As noted, I think the changes to the D jet controllers were related to the change in the cold start system. There are different throttle switches for the D jet system and I don't know whether they were product specific (VW versus Volvo versus ..) or whether they changed through the production run. I know some had over run shut off and some didn't. There are also differences between the MAP sensors; but, I don't know how that altered the fuel delivery.

Your approach to adjusting fuel pressure to adjust the fuel mixture won't work very well with the Bosch fixed fuel pressure regulator. The original base pressure on the B20E was 28 psi. At wide open throttle the manifold pressure will be about 14 psi absolute which results in an injector pressure differential of 14 psi. If you crank the base fuel pressure up to 35 psi you will have a pressure differential of 21 psi. You will have increased the fuel delivery in each pulse by (21/14)**0.5 = 22%. I would expect that you should be running rich based upon your modest 5% capacity increase. At idle, a B20E with a D cam runs with a MAP pressure of about 7 psi absolute. The injector pressure differential with 28 psi fuel pressure would be about 21 psi; but, would be 28 psi with your 35 psi fuel pressure. Fuel delivery at idle will increase about 15% so you still should be running slightly rich.

The engine coolant temperature has minimal effect on fuel mixture at operating temperature. It provides enrichment when cold and some people have added constant enrichment by adding a resistance in series with the coolant sensor to cause it to operate in the cold enrichment zone all the time. The primary temperature based fuel compensation correction occurs through the intake air temperature sensor.

On a 1971 B20E engine running all the correctly matched components for a 1971 D jet system, you would typically be running with an AFR somewhere in the 13s at idle. Your modest 5% capacity increase should not cause you to go really lean, particularly since you have cranked up the fuel pressure. I would check to make sure that you don't have mismatched components or that you have other fuel delivery problems. Be aware that on the 1971 and up D jets, there will be a adjustment potentiometer on the back of the D jet controller that is specifically for adjusting mixture at idle. If you are running the batch fire D jet system, your AFR needs to be in the low 14s to 13s to get a reasonable idle.

If you want to run the D jet system, I would not mix an match components. I would stick with all the matched parts for a 1971 D jet system as used on the B20E. Cranking up fuel pressure will increase fuel delivery. You could use it to make small tweaks; but, if you need big corrections then you are going to skew fuel delivery at idle or at WOT depending on which way the tweaks go.

It is unlikely that you will get fuel delivery exactly right using the D jet system. If you want to get fuel delivery closer to ideal I would switch to an aftermarket fuel injection system while retaining the D jet manifold and fuel delivery components.

How many of these parts do you have? What drives the car around at the moment?

I would NOT buy the first version of a 123, it is relatively useless.

Although D-Jet produces more power & economy compared to most carburettors systems seen on various cars, it's actual intent was for meeting USA emissions standards, specifically on a Type 3 VW.

It's also important to note that the B20E/F intake manifold was designed to fit under the bonnet & look good in a P1800E engine bay. It is actually too short & too large in diameter & is a long way short of what a pair of 45DCOE or an ITB system can deliver. (O2 meters give you a very good answer for AFR W/W but not V/V)

What is a 'Flowed head'?

What is a 'Street performance cam'?

If you have the $$$$$ for 'NOS Bosch fuel injectors', then you would buy modern ones of the correct size, ITBs, the good Twin DCOE manifold & your preffered ECU, etc.

Upping the fuel pressure on D-Jet doesn't work like you think either because it sprays directly on the backs of the intake valves. The fuel vaporizes which greatly increases the area it takes up, but that leaves less space for air. (Did I mention it was built for Emissions?) That's why the injector is moved back on a performance injection system.

B20F(xx034) vs B20E(xx017) ECU's: There is no B20F specific ECU.

0 280 000 009 is for 1970 models, 017 is is for 1971, 034 is for 1972 & 1973, and for general interest 0 280 001 009 is for 1971 to 1975 B30E/F powered cars.

I wouldn't buy a K&N air filter for it either because the original filter is HUGE. I had an early model 140 running K-Jet for more than a decade & if the rodents have not eaten it, I would still use the original aircleaner.

If I had a 2nd K-Jet system for spares, I would build a decent intake manifold to go with it. I have several K-Jet systems from B21E's, but the injectors & metering head are not interchangeable.

To sum up, if you only want B20E power, then twin SU's will get you there. I made a .0625" OS B20B with low compression slaughter a D-Jet B20E car, so even going to +.125" and the hassle of having to source bigbore headgaskets & the problem of no OS after +.125". I know some people go to 92.5mm BUT I wouldn't, atleast not in a conventional way. Not all blocks got to the bigbore size successfully.

If you want serious performance, the cylinder head is the heart of the engine. Finding someone to do it for you is hard. It sounds like you need someone to built it & tune it too.

Most people don't know that the stock B20A/B head is a better choice with cams of less than .25D lift such as all the stock ones & VV-71 Isky that ipd also sells. The inlet ports the B30A/E/F has are 1st choice for a daily driver with a cam that has .280" lobe lift, which is around .400" valve lift (less than .25D)

Spec fuel pressure is 30 Psi, not 35. B20E has higher HP spec due to higher compression than B20F. How did you determine 9.9 CR? One difference is the 72 has a thermo time sensor for the cold start valve and the 71 doesn’t. What do you mean about water temp in steady state?