Good thing you made a point of the plenum ports assisting velocity/decreasing turbulence noahwins, as it does help. I've performed a fair bit of work on this inlet manifold partly due to my obsessive hatred of the EGR garbage - the rest is as a result of the generally poor quality of the casting, a corroded out base plate in the coolant chamber (), a blocked coolant gallery (if you've wondered where that coolant gallery that ramps down over #2 inlet runner goes, it feeds into the coolant basin under the plenum chamber) and my observation of how restricted that gallery actually is (once I'd chipped out the debris with a needle file the passage is barely big enough to poke a colour pencil through - I took to it with a hefty HSS drill and opened her right up ) I'm not 100% sure of the side effects of completely killing off the coolant basin would have but it might cause issues with coolant passing from the rear of the head to the front - plus if I do go down the Weber adapter path, part of the coolant flow that would normally enter the base of the factory carby will be blocked off so I figured I wasn't going to lose anything by improving coolant flow back into the thermostat housing. I've done my best to remove deflections inside the inlet runners but due to the castings' porousity it'll be impossible to get them looking flawless (plus the seams of the casting plugs leave a pretty ragged line that was going to require the removal of more metal than I was prepared to take out...) . This is what it ended up like after cleaning up the walls of the runners -