Webers need a tweek?

[Neilnaz]

Hi, I have a 71 911T Targa that drives really well. It is of German origin imported to the UK in the mid-70s then onwards to NZ in 1976 (if I remember the documentation correctly).

The minor issue I have is that the car stutters at revs of 4,000-5,000 before really lifting up its skirts and screaming up to 7,000. If I apply 3/4 throttle and accelerate more gently the phenomenon is more exagerated. If I floor the throttle pedal it seems to blast through with less of an issue.

I am no expert with Weber carburettors hence the post. I am sure that a service kit would be a good idea and to clean the carbs and set the float levels (etc.). But if those symptoms persisted following a good service is that symptomatic of going slightly lean?

The engine did receive some work in 1988 where the pistons and cylinders were changed and an S cam and timing adopted. I haven't established exactly the specification of the engine although this is probably not relevant to the problem. It may however be relevant when comparing Weber jetting where my engine may need something different from the standard 2,200 T engine.

Your thoughts would be greatfly apperciated.

Regards, Neil

[wayner]

Neil, my 3.0 on webbers in my 73 does exactly what you are describing so I am all ears.

So far with help from the board I think mine is rich but I have a gauge to install and verify.

Mine worked well in the early cold Canadian sportscar season, but then developed a slight flutter at that RPM range as the weather warmed. By the heat of summer it was almost undrivable. This cold spring it was fine again but I haven't been using it yet so not sure.

I will stay tuned to learn what you find out, and when mine comes out I will compare notes.

[Trackrash]

The engine did receive some work in 1988 where the pistons and cylinders were changed and an S cam and timing adopted. I haven't established exactly the specification of the engine although this is probably not relevant to the problem. It may however be relevant when comparing Weber jetting where my engine may need something different from the standard 2,200 T engine.

So the Webers are the originals to the T? Any mods to the Webers? What about mods to the heads and distributor? If you change one thing on these motors, other things need to be adapted to match.

Post the details of the Webers. Venturi sizes, jets, etc.

There is a big step from T to S cams.

[gsjohnson]

S Timing should be the same as the T engine. Total advance of 30 degrees is what you want. Make sure your valves are adjusted properly as well. Both can cause the drivability issue that you are experiencing. Low float levels can also contribute to this as well...possibly the air correction jets are too large? I would set your total timing advance first, then I would move to the valve adjustment...

[Trackrash]

Originally Posted by gsjohnson

S Timing should be the same as the T engine. Total advance of 30 degrees is what you want. Make sure your valves are adjusted properly as well. Both can cause the drivability issue that you are experiencing. Low float levels can also contribute to this as well...possibly the air correction jets are too large? I would set your total timing advance first, then I would move to the valve adjustment...

2,2 T timing is 35 deg BTDC. A T distributor has more internal advance than an S, which could cause problems with the S cams.

Again, without knowing how the POs carbs and motor are set up, we are shooting in the dark.

[gsjohnson]

I stand corrected on the T timing spec, but with his S cams the total advance should still be at 30 degrees. The curve between the S and T distributors is also different, not just the total advance. Although we are shooting in the dark, there are certain things that we can look at relating to his drivability complaint...such as tight valves and low float levels, both of which should be checked anyway...as well as his total advance.

[Neilnaz]

Thanks for all your responses. I do not have detail of the exact engine modifications but recall reading a receipt from the engine work where the Porsche dealer stated that they had trouble setting the timing to the S specs. I will have to hunt down the actual words. The trail of paperwork I have is difficult for me to interpret (for instance part numbers of cylinders used during the engine rebuild). I will post it on this thread so that someone can shed some light.

As far as I understand (I have no evidence to the contrary) the Webers are original to the engine and the engine is definitely original (in terms of the core) to the car. I do not have any evidence of modification to the distributor or the heads but nothing that disproves it either.

If I did know what had been done to the engine then I presume people would point to a spec of timing, choke sizes, venturis (etc) that would work. As I do not, as it stands, have much or any of this then there is little merit in pointing me towards a likely set up that would work.

Maybe a different approach would be:
1) check and adjust the float levels
2) confirm what the carb specs are currently
3) clean the carbs
4) establish whether the problem persists
5) post my findings

Incidentally, how easy is it to confirm the carb specs and identify components? I will do a search to see how it is done - I am used to SU carbs!!!

Cheers and thanks for your input,

Neil

[gsjohnson]

Sounds like Gordon may have hit on it by the comments you received by the dealer regarding your timing difficulty. 35 degrees is too much for S cams and will also cause your symptom.

[Trackrash]

Are your Webers 40 IDTP ? It's stamped on the side of the carb.

[1QuickS]

All of our Webers are identical in performance potential once the main circuit is activated. The IDTP carburetors have a progression circuit design that is mated to suit smaller main venturis (27mm to 30mm) and all other progression circuits used in the various type permutations for our cars are similar and more appropriate for larger main venturi sizes.

When I say "performance potential" I mean you may install main jets, emulsion tubes, air correction jets and main venturis and with each item you have the responsibility to decide if that component works harmoniously with the other bits in your Webers and with your particular engine and driving style/application.

If your engine was modified with S-cams and other "upgrades" and your Webers are as they were as OEM for a 911T then you have work to do. However it is difficult to map a course of action without knowing where you are now. The particulars of your Webers may not be important if you do not care for cost impact of component changing but if you do want to control costs then it would be best to list what is currently installed in your Webers.

Those items of interest in your Webers are:

• main venturi size
• idle jet size
• main jet size
• emulsion tube type
• main air corrector size
• part number of your Weber (IDA, IDS, IDT, IDTP or IDAP)

Since your issue occurs well within the range of main circuit operation the details of the idle/progression circuit are moot. Emulsion tube type may be the culprit since WOT is mostly the interaction of the main jet size and the main air correction jet size. When you are at partial throttle the emulsion tube has more contribution to fuel mixture than when at WOT.

I believe the OEM emulsion tube for your Webers is a F1 which has a rather large outside diameter that extends for much of the length of the tube. This large/long diameter "fills" the emulsion tube well with metal that restricts the flow of gas past and also excludes gas from the well. Since partial throttle opening acceleration performance is the domain of the emulsion tube it stands to reason that your emulsion tubes are hindering fuel delivery. Typical emulsion tube selection for "S" specification pistons/cylinders is F3 but that assumes IDS carburetors with the high speed enrichment tubes. F26 emulsion tubes are also a good choice but require larger main jets and smaller air correction jets to work than when F3s are selected.

Of course, your distributor is worthy of inspection as well but I believe most of the dizzys are at full advance at your RPM range so for there to be an advance issue that occurs within the 3000 to 7000 RPM range but does not cause trouble at 3000 RPM or at 7000 RPM would be difficult to imagine. Someone provide more sage input here, please.

[Neilnaz]

Thanks for all your helpful responses. I looked at the carbs but couldn't see the IDTP stamp. I may not have looked in the right place. I attach a picture showing type number and serial number(?) of one of the carbs.
Hopefully it will make sense - if not let me know where to look. Apologies about the photo being upside down I don't know why that has happened.

Again thanks very much for your assistance with this.

Cheers, Neil

[1QuickS]

40IDTP13C1 from a 914/6.

Weber part number is below the "TIPO" and above the serial number "3870"

[Trackrash]

Advance curves for various 911 distributors. The problem with the T distributor, * *** *** 008, is they will not have enough advance at idle or low speeds if the total is set at 30 deg BTDC. It shouldn't be an issue at 4000 rpm or above. You might want to see how much advance your motor has at 4000 rpm, just to rule out a timing issue.

EDIT: That's for the distributor advance not the advance at the crank, maybe someone has that chart.

[Neilnaz]

Great information - appreciated.

I checked the other carburettor 40IDTP13C serial number 4303 just to make sure they are consistent. This probably means that at least one of the carbs does not originally belong to the car.

Maybe I need to check the jets and emulsion tubes (etc.) to see what I have installed. I feel the need to research cleaning the carbs and recording each component to see what I have. I just looked at the distributor but could not find any identifying words/code. Is there an easy way to distinguish the distributors?

You are all kindly providing information and for me to move this onwards I need to check things so I will also check the float chambers as they could be incorrectly set.

Blimey I am in need of lots of assistance - blinking Noob!!

Cheers, Neil

[1QuickS]

Don't worry the serial numbers on the carbs, I've never seen two with the same serial number on a common project.

What is important:

• the carbs are of the same type (which yours are)
• they are jetted similarly
• the jets are actually the size that is indicated (easy to ream them larger and to not "erase" markings or re-identify)
• Webers are tuned thoroughly
• engine is in good condition with valves set
• electrical system is in good condition and with correct components

[dicklague]

think about getting an AFR instrument so you can know what is going on under varying loads and RPM.

Mine sure helped me fix my AFR after years of frustration.

If you are thinking of one think beyond Innovate.....there are lots better.

[Trackrash]

Originally Posted by Neilnaz

Great information - appreciated.

I checked the other carburettor 40IDTP13C serial number 4303 just to make sure they are consistent. This probably means that at least one of the carbs does not originally belong to the car.

Maybe I need to check the jets and emulsion tubes (etc.) to see what I have installed. I feel the need to research cleaning the carbs and recording each component to see what I have. I just looked at the distributor but could not find any identifying words/code. Is there an easy way to distinguish the distributors?

You are all kindly providing information and for me to move this onwards I need to check things so I will also check the float chambers as they could be incorrectly set.

Blimey I am in need of lots of assistance - blinking Noob!!

Cheers, Neil

You are on the right track. It may very well be the float levels, main jets or emulsion tubes.

Let us know what you find.

BTW, there is a number stamped on the side of the distributor. You may need a mirror to read it. The last three digits are telling. 006 for Es. 007 for Ss, and 008 for the T distributors, assuming it's original to a 2,2.