Weber IDA 'S's carburetor problems

[Fritz A. Ficke]

I am helping some friends with the engine in their Early 67S it still has the original IDA 'S's carbs and it has the damage caused by this set up Porsche used.
Porsche solved this problem with the Band-aid fix of the IDS carburetors, which of course do not make more power (actually less than IDA's) but they allowed the compression ratio on the 2.0L engine with single plug and no detention at high speed.
This is the damage from running these IDA S carbs and no twin plugging, goes lean Detonation at high-speed causing Scored cylinders and a piston with a broken compression ring. Of course if you have a "cars and coffee" early 67S with the IDA S carbs you will be fine, just the wide-open throttle people will suffer running these without twin plugging the engine.

[1QuickS]

Could you explain why the IDS Webers make less power than IDAs?

IDS is same as IDA except for additional blockage of air flow in the intake throat due to area of the tube of the High Speed Enrichment Tube.

Jetting can be tailored for mixture strength on main circuit.

Perhaps you compared on a dyno OEM jetted IDA to OEM jetted IDS and found a deficit?

[Fritz A. Ficke]

The IDS carb is the same as an IDA, with the exception of the enrichment circuit. By design the IDS was meant to run over-rich at wide open throttle to save these early 67S engines. An over-rich mixture will make less power than a correct mixture. The best thing you can do with an IDS carb to run on any engine other than the 2.0L 67S is turn it into an IDA by blocking off the enrichment circuit.

[1QuickS]

Yes regarding 67S engines but mechanically the IDS is capable of jetting changes as any other IDA.

I have dyno data from Jeff Gamroth and a couple of others with IDS carbs on engines with later model heads (narrow valve separation angle) and larger piston diameters, both helping flame propagation issues. By performing jetting revisions during dyno tuning the AFR curves were as good as any IDA, just a matter of tuning them for the application.

My sense from your original posting was that IDS Webers were inferior to IDAs but I now understand your comment was intended for only the carbureted, 2.0 S-engines.

The engine in my 67S has late heads, 85mm pistons, S-cams and is twin-plugged. I initially disabled my IDS Webers (now 43mm) but have since re-activated the High Speed Enrichment Jets and re-jetted. I modified the HSE Jets to have tunable jet size which helped adjust contribution at WOT. AFRs at WOT at upper RPMs are in the 12s.

By the way, it is rather easy to modify most triple throat Webers to add the High Speed Enrichment Circuit, the throttle bodies must have the casting feature as adopted in late 1966 for the IDS version, all subsequent Webers have this casting feature although it is not utilized. Obviously, IDA (S) and early IDA Webers do not have this casting feature.

[Fritz A. Ficke]

Let us be very clear here Paul, so that people who perhaps might not know as much as you can understand these very early 2.0l S cars and why the changes were made by Porsche and what are our options with them today.
The first S had IDA carburetors with S jetting and had a S stamp on them, picture in 1st post. Porsche experienced high engine failure with autobahn travelers with these early S engines, pictures in 1st post of the type of failure Porsche had experienced.
The problem was lean detitanation, the best answer would have been twin plugging the 'S' engine like the race cars. The less expensive answer was to dump excessive amount of gas in at high speed. Webber added a circuit to the IDA carburetor to do that and called it the "IDS"
It was a less than ideal way to solve the frequency of warranty claims, but it was the least expensive way.

I gave the owners of this early 67S three options so this will not happen again,

1)Twin plug the engine.

2) buy later model IDS carbs and shelve your OEM IDA S carbs.

3) do not use WOT for sustained periods of time.

They went with option 1

[1QuickS]

Cool, twin plugging is great!

[66S]

Motors make the most power with a lean mixture so IDa(s) wins at high rpm, right? For a while maybe. I wonder if an emulsion tube revision might give a more rich mixture at high rpm?

[1QuickS]

The following is based upon what I think I know about e-tubes:

Emulsion tubes are a primary factor in when the main circuit starts and for adjusting fuel mixture during most of the mid-range operation. However, by the time the top RPM range is achieved the E-tube is completely exposed without further mixture adjustment. So, top RPM mixture adjustment is dependent upon main air correction jet in combination with fuel delivered through main jet with E-tube emulsification of the air.

The main air correction jet is not that powerful for top end adjustment of mixture, it is 1/3 as effective in adjusting mixture as a fuel jet is.

Porsche revised 911S jetting with the IDS carbs to actually use a smaller main fuel jet than the IDA (S) Webers (125 vs. 135) but this reduction was made in conjunction with an earlier acting E-tube (F3 vs. F26). Also, the F3s had fewer holes than the F26s which helped mid-range mixtures remain adequate for power requirements. The IDS Webers added the High Speed Enrichment Tubes as Fritz pointed out to assure a fat mixture at WOT and peak RPM.

If Porsche or Weber had an easy E-tube fix for the detonation issue I assume it would have been incorporated as opposed to revising the castings for the Webers. A simple E-tube change would have been far less expensive than revising the castings.

High Speed Enrichment is not unique to the Weber IDS carbs. Solex carbs (for 356 and early 911 engines) and the Weber DCO side-draft carbs had this feature as well, the DCOs achieved this via fuel being drawn through the accelerator squirter nozzles at high engine RPM.