2003 yz250f stalls when warm/hot

when the engine gets hot or warm from riding maybe 3-4minutes in 85 deg f. the bike progressively starts to cut out, sounds like its leaning.

I found a large washer in the intake ports of the head rattling around, found this after cleaning the carb and valve clearance check, which both came back good. The washer cause alot of head aluminum to be flaked off and ingested.

When the bike is warm the kickstarter seems alot easier to kick than cold, thus compression is being loss. Will this cause the bike not to run or should i look into the electrical system.

whats the proper tool or way to check compression?

just helping out a friend here!



Very hard to check compression on a bike with auto decompression cam. Where did the washer come from!? Have you adjusted the idle mixture screw? Is the jetting correct? I would go with those and pull your pilot jet and take a look at it and replace it if it's clogged

the pilot is completely clean~

i dont know were the washer came from, it was very large and has been there since the last rebuild.

bump to the top!

I assume the bike runs fine above idle. If so, you must be too rich on the pilot or something is opening up dei to the heat and causing a vacuum leak. One extreme of the jetting spectrum or the other.

Use electrical parts cleaner (or other Non-volitale spray liquid)and spray the carb and intake book. Ensure ht e hot start fully seats.

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

i appreciate all the input and carb tuning, which i'm aware of, but if the engine loses compression when it gets warm, from the means of a valves going bad or piston/rings being shot, the loss of compression causes a difference intake air/fuel vacuum good compression normally provides, thus leaning out.

Again when the engine gets warm, 2-4minutes the compression is noticeably less when using the kick starter.

but william u still think its a jetting issue? The hotstart plastic nut is seated/sealed, but i should check to see if the metal inner barrel is seated all the way in the port?


Absolutely a jetting issue. Pistons swell when hot, compression goes up. Valve clearences increase as temp rises due to alloy head expansion, valves are seated longer. I imagine what feels like less compression is merely hot oil and less drag. Electrical parts rear thier ugly heads as misfires at high speed, and slowly get worse until the bike simply will either not run hot or at all.

To elimnate fuel issues, lets cover what it cannot be.

Bike runs above idle fine. So it is not the tank breather or the needle jet emusion tube or main jet.

Debris in the carb is a probelm either all the time or at high speed (the vibes agitage the fuel, the relatively high flow rate makes it easy for a flake of debris to cover a main jet).

You know the float level is probably acceptable and not leaking.

You know it has a pilot jet. No is that jet spotleess like new or 'sort of cleaned'?

You know the bike has a fuel screw, have you checked the integrity of the oring?

However, I doubt it is any of these items.

Good that the hot start nut is seated, have you confirmed the plunger moves freely, is in good condition and fully seats?

Have you confirmed the fuel screw is in perfect condition?

Have you spray non-volitale (non-flammeable) liquid at the carb, manifold when hot to see if there is a leak?

i will double check the carb and look for leaks, but i have a strong feeling that the large steel washer i found in the intake track/ports causing aluminum flakes to be ingested is the culprit. but i will double check what i should look for in the fuel system! thanks :smirk:

Any idea how that got there? Think it was an attempt to slow the bike down for the previous owner? I know in some euro countries, they use things that llok like washers to reduce the power output for licensing reasons.

You got me thinking that it might be a good idea to do a leak down test too.

william, found the air screw oring shot out and also found that the leak jet was replaced with a cap head screw!lol so i'm getting a new oring and leakjet and see what happens.

the washer looks like a gas tank mounting washer that was loosly left in the intake, and was most likely left there by accident from the last person who susposibly rebuilt the engine, big mistake!lol


Well, the trashed fuel screw oring may of been your issue.

I bet the screw in the leak jet spot was an attepmt to improve the squirt, which on a 250, is not the way to go, it does need to leak some off. The 450's can use it all, not a 250.

william u were right! fuel delivery problem! fuel screw oring:bonk:


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