Adjusting Idle Speed

[mluder]

I just finished re-assembling the top side of my engine after replacing the water pump and repaining the intake, w-pipe, etc. as well as the fuel distributor, etc.

In the process, the curb idle screw and the screw that contacts the idle speed microswitch have been mucked up.

Is there a process for resetting them? I know the microswitch screw is designed to engage the switch when the throtle is released. However, how do I adjust the curb idle screw to set the rest position?

Right now the engine runs but there is an intermittant scraping sound that seems to come from the pressure plate on fuel mixture control unit. I think it is actually air being sucked into the small gap as when the engine is idling, the plate is being pulled down ever so slightly and there is about a 1/16 to 1/32 gap.

Any thoughts?

Cheers.
Steve

[Bitsyncmaster]

Just back it out till no contact, turn in untill contact and then add maybe a half turn. I have not checked the amount to turn it in but my guess is between 1/2 to 1 turn.

[DMCMW Dave]

I

Is there a process for resetting them? I know the microswitch screw is designed to engage the switch when the throtle is released. However, how do I adjust the curb idle screw to set the rest position?



Cheers.
Steve

Un-official but it works:

1 - Back both screws out so that the throttle is really closed.
2 - Turn the bottom screw in so that it just contacts the stop. Go another 1/2 turn.
3 - Turn the top screw in until the microswitch clicks. Go another 3/4 turn.
4 - Start the engine and let it get completely warmed up. Fans should cycle at least twice.
5 - Turn the top screw in slowly until the idle speed starts to increase. Every 1/2 turn, turn the top screw in the same amount to maintain pressure on the microswitch. Now back the lower screw out until it is just stops slowing down, i.e. just at 775 RPM.
6 - Back the top screw out until the microswitch clicks, then turn it back in until the switch just clicks and then another 3/4 turn.
7 - Lock the lock-nuts without turning the screws.

This makes sure that, when warm, the idle speed motor is more or less closed. This gives the best performance (margin for opening) when cold, and tends to minimize instances of the engine dying when the throttle is slammed shut at high rpm.

[mluder]

Thanks, Daves.

I will let you know how it goes.

Coincidentlally - I've got a nice idle hunt going right now too. I'm assuming that it's due to the engine being cold as well as probably a need to adjust the mixture. I replaced all the vacuum lines and the timing chain gasket and the rocker cover gaskets, as well as the spark plugs so I'm sure things are off from my previous conditions.

If I allow the engine to come up to temp - i.e. cycling the fans twice - I assume I adjust the idle speed first. Then the mixture with a dwell measure?

Cheers
Steve

[Bitsyncmaster]

Adjust the curb idle screw now with the engine off. You don't adjust the normal idle speed. That is set to 775 RPM via the idle ECU. You will need to adjust the mixture with a dwell meter after doing all that work.

[DeLorean03]

5 - Turn the top screw in slowly until the idle speed starts to increase. Every 1/2 turn, turn the top screw in the same amount to maintain pressure on the microswitch. Now back the lower screw out until it is just stops slowing down, i.e. just at 775 RPM.

Hey Dave,

Excuse me for asking. For the second part, did you mean "Every 1/2 turn, turn the bottom screw in the same amount..." ?

[DMCMW Dave]

Hey Dave,

Excuse me for asking. For the second part, did you mean "Every 1/2 turn, turn the bottom screw in the same amount..." ?

No - the bottom screw sets the throttle stop, the top one is the microswitch.

[mluder]

No - the bottom screw sets the throttle stop, the top one is the microswitch.

I was confused about that too. Dave said, "5 - Turn the top screw in slowly until the idle speed starts to increase. Every 1/2 turn, turn the top screw in the same amount to maintain pressure on the microswitch. Now back the lower screw out until it is just stops slowing down, i.e. just at 775 RPM."

It seems like you are saying the same screw - Are you sure you don't mean turn the BOTTOM screw until the idle speed... and then turn the TOP screw the same amount? I assumed that's what you meant.

Cheers
Steve

[DMCMW Dave]

Yeah - my bad. #5 is the only one wrong.

The bottom one is the stop screw, the top one is the microswitch. The idea is to open the throttle as far as possible while maintaining 775 rpm, and keep the switch engaged (pressed).

Long day I guess.

[Jonathan]

Is one aspect of that adjustment the part considering it as a lever arm, such that if the screws are both fully backed out with the throttle completely at rest, then lengthening the top screw is actually pushing the lever arm away a little more, hence the bottom screw isn't touching anymore? Where you are aiming to have it adjusted to satisfy both of the main conditions (being that the idle speed microswitch is engaged when it should be, and that the lever arm is completely at rest when it should be) BUT your adjustment is trying to get it very close to the "switching point" for each of these conditions? i.e. cutting out the lag/delay/slack.