Vacuum measurement at SU carb

As Steve said, do not modify the hole

You’re not thinking straight… the purpose is not to “consume” or use up vacuum, but only to find a place to tap into so it can be measured by a sensor. It’s the number you enter into the software for the amount of advance you want to add on at low engine load which matters most. If you don’t add any ignition advance, then nothing comes of having “too much vacuum”.

Your problem appears to be that you haven’t found a reliable place to tap into to get a signal to measure how loaded up the engine is. Deciding how much ignition advance to dial in once you have that is secondary to that. “Vacuum” everywhere isn’t equal. How much vacuum you are measuring depends on where you measure it, as putting the tap close to the throttle plate exposes you to the air pressure in the airbox when the throttle position makes that accessible; putting it in a throttle body itself means it only sees one carburettor’s air pulsing through; putting it on the main manifold branch gives an averaged out steady signal; putting it on the brake vacuum tank (DON’T!) gives a “peak” signal only (as it it’d be behind a one-way valve).

A vacuum retard unit will have taken a signal from somewhere designed to give a strong (almost digital) differentiation in signal when the throttle was shut or open; a vacuum advance capsule will have taken its source from somewhere which gives a strong reliable signal when the total air usage can vary over a wide range of values.

One simple question:- did your distributor previously have a vacuum retard capsule fitted and use a carburettor take-off port for that? That’s not a suitable take-off port for measuring for advance.

kind regards
Marek

P.S. Philip Lochner (who posts here occasionally) led a discussion about ported versus manifold vacuum on the UK forum a while ago. If interested, see Ported vacuum advance vs manifold vacuum advance - The 'E' Type Forum

Marek. The previous distributor did not have a vacuum advance on it. Hopefully I’ll be able to get the Jag out of the garage and on the street this weekend to see how it runs. I’ll use the carb port and the settings you provided for the MAP curve. I’ve had a lot to do to get to this point. Replace fuel tank, replace fuel lines, replace fuel pump, rebuild carbs, and now the new 123 distributor. This work has been done over several months. The car was not running well two years ago so it sat in my airplane hangar until I could start addressing the problems. After doing all the fuel system, I found the distributor was corroded and not turning freely. Hence the new 123. Your help on the distributor issue is appreciated. Don