My idle drops

[Willoughby Z]

You're welcome.

Just remember to 'pay it forward' by reporting back, helps keep the knowledge base going.

[rcb280z]

If the "BCDD" is at fault...there is an adjustment. It is in the FSM. It's my understanding that it is there to help reduce HC's. When coasting, mainly to a stop, from about 25 mph your throttle should "hang up" before dropping to it's factory setting. Mine does. Mine even has more hang time if I'm driving or braking harder than usual. That's when the BCDD is working.

[Zed Head]

My 76 does that. It got worse as I advanced the timing for a little more performance. It was also worse with my old N42/N42 engine which had a fair amount of blowby, and worn valve seals, probably leading to a little bit lower intake vacuum when cold.

Also, the engines with dual pickups in the distributor run advanced timing when cold, maybe to keep the idle up, or for emissions (but it would still keep the idle up - 6 degrees of extra timing advance when cold). I only have one pickup now, but if you have two, getting the water temp switch to work with your ignition module like it was designed to will help.

With a better N42/N47 engine my idle still drops way down when cold, but manages to keep running. And I'm fairly certain that everything is working correctly. One work-around is to bump the idle up to 900 - 1000 to get further from stall RPM.

Edit - forgot to ask for curiosity, but are you running flat top pistons with that N47 head?

Edited April 6, 2012 by Zed Head

[UR2H]

Paying it forward...

Bought some carb cleaner, spray it thoroughly through the vacuum inlet. Seemed to have helped a little but didn't solve the issue. However my vacuum is now at 18Hg when before it was at 17Hg.

According to the FSM, there is an adjustment that can be made as rcb280z stated. I followed the instructions (FSM EC7)-

1. Disconnect the BCDD harness

2. Connect vacuum gauge at intake manifold

3. Start car and warm up

4. Set RPM to correct idle speed, 800RPM for manual

5. Rev engine to 3,000-3,5000 RPM and then let off quickly. Vacuum should jump to 23-24Hg before settling back down to set vacuum, which in my case was 18Hg.

6. If the vacuum did not jump to 23-24Hg, then adjust the BCDD screw accordingly

I didn't have to make any adjustments as my readings were exactly what the FSM has.

This means my BCDD is okay?

[UR2H]

My 76 does that. It got worse as I advanced the timing for a little more performance. It was also worse with my old N42/N42 engine which had a fair amount of blowby, and worn valve seals, probably leading to a little bit lower intake vacuum when cold.

Also, the engines with dual pickups in the distributor run advanced timing when cold, maybe to keep the idle up, or for emissions (but it would still keep the idle up - 6 degrees of extra timing advance when cold). I only have one pickup now, but if you have two, getting the water temp switch to work with your ignition module like it was designed to will help.

With a better N42/N47 engine my idle still drops way down when cold, but manages to keep running. And I'm fairly certain that everything is working correctly. One work-around is to bump the idle up to 900 - 1000 to get further from stall RPM.

Edit - forgot to ask for curiosity, but are you running flat top pistons with that N47 head?

I'm not positively sure but it's N/A so I'm guessing the pistons are flat top.

[rcb280z]

So did this happen after the AFM adjustment?