I am familiar with sportbikes...Not with the dirtbikes. Glad this one is a 4 stroke! I had the carb COMPLETELY apart twice...No noticeable debris. Cleaned it anyway. Tried the air/fuel screw at almost three turns out (that's where it was!) and now at 1-1/2 out. No difference in the running. I noticed the idle screw was about ALL the way out. I am sure that messes with the TPS also. Turned it back a bunch and now it won't idle AT ALL. When you hold the throttle cracked to keep it running...POP city. Even when holding the throttle steady, it varies in RPM. My roadrace mechanic thinks it's the valves. He hasn't seen the bike yet.

I was even wondering about the coil, CDI, etc. I did change the plug.

We'll see this weekend...I'm taking it to my roadrace mechanic...

First SM race in two weeks! Bought some wheels / brake from Wicked_Crash...Should have them soon...

Thanks again for the feedback!

Mark Z.

AMA #196

FUSA #96

CCS #196

WERA #196

My 450 did exactly the same thing you describe after I cleaned the carb and put it back together incorrectly. Check to make sure the removable plate on the slide is installed with hole facing down, otherwise it won't idle.

I never took the slide out...Just the bowl side apart...

BUT maybe the other owner did. Where is this removable part?


On my 450, which I am assuming has a similiar carb, there is a plate that fits onto the cylinder side of the carburetor slide. Its nearly the same size as the slide itself and it will fall off when you take the slide out. As I said before make sure the hole in this plate is facing down, even though it fits either way.

Hope this helps.

my bike was popping under throttle until the engine got warm.. i replaced the spark plug with a ngk type r cr8e and now it idles great and does not pop anymore

my bike was popping under throttle until the engine got warm.. i replaced the spark plug with a ngk type r cr8e and now it idles great and does not pop anymore

400f has a point here. Have you replaced the plug?? These bikes are very sensitive to a bad plug. If the jetting is off a little it can kill a plug right quick. Some people can run the same plug all year if the jetting is on while others replace them left and right. Its all in the jetting.

If this bike has never been apart (engine) and has a fair amount of miles on it I would suggest new rings, Shim the valves, and replace the timing chain. If for no other reason than peice of mind. You will then know that the engine is sound. It will cost under $150 to do all the above if you do it yourself. The 1st time tearing into the valvetrain is a little intimidating. But after that you can do it in your sleep.

But id the valves are outta adjustment and too tight it can cause problems like hard starting and wandering idle. If you hapened to jump 1 tooth on the cams that would cause idle problems as well as lack of power. A spun gear on the cam is also a possibility. I did it on mine. So its not unheard of, just rare.

When shiming the valves:


Bad things can happen to your cam journals if you dont.

Trust me I know :thumbsup:

Thankfully just the carb...

Don;t know what exactly...Some sort of an air leak...

Bolted on an OEM 426 carb...Good as new! Just need to redo the cables & a couple of other little details...

Thanks for the help!

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