Weber strange idling settings

[Bart de Bruijn]

Hi,

I completely lost it.......

- Porsche 911 2.0T 1969
- Engine completely rebuild with some mods ( no power increase)
- Before rebuild engine was running relatively fine but many oil leaks
- Webers (40 IDTP)are been taken apart completely cleaned and rebuild (throttle shafts have 'some' play car was running fine with them)
- carbs are 100% original)
Fuel pump is old
Float heights are set NOT while car is running ( took me long to get it running idle)
- distributor has been rebuild and tested by professional German company
- new spark plugs correct electrode gap

I followed the basic carburetor setup described in the great "essay" by Paul Abbott!
When I Start the car I have to nurse the pedal for a while and then I can find a steady idle around 1000 rpms using the hand throttle (beside handbrake). At this steady idle it (almost) doesn't spit through intake.

Now the strange thing is that my STE synchro meter measures around 10(kg/h)!!! At +/- 1000 rpms. See pictures.
One picture shows how far the hand gas handle opens the throttle valves, the other shows how much air is been sucked in.....
What I remember is that 5 kg/h @ 1000 rpms is normal.
Not all exhaust pipes seem to have the same temperature.....

Does anyone has a clue what is wrong here's?
I am running out of ideas.
Maybe Paul Abbott or another Weber guru can help

Hope someone can help to get me on the road after a 18 years full car restoration ;-))
Regards,
Bart

[1QuickS]

High STE readings indicate rich idle mixture. If idle jets are too large or mixture screws too open then it will take more air for the rich mixture to burn which will require larger throttle openings & higher STE readings. If throttles are too open then the first progression holes are exposed which adds fuel to the system thereby requiring more air and larger throttle openings. Typical STE reading will be around 5 when Lean Best is achieved.

IDTP carbs actually have a different progression pattern than IDA Webers. The first progression hole is typically half exposed at idle! This is probably matched to the small idle jets on these carbs, also, the venturis are 27mm which initiates the mains very early. You can help keep throttle plates closed by drilling a 1.5mm hole into the edge of the throttle plates, about 6mm from the edge of the plate, hole to be located on the plate diametrically opposite from where the progression holes are. If the holes don't help they are easy to solder shut. You can also purposely open the air correction screws to allow air into the engine while keeping throttle plates closed as much as possible. If you try this method then you will want to select the barrel with lowest STE reading and then set the other two in that bank to match. This is opposite to typical air correction screw setting procedure.

Another IDTP issue, the mixture screws have a stepped tip where the IDA screws have a continuous taper. I assume the stepped tip was to keep from wedging the taper-tipped IDA screws into the hole and having it twist off, the stepped tip screws will not do that. They also will not adjust mixture easily, pretty much open or closed. If you open these screws the standard 1 3/4 turn you will have much more fuel than the same adjustment using IDA mixture screws.

Be sure throttle cross bar is disconnected from drop links when performing idle mixture, air correction and speed adjustments.

When my engine was rebuilt and I slapped my Webers back onto it, it was VERY irritating and erratic when it was plenty fine before the rebuild. I became my own first customer for shaft rebushing as a result of that. I assume the fresh engine generated more vacuum and disturbed my tired Webers more than before the build.

[Bart de Bruijn]

High STE readings indicate rich idle mixture. If idle jets are too large or mixture screws too open then it will take more air for the rich mixture to burn which will require larger throttle openings & higher STE readings. If throttles are too open then the first progression holes are exposed which adds fuel to the system thereby requiring more air and larger throttle openings. Typical STE reading will be around 5 when Lean Best is achieved.

IDTP carbs actually have a different progression pattern than IDA Webers. The first progression hole is typically half exposed at idle! This is probably matched to the small idle jets on these carbs, also, the venturis are 27mm which initiates the mains very early. You can help keep throttle plates closed by drilling a 1.5mm hole into the edge of the throttle plates, about 6mm from the edge of the plate, hole to be located on the plate diametrically opposite from where the progression holes are. If the holes don't help they are easy to solder shut. You can also purposely open the air correction screws to allow air into the engine while keeping throttle plates closed as much as possible. If you try this method then you will want to select the barrel with lowest STE reading and then set the other two in that bank to match. This is opposite to typical air correction screw setting procedure.

Another IDTP issue, the mixture screws have a stepped tip where the IDA screws have a continuous taper. I assume the stepped tip was to keep from wedging the taper-tipped IDA screws into the hole and having it twist off, the stepped tip screws will not do that. They also will not adjust mixture easily, pretty much open or closed. If you open these screws the standard 1 3/4 turn you will have much more fuel than the same adjustment using IDA mixture screws.

Be sure throttle cross bar is disconnected from drop links when performing idle mixture, air correction and speed adjustments.

When my engine was rebuilt and I slapped my Webers back onto it, it was VERY irritating and erratic when it was plenty fine before the rebuild. I became my own first customer for shaft rebushing as a result of that. I assume the fresh engine generated more vacuum and disturbed my tired Webers more than before the build.

Hi Paul,

Was hoping you would react on my mail ;-)
Your explanation makes totally sense (of course)! There is one thing that I don' t get.....
When mixture is to rich due to idle jets (std) or mixture screws to far open (2 turns) carb needs more air to get combustible air/fuel mixture, right!? Consequence is more open throttle plates. How is it possible that it still runs at +/- 1000 rpms?
Another phenomena is that trying to find steady idling to start "lean best method " not all the exhaust are equally hot(I did check my emulsion tubes and idle jest for debris many many times....

My fuel pump is still the same as when I started restoration 18! Years back. It's the square box shape one that starts clicking to build up pressure and then becomes quiet. However sometimes when I stop the car, I hear the pump clicking again....would that mean that even during idling it has to (re)fill the float chambers....?

Regards,
Bart

[edmayo]

What is the ignition timing set at. Have you checked resistance in all spark plug wires? Since engine has just been into verify all is correct by doing leakdown or compression test. Your problem may be other than the carbs.

[1QuickS]

I always am narrow in my responses and need someone like Ed to balance me out, Thank you Ed!

Your STE readings of 10 are quite high, I still think you are rich. Your idle speed will increase without making adjustment to the idle speed stop screw (at the throttle lever arm) when a rich mixture is leaned out. I see this frequently when testing a newly remanufactured set of Webers. I see a STE reading of 7 with rich & uneven idle mixtures to get the engine to behave and as I adjust mixtures to balanced and leaner settings the idle speed increases without adjusting the stop screw at the throttle lever arm. The higher engine speed requires me to adjust the throttle stop screw to allow butterflies to close more and typically this will richen idle mixture. By continuing to perform the "Lean Best" procedure the throttle speed stop screw is adjusted more and more to allow throttles to close and idle mixtures are adjusted leaner until a slow, smooth idle occurs at 4 3/4 to 5 kg/h.

I have it easy since I use three Colortune devices in my test engine so I can see mixtures as opposed to "Lean Best" adjust them.



It would be good to check float levels also. You can do this in a simulation of running engine by cracking open a main jet carrier and using it as a flow control valve to let fuel leak out of the float bowl while the fuel pump is running. In this fashion you simulate a dynamic fuel feed although you do not have the vibration of a running engine to keep everything "loose". (You could tap on the Weber to provide this release of "stiction".) Obviously, you would perform this with engine off, catch the leaking fuel and the assumption is that you are using the external float level gauge. I have set float levels with the external vial and have disassembled the carb to measure float geometry and have discovered the mechanical settings of the float do not really correspond to the dimensions provided by Weber.

Another item regarding floats: the needles make divots on the tabs where the contact the floats. These should be removed so the interface is flat. Easy enough to do by peening the back surface of the tab which levels out the surface on the opposite side. If the divot does not match the location of where the float touches the tab then there is the real possibility of the mis-match causing the needle to not sit in the seat correctly which will allow fuel to continue to flow. You can check for this by using your external fuel level vial and see if the level is constant while the pump is running. In this test you might want to allow fuel to bleed past a main jet carrier in another float bowl than the one you are observing.

Cheers, Paul

[Bart de Bruijn]

Hi Paul and edmayo,

Sorry for late response on your quick response!

Regarding to ignition:
- plugs new and gap distance set 0.7 - 0.8 mm (sorry, I am an engineer educated with the metric system ;-))
- new wires (not measured though...)
- original Magneti Marelli distributer checked and recurved for '69 911T 2.0
- checked point gap but not dwell angle as I read many times that it can ruin the electronic HKZ device!?

Paul,

I timed ignition static Z1, then with engine running with timing light it is about +/- 5 degrees ATDC as it should.
However in my search for an solution I turned the distributor once and then the revs went up (will try again tomorrow to be sure as this could be indication too?)

When I restored the carbs I had them ultrasonic cleaned. I had them off a few times to check for debris and only the first time I found crud in the emulsion tubes.
3 days ago I checked idle jets and emulsion tubes and they were completely clean
Is there a proper way checking the idle circuit if it is completely free, no blockage?
I checked the float levels with the external outside float level gauge.
However it was not with the engine running.
Fuel pressure is not known/measured yet.

As I mentioned 2 (one on each side) of the exhaust pipes are significantly cooler then the others!?
Could this be a clue for two cilinders not idling/firing well?
Just a brain dump and please correct me when I am wrong.
the idle circuit is for these two cilinders not working well -> debris!?
the throttle has to be opened this far(see picture) to compensate for two cilinders not firing. By opening the throttle this far the progression holes are been freed to get enough fuel and compensate the two dysfunctional idle circuits....!?

Otherwise I can not understand that with this throttle valve position the car is running +/- 1000 rpm's.


Thanks again!
cheers,
Bart

[1QuickS]

Hi Bart,

Fuel is delivered to the idle jet from the float bowl, through the main jet, into the emulsion tube well and then into a vertical passageway that feeds the tip of the idle jet. I refer to this vertical passageway as the "Hidden Gallery" since it is very difficult to blow compressed air through. This "Hidden Gallery" has a reduced diameter section towards the bottom which loves to become clogged with old fuel residue. I provide a method for isolating this gallery and directing air through it to check for freedom of fuel passage. Since your carbs were sitting for awhile they may well be suffering from blockage of one or more of the Hidden Galleries.

Try this procedure "Clearing Hidden Fuel Gallery" on my web page and see if it helps: http://www.performanceoriented.com/special-procedures/

[BartdeBruijn]

Hey Paul,

How quick can someone reply and how difficult it must have been restoring a car befoe the age of newsgroups ;-))
Anyway, I will read your link in a minute, I read it many times and has been a great help!
If I need rebuilding -> shafts I definitely sent it to you!

I would like to hear your opinion on my 'theory' on the two colder exhaust tubes, open valves, low rpms and blocked idle circuits ?
Or could wrong timing cause the same trouble?

Thnx!
Will keep you updated

[Chris E]

Although its a lot of work you might want to check/adjust your valves all over again. Say you have two cylinders on which the valve gaps are off - that would account for cold exhaust pipes and a rough idle/ cylinders not responding to mixture adjustments. And from the pictures (phone display) I dont understand your idle-stop screws? The arms doesnt seem to seat on the stop screw - which, if correct, would mean your butterflies are open at idle and account for the 10 kgs/hr. I suppose this is the hand gas that advances the arm - but adjusting for best lean with butterflies open and progression holes exposed will never get it right.

Also - something as powerful as the stock CDI should easily cope with plug gaps of 1mm - which would give a better burn throughout the range and maybe help improve the idle.

[BartdeBruijn]

Although its a lot of work you might want to check/adjust your valves all over again. Say you have two cylinders on which the valve gaps are off - that would account for cold exhaust pipes and a rough idle/ cylinders not responding to mixture adjustments. And from the pictures (phone display) I dont understand your idle-stop screws? The arms doesnt seem to seat on the stop screw - which, if correct, would mean your butterflies are open at idle and account for the 10 kgs/hr. I suppose this is the hand gas that advances the arm - but adjusting for best lean with butterflies open and progression holes exposed will never get it right.

Also - something as powerful as the stock CDI should easily cope with plug gaps of 1mm - which would give a better burn throughout the range and maybe help improve the idle.

Hi Chris,
Valves are checked multiple times and are okay.
Spark plug gaps are std (I thought)

Concerning your question on the position of the shaft arms, this is the hand gas position needed to get a stable idle at 1000 rpms......
I know this is wrong so my suggestion was that if some of the idle circuits are blocked only this position gets enough fuel into the cylinder to compensate for the blocked circuits. Any other ( better ;-)) idea?
I checked spark plug wires and they have all the same resistance from distributor cap to spark plug.