If I had 1 pair running lean I'd look at the carbs, but both failing with similar faults at the same time. I fully rebuilt the carbs just before the engine build, bodies faced and bushed for new spindles. Jets converted to old style (no wax stats)  I'll check them over before I spend a fortune of an electric fuel pump setup, but I just don't see it being the likely cause.

Have some wild speculation, based on my personal carburettor woes.  Information very much worth what you paid for it  Could it be the plastic hoses to the jets?  My old ones went brittle and kinked causing fuel starvation.  Difficult see under the sleeves, but obvious once you move them out of the way. One other option is something wearing and causing a vacuum leak.  Throttle spindle being a likely culprit.  I understand o rings and/or vaseline can temporarily improve these.

Excellent photo, thank you.  Does that mean you don't have a coolant bypass unless the heater is on?

https://www.amazon.de/Tyenaza-Bistabiler-Selbstverriegelungsschalter-Niedriger-Boarding/dp/B0983MCWWK  

From what you're saying - and isn't hindsight a marvelous thing - I'm with you on the pump as a prime suspect here.

28%

Basically yes. This is the arrangement on my engine. Hose (not fitted her) to the heater from the connection at the back of the head then hose from the heater to the return pipe - behind the manifold - to the water pump housing. Makes it all fairly tidy.

Banged it into AI, flip flop out put from a momentary switch.  ## Flip-Flop Circuit for Momentary Switch A **flip-flop circuit** that changes state with a momentary switch is often designed using an SR (Set-Reset) flip-flop or a JK flip-flop. Below is a description of how to create this circuit. ### Components Needed: - **SR Flip-Flop or JK Flip-Flop Integrated Circuit** (e.g., 74HC74 for SR or 74HC109 for JK) - **Momentary Push Button Switch** - **Resistors** (e.g., 10 kΩ) - **Capacitor** (optional, for debouncing) - **Power Supply** (typically +5V) ### Basic Diagram Explanation: 1. **Power and Ground Connections**: Connect the VCC pin to +5V and the GND pin to the ground. 2. **Switch Connection**: Connect one terminal of the momentary switch to the **Set (S)** input of the flip-flop. Connect the other terminal to ground. 3. **Reset Connection (Optional)**: - If using an SR flip-flop, connect a resistor to the **Reset (R)** input to pull it high when the switch is not pressed. - If using a JK flip-flop, connect the **J** and **K** inputs appropriately. 4. **Debouncing (Optional)**: To prevent multiple toggles from a single press, add a small capacitor in parallel with the switch. 5. **Output Connection**: Connect an LED or other output device to the output pin (Q) to visually see the state change. ### Example Connection for SR Flip-Flop: 1. **VCC**: Connect to +5V. 2. **GND**: Connect to ground. 3. **S (Set)**: Connect to one terminal of the push button; the other goes to ground. 4. **R (Reset)**: Connect a pull-up resistor (10 kΩ) to +5V. 5. **Q**: Connect to an LED with a current-limiting resistor. ### Operation: - Pressing the momentary switch sets the flip-flop, changing the output state from low to high. - Releasing the switch retains the state until the switch is pressed again (if designed for toggling). --- This setup allows the flip-flop circuit to toggle states with each press of the momentary switch. If you need further elaborations or specific circuitry for JK flip-flops, let me know!

So, with the distance of hind-sight I'm starting to think that the car/engine was running progressively leaner over the last 2 years.  Before the 2023 RBRR it had a rolling road session, and at top RPM it started to lean off. I changed the fuel filter for a free-er flowing one and that seemed to fix the issue.  Since then the prevalence of what sounded like "pinking" under heavy load/throttle has got progressively worse (and MPG has gone up on long runs) - I should have stopped and thought about it and done some investigation before the RBRR this year.  The thing that drives me towards the fuel pump is - what could be a problem that's going to affect both carbs equally. It's not impossible that both carbs have developed the same fault - but I doubt it. So although expensive I'm starting with the most probably cause (IMHO) - the fuel pump.   

I had a look if the splines was the same between the 25 and 28% unit.. And... just the same !   Now im stuck.. build it up with 25% or the 28% 🫣😅