[torpedo]

Hi B&S, Kurt and Pete. Thanks for providing opinion and tips.
Carb visual test that you suggested was done yesterday with engine running at diff RPM. Plenty of gas flowing, squirters operating when full throttle.

I'm being helped by a pro mechanic (his specialty are flat4/6 VW/Porsche but also services and repairs other engine types with carburetors and mechanical distributors in 6V such as Porsche 356).

We can summarise the problem as follows:
Engine starts and runs smoothly at idle on 8 cylinders. Did the ignition wire test off the plug tip on all cylinders and have a clear blue plain spark.
A smooth and gradual push on throttle will increase RPM almost to the max (we measured 3400 rpm).

The problem arises when throttle is open sudden to the max. At approx 75% of rpm range the engine does not rev up. On the road that manifests by a poor response when full load is required (hill) car not going more than 45 mph.
So far we tested:
fuel filter > new
fuel pump > delivers volume at correct pressure. restored in 2021
carburetor > professionally checked. Accelerator pump OK
Distributor > installed a new unit (supplied by 3rd Gen)
Coil > new Pertronix
Condenser > New (supplied by 3rd Gen)
Ignition wires > New (copper wire)


We are guessing we may have a leak at the intake manifold gasket (I do not know when it was last installed but probably 30+ years).
We plan to work on this next week (I have a new gasket): any tips or recommendations ?
Additionnally and since we have access to the valley, are there any sanity checks related to valve train we can do such as clearance between valve stem and lifter ? Please provide as much detail as you can because this is new territory for us. Also please indicate if any special tooling will be required.
Sorry for the long message and chunky images. All the best