Weber IDTP to IDA conversion

[1QuickS]

If you suffer a flat transition with your IDTP carbs then there are a couple of ways to deal with it:

1. Install new throttle plates with a modified ellipse angle to help cover the first transition hole (which is partially exposed at idle on these carbs) and to install smaller idle air bleed jets (not a standard replacement jet)
2. Have the progression holes reworked to have the progression of IDA carbs and (again) replace idle air bleed jets with IDA sizes

Here is a picture of what the second option looks like after replacement:

[MBruns]

Looks nice Paul !
Mike Bruns

[Trackrash]

Install new throttle plates with a modified ellipse angle to help cover the first transition hole

Can you give us the details of this? I'm getting ready to go through my IDTPs

[1QuickS]

IDTP carbs are an emission era version of the IDAs. Combined with an OEM main venturi size of 27mm they rearranged the progression to have a transition overlap with the earlier initiation of the main circuit due to the smaller main venturi size. When larger main venturis are used there is a lean transition due to the early end to the IDTP progression and a delayed initiation of the main circuit actuation.

Typically this lean transition is masked with large idle jets, earlier acting emulsion tubes and smaller main air correction jets.

Another aspect of the IDTP carbs is the partial exposure of the first progression hole when at idle. (This is a Zenith 40TIN issue as well.) I believe this is purposely this way to help enrich small throttle opening richness to pass tail pipe emission testing. Classical carburetor design is to block the first progression hole with the edge of the throttle plate when at idle, leaving only the mixture screw port as the only source of fuel at idle. Since progression fuel delivery is the summation of all progression (and idle mixture screw) ports below the throttle plate and the since the idle mixture screw is set to achieve "Lean Best " when idling; it follows that the idle mixture screw is closed down a bit to compensate for the extra fuel coming from the partially exposed first progression port. The result is a progression that is a bit leaner than it should be since the idle mixture screw isn't supplying all the fuel it would if the first progression hole was blocked.

A throttle plate with a different ellipse angle will effectively block the first progression hole which will then enable an idle mixture screw setting that will provide more fuel during progression.

The second part of the equation is to look to the idle air bleed jet as a tuning aid. PMO carbs and virtually all Weber IDA (two barrel down draft) and DCOE side draft carburetors provide for idle air bleed adjustment but the OEM Webers and Zeniths on the 911s used fixed bleeds. The idle air bleed provides air to emulsify the idle mixture during idle/progression circuit operation AND it also affects duration of the circuit's operation. A smaller bleed prolongs progression AND enriches mixture strength with the effect being more pronounced on the upper RPM range of operation; much like the air correction jet for the main circuit.

The idle air bleed for the IDTP carbs is a 140 vs 110 for the IDA carbs. Replacement with a smaller bleed WILL help bridge the lean transition issue.

Of course the optimal solution is to rework progression and idle air bleeds to have IDA specifications during a more extensive service of your IDTP carbs like I presented in my first posting.

The following two pictures show the difference between IDA and IDTP progression; first the IDA then the IDTP:

[Trackrash]

Thanks for the explination. I have the opposite problem with my Webers. I am too rich during progression. At least according to the tech who recently dynoed my car.

I have modded my carbs. I have 110 idle air jets and re-drilled my progression ports to match the size on the IDAs. I have 32 venturis, F3 imulsions, 135 mains, 180 air corrections, and 60 idle jets.

I did notice that someone had thinned down my throttle plates on the top by the transition ports. I think this may be my problem, so I am thinking of replacing the throttle plates, along with a re-bush.

A throttle plate with a different ellipse angle will effectively block the first progression hole which will then enable an idle mixture screw setting that will provide more fuel during progression.

Can you show pictures showing this? Or one comparing the IDPT with the IDA in idle position?

thanks,

[1QuickS]

My "opinion" is that enlarging IDTP progression holes added more fuel at a time when the IDA progression was only partially exposed; the top hole of the IDA progression is somewhat above the center of the port compared with that of the IDTP (see photo comparison in previous post). The progression ports above the throttle plate act as supplemental air bleeds and continue to dilute mixture strength until the throttle plate edge sweeps past them whereby they become fuel emitters as opposed to fuel emulsification air sources.

Also, the middle progression hole in the IDTP is somewhat larger than on the IDA so if you increased the bottom and top holes on the IDTP you have effectively made a large increase in fuel flow without much air to emulsify it. Add to that the 60 idle jets (typically needed for 2.7+ engines) and 110 idle bleeds (smaller is richer progression) then you may be suffering several progression enriching issues.

IDTP progression hole sizes from first to last are: 0.70mm, 1.25mm and 1.10mm for a combined flow area of 2.56 excluding idle mixture contribution
IDA progression hole sizes from first to last are: 0.80mm, 1.10mm and 1.35mm for a combined flow area of 2.88 excluding idle mixture contribution
Modified IDTP progression holes per my guess as to what was changed on your carbs are: 0.80mm, 1.25mm and 1.35mm for a combined flow area of 3.16 excluding idle mixture contribution

Throttle plates are commonly filed to help "time" the exposure of the first progression port simultaneously in all barrels to compensate for manufacturing tolerances. I haven't seen where filing on the top side provides a benefit here.

[Trackrash]

Again, thanks for the help.

IDTP progression hole sizes from first to last are: 0.70mm, 1.25mm and 1.10mm for a combined flow area of 2.56 excluding idle mixture contribution
IDA progression hole sizes from first to last are: 0.80mm, 1.10mm and 1.35mm for a combined flow area of 2.88 excluding idle mixture contribution
Modified IDTP progression holes per my guess as to what was changed on your carbs are: 0.80mm, 1.25mm and 1.35mm

You are correct that larger progressive holes add air into the circuit, I found that out the hard way on some Zeniths.
However, on my Webers, I peened the middle progressive hole almost closed and redrilled it so my progressive ports are all the same size as the IDA. Perhaps not in the same exact location, however.

I'll be adding this discussion to my archives.