Too much carb? 5 mpg at cruising

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....

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

This is exactly right, my built by the factory 383, ran much harder all through the range with a 3310 versus the avs. Mileage was basically unchanged, if I kept my foot out of the power. Alas, guys that have not done such a back to back comparison continue to say it’s not possible. Why I don’t know.

The 750 eddy is the only carb I couldn’t tame. Total pos. A TQ is a great carb too. Love those.

It's a vacuum carb for gods sake. It's a 2 barrel most of the time.

Amen

You can argue that with Holley:

'Mechanical secondary carbs (double pumpers) are the better option for race cars, hot rods, and performance vehicles with manual transmission, automatic transmissions with looser torque converters, and lower (higher numerically) rear end gears. They are designed for engines that rev quickly, demand air flow more quickly, and are more focuses on quick acceleration.

Vacuum secondary carbs make excellent street carburetors. They open the secondaries progressively and are designed for stock-ish to mildly built engines that are heavy, have taller (lower numerically) rear end gears and automatic transmissions with tight converters. By keeping the secondaries closed until considerably higher load, these carbs also help to improve fuel economy over a similarly sized mechanical secondary carb. Holley has a plethora of vacuum secondary carbs to choose from in small and large CFM sizes.'

www.holley.com/blog/post/what_is_a_holley_double_pumper_carb_/

You might want to read his combo again

In my superduper MPG 318 I ran a 273 2bbl cam, he needs his carb and ignition tuned properly not replaced with a double pumper.

You mean this part?; "The engine is bone stock 383 2bbl engine".

If you don't believe Holley's general application guide, why don't you e-mail Holley with the OP's particular combo and preferences and ask them to recommend a carb?

Hint: It still won't be a track calibrated double pumper.

Let us know what they say.

I can make a 600 slushie on a 440. Did you tune the 750 any? Your (any) engine requires a certain amount of fuel to cruise (idle or WOT) at a certain RPM. If you have the jetting/bleed (Holley) combination right the engine doesn't know if the carb is 650 or 1000. I've never looked at a carb as being ready to run out of the box, sometimes they work great, other times it takes some work.

It always about the combination, and Your combination is terrible.
To much gear and sell that 3310 to a ricky racer. Put on a factory AVS.

What year 383 2 Bbl engine? The 68 -70 have no compression ratio and small cams.
The early 60’s are 10:1 and run way better.

I ran a box stock 3310 on a low compression, over-cammed 383 for years. It ran great, sounded great, didn't make a ton of power but got well over 5 mpg. There's something else going on with the OPs ride or his MPG calculations

all will howl at me, but for a good street carb, i just love the Thermo-Quads.
reasonable to good mpg for all engines [WAY better than 5mpg if you keep your foot out of it], big or small block, plus reasonable performance as well.
however, one must be able to tune these things with the proper metering rods and jets [which are somewhat difficult to get at times], as well as adjust the secondary air door properly.
proper vacuum advance and distributor weight [and spring] curve for your combination is also a must when using these carbs.
this is only my life experience and opinion. your MILEAGE will vary.