1978 911 SC idling issue
03-05-2006, 08:10 PM
#1
5th Gear
Thread Starter
Join Date: Mar 2006
Location: Greensboro, North Carolina
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
1978 911 SC idling issue
My 78 SC was idling at 1100 and I dropped it down to around 950.
Runs beautifully.
However, almost everytime I come to a stoplight/stop and engage the clutch, the ilde speed drops rapidly to below 700 and shuts the engine off.
On the times that it does not cut off, the engine is able to go back to its 950 idle speed.
Would love to here anyone suggestions on fixing this issue.
Runs beautifully.
However, almost everytime I come to a stoplight/stop and engage the clutch, the ilde speed drops rapidly to below 700 and shuts the engine off.
On the times that it does not cut off, the engine is able to go back to its 950 idle speed.
Would love to here anyone suggestions on fixing this issue.
03-08-2006, 11:46 PM
#2
Instructor
Join Date: May 2003
Location: Edmonton, AB, Canada
Posts: 117
Likes: 0
Received 0 Likes
on
0 Posts
One suggestion would be to speed the idle back up. Of course that's probably not what you wanted to hear, so the other suggestion would be to adjust the throttle stabilization servo on the CIS.
On CIS, when engine speed drops suddenly (letting off the throttle) the inertial of the air flap closing can result in the air flap overshooting its normal idle (rest) position. This pushes the plunger too far into the fuel distributor and leans out the mixture to the point the car stumbles or quits. This can also exacerbate a hunting throttle at idle, especially when cold.
Throttle stabilization is controlled by an adjustable vacuum servo found just behind (meaning further into the engine compartment) where the throttle linkage connects to the CIS. Remove the top vacuum line, loosen the lock nut and adjust the screw on top. Reconnect the vacuum line, test drive, and continue until the problem goes away. If you go too far, the throttle will seem to hang momentarily at about 1500 RPM when you let off the gas. Finding the right spot is really a trial and error thing.
Just a bit of warning ... the lock nut on the vacuum servo can be difficult to loosen. These servos seldom need adjustment, and yours may never have been touched.
Have fun.
On CIS, when engine speed drops suddenly (letting off the throttle) the inertial of the air flap closing can result in the air flap overshooting its normal idle (rest) position. This pushes the plunger too far into the fuel distributor and leans out the mixture to the point the car stumbles or quits. This can also exacerbate a hunting throttle at idle, especially when cold.
Throttle stabilization is controlled by an adjustable vacuum servo found just behind (meaning further into the engine compartment) where the throttle linkage connects to the CIS. Remove the top vacuum line, loosen the lock nut and adjust the screw on top. Reconnect the vacuum line, test drive, and continue until the problem goes away. If you go too far, the throttle will seem to hang momentarily at about 1500 RPM when you let off the gas. Finding the right spot is really a trial and error thing.
Just a bit of warning ... the lock nut on the vacuum servo can be difficult to loosen. These servos seldom need adjustment, and yours may never have been touched.
Have fun.
03-09-2006, 01:00 PM
#3
Rennlist Member
Matt: Sometimes the cure for your symptom is a simple mixture adjustment. If you get your model SC a little too rich the result will be exactly what you're describing. Keep in mind, mixture must be set with the car at full temp, before the cat, at the correct idle speed.
Pete
Pete
