YZ250 - Fouling plugs

Ok, well I have a 1999 YZ250. Its been used for racing it had a Pro Circuit Platinum Pipe on it and was jetted for it. I got it the guy had taken the pipe off and left me with the stock but didn't rejet. It fouled plugs left and right. I got the stock ones and it still fouls plugs left and right after putting them in. Any ideas on how to get it running right?? Someone said maybe there i gunky oil in the bottom of the tank? But can that be the reason?

I would check the jetting. Who knows if it was really jetted right. Find out what stock is supposed to be and see if your jetting is richer than that. If so you have likely found your problem. Make sure the air filter is not clogged also.

replace the left side crank seal ,this is always over looked and causes many problems with jetting .its a $4 seal and while you have it down check the flywheel for up &down movement it must have 0 play .if it moves you have a crank bearing worn out{not likely} next have someone to help you get the jetting back to stock .and check your needle clip setting .if you still have problems replace the needle and seat about $35 from yamaha,this helped my 97 250 more than anything i have ever done to elimnate plug fouling. oh yeah yamahas love sunoco race fuel!

What is your jetting?

Pilot jet

Air screw

Needle and clip position

Main jet

Without this information, it's not possible to make suggestions.


I am not sure..I will check though.

How hard is that seal to replace..I doubt I can do it. I'm 15 and not a mechical type of kid lol

if its a 99 yz 250 im thinking its a two stroke right? what ratio are you mixing your fuel/oil at. i have a 97 yz250 and i mix at 40:1. that could also be part of the problem. the manufacturer suggests 32:1. if you call the dealer or check the repair manual to find what stock jetting is and it happens that your bike is jetted close to stock you may want to try playing with the mix ratio. depending on temp, jetting, and altitude i have run the mix as lean as 50:1. just make sure not to go too lean too quickly, or you may kill the motor.

I have the manual. I do run it @ about 40:1. I dont usually measure to well. I go to the gas station put gas in. I look how much I put in say like I put 2 gal in I put the amount of yamalube in there for 2 gal 40:1 My dad siad maybe there is gunked up oil in the bottom of the tank?

This damn YZ i want a new one lol this one is nothing but problems soo far :)

Tell me your jetting and I'll help solve the plug fouling. I've been waiting for 2 weeks... :)

If you want an estimate, take the top of the carburetor off and remove the needle from the slide. See where the "C" clip is set. Move the clip 1 step closer to the top to lower the needle. Set the air screw on the left side of the carb to 2 turns out from turned fully inwards. Be gentle when turning it inwards.

Next, clean the air filter and don't over-oil it. Use fresh gas and mix it at 45:1 (3oz per gallon).

Hope this helps,


I have a 175 main jet 50pilot needle on setting 3

sorry about the weight i dont come on often

I may be 'way off base here, but my normally rock-solid, 1-2 kick YZ250 started fouling plugs early last year. A fresh top end has it back in top form (the rings were badly worn - lots of blow-by). Give your steed a compression test.

BTW, 1999 YZ250 = good bike. Wise choice!


A 50 pilot is pretty rich. I run a 45 in mine at around 500 ft. with the stock needle and clip settings and a 175 or 178 main depending on the temp.

i got a 2002 yz250 and i also have tons of trouble with fouling the plugs. I usually ride at about 1000ft and the carb was rejetting for fmf pipes and a little engine work. Im not sure of the exact jet sizes, but my last ride i fouled 3 plugs in about 2 hours. I had the regular br8eg in it then threw in a br9eg so it would burn hotter, but even that fouled after a little. The 9 is deff a better choice, it ran great before it fouled. I have my clip in the 2nd leanest position and the air/fuel screw totally turned out. Im thinking my float height may be to high. If thats not it does anyone have the stock jetting for an 02 yz250. Also what jets would i need to lean out?

also they say yamalube premix is the worst thing for fouling plugs. Im deff switching to a different brand. I run my gas at 30:1 or 32:1 and am not a revver but i dont keep the engine at low rpms for long. My bike just runs totally fine then all the sudden acts like it just got a huge shot of 5:1 premix and fouls. Anyone else experience this and have the sollution?

I run yamalube 2r at 40:1 and havent fouled a plug in 2 years. open the carb and get the jetting close to stock, by changing the jets. a phone call to the dealer and they should tell you what stock is.

In regard to the spark plug. With ngk as the number goes down (from 9eg to 8 eg it actually gets hotter. The higher the number the colder the plug. I Have a 2000 YZ 250. I run amsoil at 50:1 and a br8ev which seems to last longer and burn better than the EG. I have my needle in the middle position, and a 170 main. My pilot is a 50 and my air screw is 1.5 turns out. I do believe my pilot is a little rich so you may want to go with the 45. I am at sea level. Hope this helps.:)

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now