Weber Carbs Diff Between IDA IDT IDTP

[matty74]

Can anyone tell me the diff between IDA, IDT, IDTP 3 barrel carbys

[Grady Clay]

Matt,

There are basically five Weber types used on our old 911s; 40IDT, 40IDTP, 40IDA, 40 IDAP and 40IDS. Each engine has one 40IDx3C and one 40IDx3C1 carburetor for left and right respectively. The ‘3C’ refers to three chokes or intake venturi common to all 911s. The "40IDx3C" are Weber factory designations, not Porsche. Porsche commonly used the Weber designations in their technical literature. Each carburator has a Porsche part number in the form; 901.108.107.01 for a right 40IDS3C1.

While the basic carburetors are very similar, there are subtle differences. Some are significant when used in other applications (different cams, displacement, etc.)

To summarize their original applications:

40IDT3C and 40IDT3C1; 1968 911T (non-USA) 901/03 (901/13 Sportomatic).

40IDA3C and 40IDA3C1; 1966-’67 911 (normal) 901/05 and 911/06 and the 1968 911L (non-USA) 901/06 (901/07 Sportomatic). There were undocumented early 1968 911 (normal, non-USA) that used these.

40IDS3C and 40IDS3C1; 1967-’68 911S 902/02 (901/08 ’68 Sportomatic).

40IDAP3C and 40IDAP3C1; 1968 911 (USA) and 911L (USA) 901/14 (901/17 Sportomatic).

40IDTP3C and 40IDTP3C1; 1969 911T 901/03 (901/13 Sportomatic) and 901/16 (901/19 Sportomatic). Also the non-USA 1971 only 2.2 911T 911/03 (911/06 Sportomatic).


So, what are the differences? Basically they are jetting and venture size. Most of the jets (and emulsion tubes) are replaceable brass parts. The venturi are replaceable. There are some non-replaceable jets (idle air correction aka idle air bleed) pressed into the casting. The 40IDS has an additional ‘high speed enrichment’ circuit. There are subtle but important differences in the size and placement of the just-off-idle non-replaceable ‘Transition ports’ and the butterflies are different in this area.


I’ll add the individual specifications as an EDIT.

Best,
Grady

[Fritter]

Grady, thanks for the info. I have 40IDTP's on my '68 and was going to have them rebushed this winter, unless I find something out that they are not good ones to use for some unknown reason to me.

[Grady Clay]

Mike,

Is your 911 a non-USA 1968 911T?
Are there any mods?
What are the S/Ns of the two carburetors?
Do you plan a ‘rebuild’ cleaning first?


Specifications for the Weber 40IDTP3C and 40IDTP3C1

Combined from the Spec Book (4203.20), Factory Workshop Manual (pp. F33-SF5, revised March 1968), Weber manual (3rd Ed., 1973) and carburetors 40IDTP3C #2563 and matched 40IDTP3C1 #2095. (I just happen to have an apparent virgin original pair sitting here for show-‘n-tell.)


Venturi (K) 27 mm. Replaceable.
Preatomizer (we usually call it ‘auxiliary venturi’) 4.5 mm (exit diameter). Replaceable.
Main jet (Gg) 110 (1.10 mm). Replaceable. (Try larger and work down.)
Air correction jet (a) 185 (1.85 mm). Replaceable. (Try smaller and work up.)
Idle fuel jet (g) 50 (0.50 mm). Replaceable. (Try larger and work down.)
Idle air bleed (u) 110 (1.10 mm). Brass piece pressed into carb base – not easily replaceable.
Start fuel jet (Gs) - (na, common to all).
Start air jet (Ga) - (na, common to all).
Cruse power jet - (na, 911S [IDS] only).
Pump jet 50 (0.50 mm); Replaceable.
Emulsion tube F3 (description to follow).
Fuel injection nozzle (j).
Fuel jet for injection nozzle.
Air correction jet for injection nozzle.
Float weight (F); 25.5g.
Float needle valve adjustment; 12.5 to 13.0 mm. (not normally measured)
Fuel level; 20.75 mm ± 1.00 mm from top edge of carburetor housing (with spec fuel pressure).
Float needle valve; 1.75 mm.
Enrichment jet (accelerator pump); 70 (0.70 mm).
Pump valve (accelerator pump) closed
Pump cam (accelerator pump); 3 mm lift.
Pump inlet valve (accelerator pump); closed.
Injection quantity (accelerator pump); 0.5 ± 0.1 ml per stroke. (This is a VERY important number. Try larger.)
Float chamber vent; 6.0 mm.
Mixture discharge port; 5 mm.
Idle mixture discharge port; 1.0 mm.
Bypass bores:
1 = 1.0 mm.
2 = 1.1 mm.
3 = 1.35 mm.

Best,
Grady

[Fritter]

Grady, I have a USA "base" 911 coupe. Not an L or T. But it has the S brakes, tank and gauges due to original sporto config.

Car is basically an all original CA car sold at Century Motors. Biggest changes are a 902/1 #131290 AFMSX that replaced the original sporto trans, and the car also has a '70 T numbered motor 6105729 911/07 in it, but with all the '68 ancillaries on it.

I have a mag cased '68 motor I am currently rebuilding for it.

The right 40IDTP3C is numbered 4903, and the left one (40IDTP3C1) is numbered 4942.

My carbs currently have 50 idles, 125 mains, 30 vents in them, so definitely not stock.

After I take them off, I will look them over and possibly disassemble to a certain point. Because I can do the assembly/reassembly part, the only part I can't do is the rebushing of the throttle shafts. I will see whatever rebuilder I choose has to say about this.

Other than the pressed in idle air bleed size, I wonder if there are any differences in all of the Weber 40's in the basic casting? Meaning, bypass holes, etc.

[69sporto]

That it´s fun

I have a pair of webers (plus linkage, fuel line and manifolds) with this numbers:

40IDS3C1 1461

40IDTP3C1 4625


Are them from the same side???

They were taken for a running car (I suppose)

Do I have a problem to fit this carbs in my car????

Thanks

[911pcars]

One has the high speed enrichment circuit and the other one doesn't. See Grady's earlier post. This extra circuit can be deleted. Not sure how they do it, but there's an answer from the rebuilders.

Other than that, venturi and jet differences that can be brought up to speed.

Sherwood

[unclebilly]

As I understand it, you can get the same performance level out of all of the variations of these carbs provided they are jetted the same and have te same sized venturis. I bought some IDTs after doing the same research you are currently doing. I got my jets etc from Richard at PMO - give hime your engine info and he will tell you what you need. A very nice guy and very knowledgable on webers.

[tonybuono]

I have a 1968 911T engine with 40IDTP carbs. The car was purchased and lived most of its life in Denver, except for the past few years (I live in Florida). I'm getting ready to rebuild the carbs and am wondering if the factory jets were replaced due to the altitude in Denver? If this is likely, I want to replace the jets also.

[1QuickS]

IDA and IDS Webers have a richer progression circuit than IDTP and IDAP carbs do. The spacing and diameters of the progression circuit holes and the associated air correction jets (pressed in) provide a leaner mixture as you approach the transition from the progression circuit to the main circuit. Otherwise the main circuits are the same in that jets and venturis may be swapped at will for equal performance. It's not too hard to modify the air correction jets to provide a richer top end for the progression circuit but the modification of the three progression holes is a precision task.

[Grady Clay]

Tony,

Your 40IDTP3C and 40IDTP3C1 carbs were probably installed when your ’68 911T was imported to comply with USA emission laws. Your ‘68T (non-USA engine type 901/13) originally came with 40IDT3C and 40IDT3C1 carbs. The 40IDTP3C and 40IDTP3C1 were the standard for the USA 911T in 1969.

That said, you might confirm that you have ‘45’ idle gas jets (45 = 0.45 mm). Those are already very lean. I don’t recall installing smaller idle jets – almost ever, except to test. It would be unlikely that we didn’t work on your 911T at some point if it was in Denver since the late ‘60s. What color? Do you have PO names?

Best,
Grady

[tonybuono]

Thanks Grady:
The car is a Red '67 912 soft window Targa with an '68 911T engine (ser 2080872) that was replaced by Germa(n) Equipment Co, 931 W. 5th Ave, Denver on 4/5/72. This car was originally owned by Don McDonald. The carbs are jetted as follows:
Main 1.25
Idle 0.60
Air corrector 1.80
Accel jet 0.50

In the Haynes Weber Manual, I find jets for the 40IDTP13C1 (and 3C/1) as follows:
914/6 (911T)
Main 1.05 1.10
Idle 0.50 0.45
air corrector 1.70 1.85
Accel jet 0.50 0.50

I'm also checking the float levels. There is an extra washer (a shim?) on each of the needle valves, I presume to adjust the float level. I have a PMO float level gage. Is using a "shim" washer the method of adjusting the float level using this gage? And, is this a better method to adjust the float level then bending tabs? (I bought the gage when I was going to rebuild the carbs on my '65 912 but never used it as I installed Webers instead.)

Thanks for your help. Tony (btw, I used to live in the Denver area off 285 in Aspen Park near Conifer)

[911pcars]

Originally Posted by tonybuono

Thanks Grady:
The car is a Red '67 912 soft window Targa with an '68 911T engine (ser 2080872) that was replaced by Germa(n) Equipment Co, 931 W. 5th Ave, Denver on 4/5/72. This car was originally owned by Don McDonald. The carbs are jetted as follows:
Main 1.25
Idle 0.60
Air corrector 1.80
Accel jet 0.50

In the Haynes Weber Manual, I find jets for the 40IDTP13C1 (and 3C/1) as follows:
914/6 (911T)
Main 1.05 1.10
Idle 0.50 0.45
air corrector 1.70 1.85
Accel jet 0.50 0.50

I'm also checking the float levels. There is an extra washer (a shim?) on each of the needle valves, I presume to adjust the float level. I have a PMO float level gage. Is using a "shim" washer the method of adjusting the float level using this gage? And, is this a better method to adjust the float level then bending tabs? (I bought the gage when I was going to rebuild the carbs on my '65 912 but never used it as I installed Webers instead.)

Thanks for your help. Tony (btw, I used to live in the Denver area off 285 in Aspen Park near Conifer)

I find your jet settings okay. However, the idle jet might be a tad large, but they're easy to change. It will have an adverse effect on fuel mileage.

Bending the float tabs is fine for basic adjustments, but the fiber washers are the best way to fine tune the float level. Install the PMO float gauge and observe where the fuel levels are on both carbs.

If you're still in Denver, the altitude will affect these sea level jets. What's your fuel mileage? Grady is in CO so he'll have better info.

Sherwood

[tonybuono]

Thanks. I'm not so worried about fuel economy as much as performance. Will the 0.60 idle jet improve lower and midrange performance much or just waste fuel? Regarding economy, I've never really checked. My 912 and 356C are the economy cars. I bought the 911 about a year ago and got side-tracked resurrecting a dead '84 Ferrari 308QV I got a good deal on; I just got back to the 911 about a month ago.

Do you think jets were changed for altitude (although I would think the jets should be smaller not larger at 5,000 feet above sea level), or for boosting performance?

Thanks, Tony

[911pcars]

Originally Posted by tonybuono

Thanks. I'm not so worried about fuel economy as much as performance. Will the 0.60 idle jet improve lower and midrange performance much or just waste fuel? Regarding economy, I've never really checked. My 912 and 356C are the economy cars. I bought the 911 about a year ago and got side-tracked resurrecting a dead '84 Ferrari 308QV I got a good deal on; I just got back to the 911 about a month ago.

Do you think jets were changed for altitude (although I would think the jets should be smaller not larger at 5,000 feet above sea level), or for boosting performance?

Thanks, Tony

If you're at altitude, the air is less dense and thus the mixture will be rich. Even at sea level, it's on the rich side. An accurate A/F gauge is the only valid method to measure the mixture. It might be worth a trip to a chassis dyno or maybe borrow a wide band sensor setup. If you go through the archives on jetting, you can extrapolate the settings owners have used/suggested for various engine displacements and modifications and come up with a closer ball park assessment of where you are and where you need to go.

Sherwood

[naro914]

I just came across this thread... I have a set of 40IDS carbs that have the enrichment circuit in them. Is that a good thing, or better question, what is good or bad about having it? My mechanic has only ever seen one other set of these in all the years he's been working on early cars...which were not early when he started working on them!

[angelny911]

chime

[Gene Smith]

Can anyone tell me what the extra "1" between the "P" and "3" is on these Weber carbs is? I see no reference to this part ID.

40IDTP13C1 Serial # 1162

I can feel the throttle shafts vibrating when the engine is running and there is some axial play in the shafts, did these carbs come with ball bearing throttle shaft bearings?

What am I up against repairing the bushings/bearings?

I'd like to do this myself. Any suggestions???

[1QuickS]

Those were for the 914/6 engines; the "1" indicates they were reversed from the orientation used on rear engine cars.

Webers used plain bearings and all but the earliest ones used a plastic sleeve in the end bores which leads to rather quick wear-out issues.

For more info regarding this try my web site:

Weber Carbs Technical

[T77911S]

here is a good site to check out.

Performance Oriented