Good afternoon everyone
I have my 1970 Cuda, 383, 4 speed, 4:10 gear. Tires in the rear are 275/60/15.

The engine is bone stock 383 2bbl engine (now factory 4bbl intake, hp exhaust manifolds). I topped it off with a fresh rebuilt Holley 3310-1 780 cfm carb. The carb was tuned by my buddy who actually knows what to do.

The car start half a crank and usually runs good. Every once in awhile on take off the car starts bucking but clears out. The issue I'm having is sometimes it seems like it has bogs when the secondary open and horrible gas mileage. I recently took a 67 mile trip with a full tank and it's now at a quarter of a tank. I did the math and it's getting 5 mpg on side road drives just putting along.

I understand these cars are not built for gas mileage but dang.

Is the carb too much for this engine? Timing is 12 degrees advanced and spot on.

tune up is miles off, ignition system is not working correctly, too much fuel pressure, power valve issue, the list is long as to what it may be.

A stock 383 with that set up should be getting better than double that mileage in mixed driving. 3310's are usually decent carbs for all around performance, even out of the box.

I'd start with pulling a plug or two and seeing the color. What's the tailpipe look like ? Black? Gray?

Around town fuel mileage versus steady state cruising fuel mileage should be a lot different, 6 or 7 MPH around town, depending on the driver versus 14.0 to 15.5 MPH at 55 to 70 MPH fuel mileage while cruising with minimum stop and go driving
As already suggested pull the plugs and look at them, the proper heat range helps make them easier to read and the ignition timing all contribute to your MPG.
I've work on one 383 two barrel motor like you have (1965 to 1971) and it didn't have very much true compression ration, maybe between 7.5 to 8.5 to 1 compression ratio
My message is you need a hot plug and a little more total ignition timing to make it run a good as you can, I would shoot for 12 to 18 degrees before top dead center at idle rpm up to 2000 RPM and then have 34 to 38 degrees total timing with a hot plug in the motor, a # 4 or 5 NGK or a RJ 12Y or RJ14Y Champion, Old Auto lite # 65 or 75.
What jetting does it have now and what vacuum rating power valve does it have, EI 4.5 or 8.5? That number is stamped onto one side of the power valves. up:
Holley are very sensitive to how they are jetted and which power valve they have and if the power valve gasket is installed and seated properly . That carb came with an # 8.5 primary power valve originally, #72 primary jet, secondary power valve was 10.5 and the secondary jet was an # 76
On the stock jetting it was design to be ran at sea level so for every thousand feet you are above sea level you should reduce the jet size by one Holley jet number smaller
Did that carb run okay before the rebuild? If so check the gaskets and power valves as well as the jet #, if not running good before the rebuild you will need to work on that carb to find out why it is doing what it is doing to cause your issues If you don't work on carbs find someone good that does, you will be amazed and happy once it is dial in correctly
As far as getting the true gas mileage you need to know exactly how many gallons of fuel used for each trip, I fill my gas tank up to make sure their completely full so I can see the fuel level in the filler necks at the same spot each time

3310s are a generic carb. Every one I have ever worked on was pig rich on the main circuit. As bad as a double pumper. Usually took 6 steps or more to get them in the ball park for clean cruising. Then the power valve needs to open sooner and the PVCRs need opened up to bring the wide open throttle ratio back to where it needs to be.

You need an O2 sensor if you are going to start tuning the carb.

This will probably cause an argument, but a cheap (even a used one) O2 will get you close enough on the cruise circuit. Tune the gauge into the green and you will be good as long as you richen up the power circuit. Done it many times. Most guys wouldn't be able to get it any closer with a wide band if they aren't an experienced tuner already.

Then there's the timing ....................

With a 4:10 gear on the highway, it might be nice to know how fast, or what rpm the motor was running on that highway. Then are you running a steady speed, or do you keep hitting the throttle to hear the 4bbl sing? Next, determining gas mileage based on what the gas gauge reads really means nothing. We don't know how full it really was to start with, or how level the car is while you are looking at the gas gauge trying to guess how much gas it actually used.

Spark plug inspection would be on my to-do list if you suspect it's way rich or flooding out occasionally. Have access to another confirmed good working carburetor to try out?

4:10 gears are up there, but you should still be getting way more than 5mpg. If your not racing it maybe 3:55's would be a better choice for just driving?

OK so I am pulling the plugs tomorrow to give them a read.

So the car has not gone on the freeway. I am driving it on county roads and fastest speed limit is 60 mph. I am still shaking the car down and not getting on it too much. Just driving it like a normal car.

The carb was one I had on the shelf for years and it was given a full rebuild. I even had a friend at the carb shop look it over and it was set up as out of the box. I need to figure out these carbs as if I want to keep it, I have to learn how to tune it properly



All this advise is very very helpful and I appreciate it big time.

Hi TX , Can you do Us a Running Test , and check Front and Rear WET Fuel Bowl , gas level settings too ? Please have a rag under the bowl screws , as you check Fuel Carb WET Level , Idling and running on the Vehicle . ALWAYS Different than DRY Setting from a Carb Rebuild Specs . post back , Ronnie

Go to the last 10 minutes of this video, (or watch the entire video if you have an hour or so) funny in a way but not when you are looking for MPG and have a question about carburetor size for your engine or driving use. I thought it was going to be a joke but turned out to be pretty interesting. "Too much carb? 5 mpg at cruising" could be

you probably have something out of whack on the tune-up or there could be multiple issues. i'm not a holley hater and a 3310 can be made reliable and reasonably economical, but if it was mine and efficient driving is critical i'd seriously look at an edelbrock 650avs. the 650avs is nearly identical in size to an original factory 383 avs which means it will have smaller primaries to drive on which will be more efficient. holley's in general aren't the best choice for good mileage but can be made to work. make sure your vacuum advance is working too. a working vacuum advance will aid in good driving and fuel mileage.

A taller tire would reduce the 410 affect (rpm on highway) Faster than a gear swap. At idle I would also spray the manifold and carb base looking for leaks. Only takes a few minutes and takes one thing off the checklist. Vacuum leaks can cause problems.

Yeah, a 780 is too much carb here.

That's the problem with taking advice from racers, they have no concept of how that will work effectively on the street, poorly is usually the answer.

Can that 780 be made to work, well it's "working" right now.

But can it be made to work better? Yes and I suspect your friend tuned it for WOT and nothing else because it sure isn't acting like it is tuned for anything else.

It is the best choice for what you are doing? Nope. A 600 or 650 would have been a better choice, tuning it for all modes of operation would be a better choice as well. Not quick or simple though,

Someone suggested a Edelbrock AS of similar size to the stock AVS, thing about that choice is you could start with the same jetting, rods, etc as the stock carb and have been way closer to the proper tune from the start,

The activity and weight of your right foot plays into MPG too.

Do you have Vacuum advance? That will help. I would hook up a Vac gauge and get an idea of what is going on. That will tell you a lot. Pull the bowls and block and see what jets and PV you have.

Before you run it any further I would also pull the dipstick and make sure your oil isn't diluted with gasoline.

Redo the tune. A 3310 on a 383 is the correct carb. Don’t believe me and want to talk charts and theories, go run one back to back with the stock carb. You won’t want anything to do with something that small after you do. Easy thing to do back in the day was to put an avs from a 440 on your 383. Mine got good mpgs with a 3310.

A 780 on a built 383 that is racing may be the correct carb, but this isn't a built 383, and he isn't racing. Is the 780 a double accelerator pump 780? A 780 is probably too big. Mopar only delivered a 383 with a 600 carb, and that was on a Magnum, this is a 2bbl motor.

Next, my 318 truck gets 13 mpg with 3:55 gears and 235 75 15 tires on an EFI motor being monitored by o2 sensors @ a very steady and smooth 70mph without OD. That mileage drops quickly when the speed exceeds 70. He has more cubic inches, more gear, shorter tires, and a big carb.

We have no idea how fast he was driving, or how he was driving, and we really don't know how much fuel he actually used. The whole deal could be based on incorrect info. A one time 67 mile test mpg based on guesstimated fuel consumption is pretty iffy.

At any rate, 5 mph with a 383 would suck unless it was a tire ripping monster. The tune is no where near where it should be with a carb that is plenty big for the motor. Checking to see how much fuel has been mixed with the oil is a good thought on anything that is getting poor mpg.

OK as I updated before, I am driving normal and am not really getting on the car at all. I am still cautious driving it as I am still getting a feel for my newly rebuilt car. Top speed is 60 mph and I am driving it like a normal person. I have barely got into the secondarys as I am still feeling the car out.

I pulled the plug and they are black and have gas on them. I have a 600 edelbrock on the bench I know it works. I think I will box the holley up and save it for when I build another engine (rejet the carb and do a few other things mentioned

This advise has been helpful and I have screen shotted everything to help in the future.

Thank you everyone for your help, it's greatly appreciated

Don't be afraid to bump up the idle timing 4 or 5 degrees more. That low compression engine with a very bad combustion chamber (open chamber with the spark plug far off on one edge of it) will be much happier with 18 or even 20 degrees idle advance. Your total initial plus mechanical advance should be around 40 degrees and another 10 degrees or so ported vacuum advance. Sounds like your advance is too retarded and you should fine tune it before swapping carbs because whatever carb you use is going to need more advanced timing....