4 Questions about Zenith 40TIN carbs popping

[fizeus]

Hi all,
I think I need some expert carb-man help.

Here is the story.
I had a 911 2.4T from 1973 (car has only 60K km and has setter in the garage for years). Times ago I discovered cylinder 4 was not firing.
So I tested engine compression and spark/rotor. All was ok. Then I decided to remove and clean my stock pair of Zenith 40TIN.
They were never cleaned and once opened I found them full of rust.
I’ve changed gaskets and orings even if they looks fine. When I remounted the carbs cylinder 4 has started to fire and worked properly.
I’ve then tuned the carbs. Idle stops, air bypass screw (using the STE SK) and at least mixture screws as reported by manual.
Dwell and timing was checked by the mechanic less then a year ago also valves and he told me all was ok.

The engine idle ok at 950/1000 rpm and when in idle it sounds good.
The problem now is that I have constant pops thru the carbs while I left the accelerator (in deceleration with gear engaged).
Pops are better audible from 3000 rpm to lower revolutions.

QUESTION 1
I know this should be a lean condition.
Can any one tell which is a good range to turn out the mixture screws?
(in your experience) Can I consider 5.5 turns out a sure over-rich condition?

QUESTION 2
When I rebuilt the carbs I haven’t removed the enrichment circuits because I absolutely want the keep the exterior carb untouched.
Is there any way to temporary stop this infamous enrichment circuit without removing it.
I was thinking to screw in all circuit screws.
Can this be a sufficient step to mute the circuit?

QUESTION 3
I was wondering if this could be an air leak (the engine runs great thru all speed and while the accelerator is pressed).
Pops come out thru the carb _only_ on deceleration (I suppose that this behavior is due to a lean condition caused by the closed-shaft letting air flowing through the leak).
How can I detect and found the leak if it comes out only on deceleration?

QUESTION 4
When I replaced paper gaskets I’ve not used any glue or sealant. Did I have to use some ?


I’ve tried two different carburation methods without success.
Zenith manual says: 2.5 turns out and then close until reach best performance or do not hear anymore pops thru the carb or the exhaust.
I also tried to close the mixture all the way in and then open step by step until the engine rpms rise-up, then 1/4 more turn out.


If possible I’ll avoid to remove the carburetor again because it a very hard work to do without the special Porsche key that i haven’t.


Thank you all in advance for the patience to read !!!

[jstobo]

The enrichment circuit is there to stop the popping on deceleration. It is probably not working. Your RPM transducer could be bad. If it is not working the most common problem is cracked solder joints. If a component s bad you will need expert help. There is a big write up on this device in this forum about repairing it as the MFI cars also have one. Use the search function on RPM transducer or speed switch.

All you have to do to disengage the circuit is to unplug the wire going to the solenoid. I also disconnected the vacuum lines of the circuit and used rubber caps over the vacuum ports.

An air leak could also be a problem. Make sure no vacuum lines are leaking.

I have a mechanic(I really do not own him though) that tunes my Zeniths once a year. He gets them perfect. No popping and great performance. Sometimes a pro is worth the money.

Hope this helps.

[fizeus]

I 've finally found rubber caps (right size) to close the enrichment circuit without removing it.

I have a question about.
One hose goes from carburetor to the distributor.

Do I have to leave it open or is it better to place a rubber caps also to this distributor valve ?

Thanks again

[nilknarf]

Originally Posted by fizeus

...I have a question about.
One hose goes from carburetor to the distributor...

I've had the same question- I used the vacuum port right above the enrichment solenoid and routed that to the diaphragm on my distributor- it seems to work, but I'm struggling to get my carbs dialed in too, so I can't say that's the definitive answer... so I figured I'd bump this thread to the top to ask our more knowledgeable fellow-travelers for confirmation too!

_c

[911s55]

Originally Posted by fizeus

I 've finally found rubber caps (right size) to close the enrichment circuit without removing it.

I have a question about.
One hose goes from carburetor to the distributor.

Do I have to leave it open or is it better to place a rubber caps also to this distributor valve ?

Thanks again

If you aren't going to use the vacuum line from carb to distributor, cap them both. FYI it is a vacuum retard not advance.