Install a vacuum gage then drive it around, very simple tool to check out an engine.

I threw my kid a box of holley stuff to put a carb together for a mild 408 small block. He came up with a 3310-4, a secondary metering block and I put .070 idle bleeds in the primary. Works perfect. I think you just need to tune it.

Sorry, can't help the OP with the tune issue, but I thought I'd toss in my mileage tracking since I regularly log this stuff and have done a good bit of tuning on my Carter TQ. This was a built 360 motor, 4K stall 727, 4.10 gears, 275/60-R15 tires.

Sooo...avg is 11.4 MPG (over 6 yrs and 6K miles), when I'm heavy on the pedal that's normally around high 9s and low 10s, but when cruising and covering a good distance I get as high as mid-13s.

Overall I'm happy with this, but I've done the tuning with O2 logging, good amount of TQ air bleed adjusting, pretty quick mech advance and absolutely running a vac can on the distributor.

YMMV

Search for a friend who could lend an O2 meter.

An emissions testing garage that still has OBD-I equipment could read the carbon monoxide level in the exhaust and use that to estimate the air to fuel ratio.

Search for a friend who could lend a “Colortune Sparkplug”.

Much worse technique would be to “on purpose” create an air leak below the carb and see if the MPG improves on a test run.

Here's the entire kit for $39.95 on Amazon: www.amazon.com/GoolRC-Digital-Narrowband-Volmeter-Modification/dp/B09ZYJLK6N

Or you can pull a used O2 from a pick a part yard and there are lots of choices of guages for around $20: www.amazon.com/ESUPPORT-52mm-Fuel-Ratio-Gauge/dp/B00SMJ0BTQ

You would need to test the used O2 before installing it which is simple. YouTube has plenty of videos for that.

My opinion is that while your carb may not be the optimum size, it is the tune in the carb (or maybe other issues), not the size of the carb that is your mileage problem. The carb being a bit oversized could contribute to the bog when the secondaries come in, but that could be eliminated with a proper tune as well.

With all the problems that you are experiencing, I am not convinced that your buddy does know what he is doing when it comes to carbs.

It's all in the carb tune, assuming the engine is tuned correctly. You could put a 1000 cfm carb with the correct tune on that engine and it would run fine. The OP just needs to learn how to tune it.

"A/F RATION". Uh-huh.

I seen that, too.

there really isn't that many good street "tuners" out there. in my area there isn't anybody i'd even let tune a lawnmower. a total street tune has to have a dialed in ignition curve along with a proper calibrated carb no matter make or manufacturer. majority of people can't put these two together. and, the last thing anybody who wants a good street tune should do is let a bracket racer work on it.

Google "tuning carburetor with wideband"

Then read everything you can find.

The circuits that are not adjustable can be made almost infinitely adjustable with a metering block
change and/or a little drill and tap.

It's a rabbit hole

Yes, it is, but it's worth the trouble to have a clean idle and cruise. I changed the plugs in my 434 two years ago right before the Moparty at Bowling Green, the plugs in it had been there for three years and looked good. I have them in a bag for back up.

If a guy wants to got to the effort, tuning the cruise is probably the easiest of all the circuits to tune. You can actually use a measuring device (O2 sensor and guage) to get it pretty close. Try that with the idle and transition circuits. Yes, you still have to make a couple other adjustments too, but the primary main jets, their PVCRs and the power valve are usually all that needs attention if the cruise is all that is being addressed.

There are much, much better street carburetors than the antiquated 3310. But in most street applications, leaning down the cruise and making the other 2 collateral adjustments will result in a much more street friendly carb. The other easy mod that everyone with a 3310 should consider is to change the secondary spring to the combo's preference. That is a very likely source of the OP's 'bog'.

Just my opinion based on my experience.

A Holley 3310-1 is approximately 50 years old. There is no way to know how many "experts" have had their fingers and drill bits inside of it. If anything has been drilled out then you are lost and need to get back to out of the box condition. Idle feed restrictions, power valve channel restrictions, Idle air bleeds, main air bleeds, emulsion channel bleeds, and kill bleed are all targets of the "expert" Mad Driller. Some times they know what they are doing and sometimes not so much. For sure they were not thinking 383 2 barrel when they got out their box of tricks. If in fact everything is untouched (unlikely) then put the original size jets, power valves, and secondary spring in it and it will be close for most engines. Fine tune from there. GM used versions of this carb from 302ci 290hp Z28s to aluminum head 396ci 375hp. Will it be optimal for a 2 barrel 383? Probably not. Can it be close enough to be drivable and not eye watering rich? Yes it can.

That said, The Ignition timing curve has to be appropriate before you worry about the carb. The factory timing curve is a horrible compromise dictated by early attempts at reducing emissions.

Run fine? I guess if you have a very loose definition of fine.

I ran an Edelbrock Perfomer 600 and 750 on the 360 in my Diplomat as a test. That was my daily driver, go to work (75 miles each way) car. With the smaller carb I had about 5 mpg more economy (mid 20's) and better throttle response.

With the 750 I had higher top speed, I could exceed the certified 125mph speedo's ability to measure whereas the 600 wouldn't go much past 110.

The more you exceed the engine's requirements the slushier the carb becomes. I suspect if I tried to run a 1000cfm carb no tuner in the world, including you, would have made it run fine and the only way it could be made to even get close to the smaller carb's effectiveness would beif it was a spreadbore with the secondaries so restrained as to essentially limit it's total airflow to something similar to that smaller carb's flow. IOW, it would never flow 1000cfm in use.

Did the 750 work? Yep, did it work fine? Nope. At any street speed the smaller carb was better in all respects.

The 750 edelbrock is a tuning nightmare, lots of "fixes" out there but nobody can get them to work properly, they don't work great on anything, at best they just get by. The 3310 holley on the other hand can work great on that 383 engine, I barely lost anything with one on the superduper MPG 318 engine VS the highly tuned thermoquad.

The cam on a 2 barrel is different from a 4 I would run a 650 double pumper .

From the frying pan to the fire with that suggestion.

A double pumper is a race calibrated carb. It is very rich throughout the range. The OP already has that situation and wants to get away from it. Double pumpers are a poor choice for most street applications.

Perfect carb for his combo