Lots of good advice here, I will have to do some playing about and see what happens.
With your lightweight 'T' - timing, carb size, single plane inlet are all OK imho - you could have even used a double pumper with that combo - you really could do with finding-out the cam spec' - do you know anyone with a similar sized engine/carb combo? - see if you can borrow their carb - see how it runs on your car?
I've just been re-reading this article
Carburetor Tuning the Scientific Way
Is it worth fitting a couple of Lamda sensors as he describes? Could it give me a better idea of what the problem is?
The Cheap Way
Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.
Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose. All that will happen is that your reading may go away during periods of idling. On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning. Remember, your eyes are using this data, and if it stops for a while, no harm is done!
Sensor installed in Pinto exhaust.
O2 sensor in Pinto exhaust
Sensor and welded bung:
Outside view of sensor installation
Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.
Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.
O2 Sensor output
Good idea - however if you have a 'radical' cam with loads of overlap and the consequentially required rich idle mixture to get it to idle the Lamda sensor may not tell you too much at lower rpm and idle - it should give you plenty of help at cruise etc though. 'Old school' way to check mixture is to find a bit of quiet 'A' road, cruise steadily at say 50 mph - knock the transmission in to neutral and cut the engine at the same time - pull over to one side - pull ALL the plugs and note the colour - do exactly the same at wide open throttle (a 'kin good trash, say in intermediate up to high revs for 15, 20 seconds) and note the result. If the colour of the plugs is the same all through, fine - if it differs you'll know if there is a carb imbalance - (although a single plane inlet may fudge the results at lower rpm) - you should be looking for 'coffee' colour plug tips with perhaps darker brown further away from the tip. Repeating the same thing for WOT will help give you an idea whether the secondaries are lean or rich. Its unlikely with a modern inlet manifold that you will need staggered jetting. If you have a 'cam' the plug colour at idle will likely be black and sooty - it should not be like that at any other speed. Changing jets on Holleys is simple but messy - the steps in jet sizes are quite small so if changing go-up 3 or 4 sizes rather than on or two. Let us know the can spec - it will help determine things.
it would be good to know the cam spec and type, also the compression ratio..sounds like a fairly large cam in there judging by the low idle vac, but a cam large enough to have fairly low idle vac would normally go way further than 6000 rpm,, i have a large roller in my sbc and it idles at 9 inch vac at 1000 rpm, but it makes power round past 7000 rpm... what is the total timing on this motor..i would always use a double pumper on a fairly healthy motor, but one with as much adjustments as possible,, does your carb have adjustable airbleeds etc
It hasn't got adjustable air bleeds which means I'll have to order up and parts to try things out which why I thought the lamda sensor might be a wise move to get a bit more info.
so the cam is hydraulic flat tappet,, do you know the total timing on the motor.. incorrect timing does cause very low vac,, in my experience 38 degrees is a fairly safe ballpark for total timing and tune from there