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Discussion Starter · #1 ·
I am having a weird problem with my 2003 250ex. I was riding it around all day the other day, it was running a little rich but otherwise fine. I parked it in the garage and it would not start this morning. I have pulled the plug, it Sparks just fine. It was black because it was too rich but other than that it looks fine. I don’t have a guage but covering the hole with my finger, it feels like it has plenty of compression. Spark plug was dry. Fuel is getting to the carb. Brand new gas, non Ethanol. I’m not sure what else could be wrong. I took the carb off and checked to make sure the float was not sticky and it was fine. It has a brand new carb kit so I wouldn’t suspect anything else to be wrong, especially after already having it running. I also turned the Pilot screw back to factory specs (1 7/8 turns from bottom). Air cleaner is new. Not sure what else to do. Anybody got any similar problems that have been solved?
 

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I am having a weird problem with my 2003 250ex. I was riding it around all day the other day, it was running a little rich but otherwise fine. I parked it in the garage and it would not start this morning. I have pulled the plug, it Sparks just fine. It was black because it was too rich but other than that it looks fine. I don’t have a guage but covering the hole with my finger, it feels like it has plenty of compression. Spark plug was dry. Fuel is getting to the carb. Brand new gas, non Ethanol. I’m not sure what else could be wrong. I took the carb off and checked to make sure the float was not sticky and it was fine. It has a brand new carb kit so I wouldn’t suspect anything else to be wrong, especially after already having it running. I also turned the Pilot screw back to factory specs (1 7/8 turns from bottom). Air cleaner is new. Not sure what else to do. Anybody got any similar problems that have been solved?
Got to check the compression with a gauge - 30 psi will blow your finger off the plug hole, but it takes about 120 to make the engine run acceptably. Low compression can also contribute to an incomplete burn and seemingly rich running.
 

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Yes - test is supposed to be done with throttle held open.

So compression is not the problem and that rules out the valves as a potential problem - now it's down to spark, spark at the right time, fuel, air and fuel/air ratio. Have you tried a new spark plug (any brand other than Champion)?

Also, with compression that high you should be running 93 to 95 octane non ethanol fuel - if it has ethanol in the fuel, it needs to be 95-97 octane but not any higher than 100, but the compression could be artificially high if there is liquid fuel in the chamber. Was the fuel turned off the last time you rode it until the compression was checked?
 

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Discussion Starter · #7 ·
I did get a new spark plug to see what happens and it started immediately, but now I’m running into the same issue with it not starting this morning. Both spark plugs are very charred and it won’t start again. I tried cleaning them up with sandpaper to no avail. I don’t want to keep buying spark plugs so I would like to figure out why it is doing this. Also, when I take the plugs out and ground them they seem to spark just fine, but don’t work inside the cylinder.
4724
 

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Well then it's time to correct the fuel to air mixture.

I looked back through the thread and you say it has a brand new carb. What brand and who calibrated it? Amazon sells carbs the 'fit' not 'calibrated for'. Why does it have a new carb? What was wrong with the one that was on it? Any brand of carb will work on any engine, but the mechanic who installs it is responsible for calibrating it, so dig out the carb tuning manual and brush up on how to determine the mixture at each speed of the 3 circuits that the carb controls and how the circuits overlap. Now Mikuni and Keihin jets are easy to get, but they are sized differently - certain Mikuni jets are sized by ho much fuel they will flow in a specific time and others are sized by orifice size. A 140 Mikuni is not necessarily the same size as a 140 Keihin jet. Amazon carbs typically have jets that have no size indications and the jets have different threads than Mikuni and Keihin, so good luck with finding a needed jet.

Get it properly calibrated and your problem is solved unless it is actually an ignition problem being masked as a fuel problem. Again, the mechanic will determine what the cause is.
 

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Discussion Starter · #9 ·
Well then it's time to correct the fuel to air mixture.

I looked back through the thread and you say it has a brand new carb. What brand and who calibrated it? Amazon sells carbs the 'fit' not 'calibrated for'. Why does it have a new carb? What was wrong with the one that was on it? Any brand of carb will work on any engine, but the mechanic who installs it is responsible for calibrating it, so dig out the carb tuning manual and brush up on how to determine the mixture at each speed of the 3 circuits that the carb controls and how the circuits overlap. Now Mikuni and Keihin jets are easy to get, but they are sized differently - certain Mikuni jets are sized by ho much fuel they will flow in a specific time and others are sized by orifice size. A 140 Mikuni is not necessarily the same size as a 140 Keihin jet. Amazon carbs typically have jets that have no size indications and the jets have different threads than Mikuni and Keihin, so good luck with finding a needed jet.

Get it properly calibrated and your problem is solved unless it is actually an ignition problem being masked as a fuel problem. Again, the mechanic will determine what the cause is.
It has the stock carb, just a rebuild kit. It was installed and tuned by my shop teacher, however I do not believe he tuned it more than what it says to in the manual.
 

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Discussion Starter · #10 ·
What’s the best way to adjust them? My experience is with old diesel pickups so carburetors are a pretty foreign area for me.
 

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Oh Boy - tuning a carb is a step by step process unless you have a flow bench where you can suck air through the carb and adjust the fuel/air ratio by electronic sensor.

If it starts and idles well, that part is done, so you have to figure out at what throttle opening it runs rich at and adjust it.

So, with a clean air filter and the intake configured as to how you intend to operate it, with a new spark plug, take it out and run it at a certain speed (either by RPM or MPH) in about 4th gear - go about 1/4 mile and without changing the throttle setting, cut the ignition, coast to a stop and pull the plug to check it's color - if black, you need to drop the slide needle - do this until the plug comes out light tan - if you can't get it to lighten up, replace the slide needle and needle jet and try again.

Once the mid-range is set, you can start jetting for wide open throttle.
 
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