Zenith 40 TIN Return to Idle Issue

[Frederickhf]

Hello everyone,

Quick Summery of the issue:
I have the engine running at idle around 900 rpm. I have the throttle linkages disconnected. I manually open the throttle valve to rev the engine above 1500rpm. Releasing the throttle valve will not decrease engine rpm. The longest I waited for the engine to return to idle is 15min with no success. I can then turn off the engine and, without touching anything on the carburetors, start the engine and it will idle at 900 rpm. When the engine is not returning to idle, it likes to hang around 1800rpm. I can open the throttle valve and rev the engine higher (3000-4500rpm) and it will always return to 1800rpm.

Background:
I recently cleaned these carburetors (full teardown) and I have not changed anything except washers and gaskets. All the ports and jets had fuel flow through them when I tested and checked during assembly. The last time these carburetors ran was 30 years ago, if there is anything that should be replaced let me know. The engine is a 2.2L on a 1970 911T.

What I have checked so far:
I have tapped and plugged the auxiliary fuel enrichment ports. I suspected this was the issue but there was no change in behavior after I did this.
The fuel bowl level on each carburetor is within spec.
I have checked all the gaskets for the intake manifold and carburetors for air leaks (none).
Timing light shows the distributor advances correctly for the rpm the engine is running at.
The air intake synchrometer shows about 6.5 at idle and I have a rough balance on all carbs. When running at 1800rpm its about 7-7.5.

Thank you for your assistance

[Hawkeye's-911T]

Bump

[Nditiz1]

What happens if you adjust the idle screws on each carb down? Like bring them down to around 5 on the syncrometer.

[Series900]

Distributor advance not returning. Which is why it returns to idle after shutting off.

[Hi_Fi_Guy]

Ignition advance was my first thought too. Definitely worth double checking.

[Bob Ashlock]

Those stinkin' old Marelli distributors on the 2.2T don't always behave consistently due to wear and slightly stretched springs. It may not be returning fully to the stops at idle leaving a little extra advance which of course will cause the revs to come up and then more advance, etc. etc. I farted around with mine, experimenting with tighter springs to calm it, and then couldn't get enough full advance. Finally put it in the frunk and installed a 123 which works perfect, giving consistent idle, etc.

[Frederickhf]

Excellent suggestions everyone! I took apart the distributor and found the primary spring is weak (it was having trouble return to the center with just my fingers moving the weights). I got replacements on order for both springs just in case. I will get an update as soon as I can!

Originally Posted by Nditiz1

What happens if you adjust the idle screws on each carb down? Like bring them down to around 5 on the syncrometer.

The engine lowers in rpm but the issue will still appear. I even backed out the throttle plate idle so that it would barely start and the same problem would occur.

Thank you again everyone for the suggestions!

[Frederickhf]

The springs fixed the biggest issue. There is still wear on the weights causing lag for the return to idle, but at least the problem is known and I can work around it.

Thanks again everyone for the feedback!