Carb Question

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Apr 1, 2007
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#1
I bought a 05 yz 125 in march and in the first 2 months i fowled 6 or 7 plugs. i was changing plugs ever other ride. I recently bought the vforce 3 reed valves and have had much better luck with the plugs. However they still come out wet when you pull it out. Do i still need to tune the carb? if so do i adjust the pilot air screw? I can't tune any kind of carb worth a crap but i can do just about anything else. I was reading in the manual that the stock settings are "2 1/4 turns out after slightly seated".
 

IndyMX

Crash Test Dummy
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#2
Sounds like you need to rejet. Do a search for Jetting on here, you'll find more info than you can handle.
 
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#3
i tried using the "pilot method" for tuning, the exhaust was completely smoke white the whole time it ran. Granted it is rainy and humid here in texas
 
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#4
Usually, the screw adjustment is only for the idle circuit. To change the mixture for the power band you need to tear the carburetor apart and replace the main jet.

Most dirt bikes have a third adjustment which is the metering needle. This changes the mixture based on throttle setting, pulling the needle out of a jet to increase the mixture. If your bike hesitates or "bogs" when you open the throttle quickly then this is what you need to adjust.

A plug fouls because it doesn't get hot enough to burn off the oil that accumlates on it. How hot the plug gets depends on the plugs ability to transfer heat to the head (you can get "hotter" or "colder" plugs), the heat produced by the burning of the fuel (mixture makes a big difference here) and simply how hard you ride the bike (pushing it hard at higher RPMs heats it up a lot more than just putting around).

The idle mixture should be adjusted for best idle, do NOT attempt to fix hesitation issues or open throttle mixture issues with the idle circuit. If the idle mixture is way off you it may need to be adjusted in steps: set the idle speed to whatever it will idle at. Adjust the mixture until best idle is achieved. If idle speed is too high, lower the speed as low as it can go and keep running. Repeat the above steps as necessary.

For the metering needle adjustment, start with a warmed up bike. Accelerate from a stop to mid range, back to slow. Note any hesitations or bogs. Remove the top of the carburetor, where the throttle cable enters (sometimes it has a large twist cap, sometimes a couple small screws). The metering needle will be dangling from the top of the carburetor. The metering needle attaches to the throttle slide with a clip. There will be numerous places where this clip can be installed that allows the needle to be lowered (leans out the mixture) or raised (richens the mixture).

In your case I would suggest lowering the needle, make it stick out the bottom a bit farther. This will make your low and mid range mixture a little bit leaner, which will help the plug fouling issues.

After you make the adjustment, test ride the bike again. If performance improves or doesn't make any noticable change then you might want to go a notch farther. If a hesitation, bog or other form of undesired behavior is introduced then move the clip back to where it was.

Rod