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Old 10-12-2012, 09:40 PM   #70167
motolab
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Joined: Feb 2006
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Quote:
Originally Posted by neo1piv014 View Post
I've also learned a valuable lesson about why I need to get that god damned extended fuel screw. So I backed the fuel screw out about 1/2-1/4 of a turn. The bike fired up, with choke, but when I gave it throttle, the engine would sputter and die. Assuming that I had the mixture too rich, I screwed it in about 1/4 turn, put everything back in place, and fired it up again. It's about the same. The idle is still very "chuggy" and dies after about 10 seconds, and now, when I open the throttle any more than 1/3 or 1/2, the engine sputters and dies. I'm assuming that I'm running too rich, but I also know next to nothing about carburetors, so I might be a bad judge here. Does it sound like I'm running too rich?
The pilot screw should only be used to tune the idle mixture. Anything that is not idle is the responsibility of other components. Correspondences on CV carbs:

Low rpm all throttle positions: float height, needle base diameter, emulsion tube outlet size
WOT operation overall: main jet
WOT operation between HP peak and red line: main air corrector
WOT operation below red line: jet needle shape
1/4 throttle opening: jet needle clip position
1/8 throttle opening: pilot jet size
1/16 throttle opening: pilot jet size
idle: mixture screw adjustment

Realistically, the float needle condition should be verified and the float height set before the idle mixture is adjusted.

Procedure:

Hold the carb in one hand and hold the base of the float cage tightly against the carb body with the index finger of the same hand. Rotate the carb so that the float tang just contacts the float needle's spring loaded plunger, but does not depress it. With the float height setting tool set to the desired height, use the other hand to drop it down over the float so the tool's posts contact the bowl gasket surface and the posts are square to the body (front to back - the tool takes care of side to side). Flip the tool to check both sides (as the float may have some twist in it). Bend the float tang until the tang on the float height setting tool just contacts the highest point on the float, but does not depress it.

Procedure for setting idle mixture:

Start the engine and warm it up. Lower the idle speed below the factory spec. Starting from a setting that is known to be lean (1-1/2 turns is likely but not guaranteed to be), adjust the fuel screw to obtain the highest idle speed. Adjust to 1/8 - 1/4 turn richer than that. Then, adjust the idle speed back to 1500 rpm.

Once you have the correct fuel screw setting, there should not be too many reasons to have to revisit very often. I'm therefore not a big proponent of extended fuel screws. If you intend to own carbureted motorcycles in the future, I recommend procuring an angle-drive screwdriver, which will be universally applicable.

Regards,

Derek
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