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Adjusting idle screws

  • Thread starter Thread starter bbytes
  • Start date Start date
B

bbytes

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I tried to adjust the idle mixture screws today. From what I understand, you turn the screws until the engine speed gets faster. Well, the engine only got faster when I turned them all the way in. I could feel it backfire into the carbs a few times too. What should I do to fix this?
 
On your '79 550, you have two screws that will affect your idle mixture.

The pilot FUEL screw is on the bottom, at the front of the float bowl.

The pilot AIR screw is on the side of the carb, near the intake.

Depending on whether you have pod filters and/or a header exhaust pipe, the settings of these screws will vary a bit.

If you have a totally stock system, the FUEL screw should be set to about 5/8 to 3/4 turn out from lightly seated. As a starting point, the AIR screw should be double that, or 1 1/4 to 1 1/2 turns out. From this point on, all your adjustments will be with the AIR screw. Turning the AIR screw IN will richen the mixture, turning it OUT will lean the mixture. Start by turning the screw in SLOWLY to see if the engine speeds up at all. If not, start turning it OUT to lean the mixture, until the engine speed drops a bit. At that point turn the AIR screw back in about 1/8 turn and move to the next carb. Repeat until finished.

If you have pods and/or a header, the FUEL screws should be about 1 turn out. The AIR screws still start at double that, and the rest of the procedure is the same.

Siince you say that it runs best with the screws (probably the AIR screws) all the way IN, it appears that you are not getting enough fuel. Check the settings of your FUEL screws, possibly turning them OUT another 1/8 turn.

.
 
I'd guess that the fuel screws are busted off if you're unlucky or, if you're lucky, just wound in fully or the circuit is blocked.
 
Yeah, actually 2 of them are busted off, but I had it running better before I cleaned the carbs. I tried getting the busted screws out, but no luck.
 
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