911 engine with carbs - sluggish return to idle

[Porsche 2]

My 1977, 911, 2.7 engine has had a top end rebuild and cams reground to match refurbished Weber 40IDA carbs now running very well. Except after reaching normal operating temperature and throttle is returned to idle position the engine rpm's drop slowly - 5 to 7 seconds - from about 1500 to idle set at 900 rpm. Linkage is at idle position but the slow return to idle prevails. I have carefully checked linkage function and have increased the return cable spring with no perceptible improvement. Suggestions of cause and fix will be welcome.

[Norm01]

Are you sure it's not your distributor sticking up.

[jpnovak]

Agree. Check for distributor advance hangup. You can check with a timing light.

[Porsche 2]

Thanks fellows - makes sense, will let you know.

[zedsn]

Have you balanced the air in your carbs?

[Porsche 2]

Turns out the distributor is working fine _ 35 degrees BTDC at 6,000 rpm and vacuum hose connected idle at 5 degrees ATDC. Issue was too much by pass air adjusted positions - closed four and reduced two with aid of volume meter. Engine running fine and a Comfortable return to900 idle from above 1500 rpm in less than two seconds. Thanks for advice offered.