Yamaha 2003 YZ250 Nightmare

I have not ridden this bike for about a yr because if this problem.It will not start without starter fluid. I have cleaned the carb 10 times, jets are spotless, float works perfectly. New plug, cleaned the gas tank and petcock filter and put in fresh 91 octane fuel and yamalube. Compression is spot on. It just will not start up cold even with the choke. As soon as I just use a small burst of starter fluid it fires up instantly and runs. It will start up again fine after that until the next day. Now I have a another issue that the idle is way to high and I need to use the kill switch to keep it from over reeving. I am at the end of my rope and need help!!!


The idle can easily be adjusted from the ide adjustment screw on the carb. As far as the bike not starting. Might have to change the jetting for your elevation and weather

Thanks John for the response. I have only 2 screws on the carb and it seems one that is recessed changes the air/fuel mixture and the other screw that is black with a spring around is not making any changes on the idle. Could the jets be wrong on the carb? Does anyone know what jets are stock on a 03 250 with pro circuit exhaust and ss headers?

Stop using starting fluid!

That is the quickest way to kill it. Get the air screw and pilot jet dialed in and it will light off much easier.

It can also be related to timing. Besides that what jetting you are running anyway?

Stock jetting should be 50 pilot 178 main. I'm no expert here, but might he have an air leak? It does sound like the pilot jet may be too big. There was a 55 in mine when I bought it and I could not get the bike to idle down properly.

do a leak down test on the cold motor.i think you have an air leak making it too lean to start.the leak will also cause the high idle.if the bike runs good when warm,probably the air leak is sealing due to thermal expansion.don,t ride it until you fix it,could lean sieze your motor.

For cold start more fuel is better, for the you have the choke. I have 48 pilots in both my bikes and use advance setting on timing. So far so good. Also the airscrew should be at 1.5 turns if stock. I'm almost at 2 turns.



Thanks guys for all the help. I know that starter is the worst but it seams like a barely give it a burst and it starts. I am going to recheck the compression but I know I did it cold and it was spot on. I will also look again at the jets and post whay I have.

Use a manual mate the best way to learn too :thumbsup:

+1 to check for an air leak

It can also be related to timing.
I have 48 pilots in both my bikes and use advance setting on timing. So far so good.

The timing, within the stator range, will not cause his problem. The bike will start 5 deg. retarded.

Also, you do NOT want to run advanced timing on the U.S. 91 oct. he's running.

It's either an air leak or a pilot/circuit that is actually clogged.

I can still start and idle my bike at 50F sea level with a 42 in it so size won't stop it. (but it's not optimum)

Hi there

I run our so called 95. If it has that I wouldn't know. So far no problems.



Right, but he said he's running U.S. 91, which will detonate on advanced timing.

I'll have to 4th and 5th the air leak. The run-away idle is a tell-tale sign. Replace the crank seal on the stator side (may as well since you'll need to check it anyway, and while you're in there...go ahead and just do it). Head or base gasket may be leaking or around your intake manifold. You can check that by squirting a little brake cleaner or other low flammable aerosol around the area...see if it changes the idle speed.

ok I got it, my bad :thumbsup:

First update, I rechecked compression and it is 205 psi on about 5 to 6 kicks and needle did not move for about 4 hrs. I usually work on cars so I am not too well verse on a 2 stroke mechanically. Where could there be a leak? I am now going to check which jets I have.

I took out the jets and the main is 178 and other is 50. I did notice that 2 of the 4 screws the bottom where not tight. I also posted pics of the carb.

Tightened and clean everything again, checked and rechecked and 0. No start without fluid and super high idle. I think I am going to give up and I don't give up too easily:foul:!!

Sorry to hear that, look have you got a spark? And how do you start the bike, with the throttle at 15% or withour the throttle?

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