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Losing idle while warmed up

Geoff's 240z

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Tough one here has me scratching my head. As always, thanks in advance to the group for helping think through the issues.

  1. Full tune-up and carb balance completed. Based on my cam, the car likes to sit at idle at 900-1000 RPM. Holds this idle while in the garage.
  2. After driving for a bit, starts to lose idle at stop lights. Will drop down to 600-700 RPM, which is rough given how lumpy my cam is. Maintains idle in the range, does not fall further or stall.
  3. No other loss of power or performance issues. Plugs, wires, etc. in good shape.

Any ideas?

Edited by Geoff's 240z
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19 minutes ago, Mark Maras said:

Did the idle drop occur before the tune-up?

No, it did not. Seems to have happened after I painted the air box and changed spark plug wires. Swapped the old wires back just to rule that out and no change. 

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I'd suggest going over every aspect of the tune-up again to ensure each setting is correct.  Further, rebalance the carbs to ensure you haven't a linkage problem.  If you're using choke until it's warm, that could mask the imbalance.

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  • 3 weeks later...

So, sitting at a red light...

Does the idle stumble with your foot on the brake?

What about stopped at a red light using the hand brake instead?

If it stumbles only when you have your foot on the brake pedal... check the brake booster.

When sitting at a light with foot on brake, can you hear a hissing / air leak at your feet?

Plug vacuum line to booster and test idle (careful if you drive).

I have found that the booster can still work but leak at idle with brake depressed....which will make your idle drop.






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My only other thought in addition to what has been suggested is: have you checked the torque / tightness of your carb to manifold nuts, manifold to head nuts etc?

Sometimes you can get a vac leak at idle which results in a higher idle than when it heats up and things expand / get tighter.

I wonder if any of those gaskets may have been loosely sealing previous and have been disturbed by the air-box going on and off, due to a nut or two not being fully tight?

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