Plug Fouling, all kinds of weird stuff

Hey guys, wondering if you can help out a 2 stroke convert. I Just bought a 99 yz 400. I've been having trouble starting it and keeping it running. In the 3 weeks I've owned it I've gone through as many plugs. All blackened up (running too rich I assume). It will start with the choke on, but after it warms up, it will stall when you try to put the choke off. Has anyone seen this happen to them? I don't want to touch the carb untill I know what the hell I'm trying to do with it. Help!!!

make sure your idle is set high enough. :)

do a search on jetting, i am willing to bet that you are off.........

Is the bike stock? Piped? Other mods? List the mods and we should be able to get you in the ball park. I cjange my plug every six months or so. Only had one foul.

As far as the engine, pipe, and carb goes, it's all stock. Last night I tightened the carb boots, I had one that was a little loose. A friend told me to adjust the pilot jet screw, but I didn't. I'm thinking that the pilot would not effect my idle and that where my issues are. I have to get another plug today before I can start toying with it again. I only had 2 extra when I bought the bike, the guy said that they would last me a year.

F@*#ing @$$hole.

You can often save the plugs by cleaning them with carb cleaner.

You can easily foul a plug if you flood these things, which is easy. Over chokeing or twisting the grip before/during starting can do it. Typically when I start mine I let it run with the choke for about 10 seconds and then it will idle when I turn the choke off, I assume others are similar. If your bike won't idle then my personal guess for the most likely causes are: 1) the idle is too low 2) the carb is dirty or adjusted incorrectly 3) your valves need adjustment pretty badly. If you start by cleaning the carb and setting the fuel screw back to stock settings then that would be a good start IMHO.

Several of us had terrible fouling problems on ours. If you search under "fouled plugs = foul mouth" you will find a Russian novel.

In the end it turn out for me and several friends that it was the GAS. They put ethanol in our gas during the winter and it fouls plugs like crazy. No amount of jetting will fix it.

I suggest you buy a good quality race gas and run that in you bike for a while to see if the problem goes away. Assuming your jetting is OK, I bet it will. Then find a station that does not use ethanol in the gas if you want to use pump gas.


You bought a used bike, probably no history with the owner as well. You have to do some maintenance. (You can't trust ANYBODY, unless he was a factory Wrench)

Purchase Owners Manual.

1. Clean air filter (do not install a freshly oiled filter, let dry over night. Why? Oil might get sucked thru jets and clog)

2. Change engine oil, Oil Filter, Brake fluid, radiator fluid. (Buy a Owners manual)Note on changing Oil, make sure you bleed or loosen oil galley bolt and oil slightly oozes out, tighten, and ready to go.

3. Remove Carb & CLEAN. Clean jets also. Note and write down jet sizes. Adjust pilot screw; (stock setting 1 3/8 turn out). Adjust according.

4. Adjust idle setting (Stock 1,700-1,900)

5. Install new plug, with proper gap clearance.

6. Now its Ready to ride. If bike won’t idle still, change to stock jetting.

Stock setting is based upon So-Cal tracks. 1,000-3,500 alt.

Stock Jets

Main Jet 175

Pilot jet 45

Needle jet 0bdvr-4

Pilot screw 0.9

Starter jet 65

Main air jet 200

Pilot air jet 100

My jetting is based upon the opinion of a Yamaha Factory Test rider;

Alt 1,000-2,000

80 degrees Air Temp

182 main, 50 pilot, next needle richer.

He told me the 1999 where jetted leaned to equal Cali, EPA?

It works for me, PS I’m on my second spark plug since 1999.

Did I mention, buy an Owners Manual?

Enjoy, have fun.

I have a 2002 426 that has fouled 3 plugs on about 3 gallons of gas (and no I am not an idiot It's not my fault) I feel for you buddy.

I have a 2001 and the dealer replaced the carb after I fouled a dozen plugs the first three weekends riding. Helped but still fouled two plugs with the new carb after I found this site and read about not leaving the gas on when transporting (definetly will flood) and the blue wire mod (clip wire that limits ignition voltage at neutral) I haven't fouled a plug since. No jetting changes or anything else besides the blue wire mod. Might want to check that it applies to 99's as I have not seen anyone else suggest that to you yet.

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