Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 911 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered
 
Join Date: May 2014
Location: Italy
Posts: 275
Garage
Question 4 Questions about Zenith 40TIN carbs popping

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.
Iíve changed gaskets and orings even if they looks fine. When I remounted the carbs cylinder 4 has started to fire and worked properly.
Iíve 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 havenít 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 Iíve not used any glue or sealant. Did I have to use some ?


Iíve 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 Iíll avoid to remove the carburetor again because it a very hard work to do without the special Porsche key that i havenít.


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

Old 06-29-2014, 12:59 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
Join Date: Jul 2001
Location: Wilmington, NC USA
Posts: 635
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.
__________________
69 911 2.3Ez
85 928S
Old 06-29-2014, 06:50 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered
 
Join Date: May 2014
Location: Italy
Posts: 275
Garage
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
Old 07-09-2014, 02:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered
 
nilknarf's Avatar
 
Join Date: Mar 2004
Location: Beaverton, OR
Posts: 771
Garage
Quote:
Originally Posted by fizeus View Post
...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
__________________
66 912 ... 2.7 conversion (What a blast!)
79 928 5-spd (sadly, sold)
85.5 944 5-spd (sadly, traded for 928)

www.curtfranklincreative.com
Old 07-11-2014, 03:28 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Regis turd ab user
 
Join Date: Dec 2006
Location: Tacomatose, Wa USA
Posts: 1,489
Quote:
Originally Posted by fizeus View Post
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.

Old 07-11-2014, 03:52 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 

Tags
carburator , leaks , lean , zenith 40 tin


All times are GMT -8. The time now is 05:55 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2020 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.