I have a 350 with the following setup in a 69 Camaro:

76' Caddy 500 Q-jet
Performer intake
roller rockers (0 lash)
1 5/8" headers
186 Double humps
Lunati Voodoo cam 268/276 228/232@.050 489"/.504"
Accel billet distributor
3.36 rear gears
M22 trans

I have only been getting 8mpg around town with this setup and a very light foot. I had a eddy 650avs that got 7 mpg and decided to upgrade to a Q-jet for a big change that only resulted in a disappointing 1mpg increase. I have had the Q-jet on a 68 that had a bone stock 350 and got about 13 around town and being from a 500 caddy I know it could do better (it's a little rich)

I was expecting around 12-14 around town with my setup. I have a very light foot and the whole time I have been testing the mileage I never got above 55. This is a daily driver year round so mileage is a somewhat a concern especially when it's as bad as it is.

Thanks for any help.

What is your timing curve? Vacuum advance on full time port? How do the plugs look after a part throttle cruise? Choke is open?

16 degrees initial w/36 total by 2800rpm. I've had the vacuum advance on full manifold with the Q-jet and both timed and full with the Eddy. Choke is wide open. I haven't checked the plugs the past week but they've always been clean with the Eddy besides the oil they encountered breaking in the moly rings the past 2000 miles.

Thats a pretty decent cam when your hopes are fuel mileage. ditch the cam for something much milder and it will improve.

What are you getting on the highway? I would guess 12-14? If so that's not too bad.

I find that strange since I get 7-8 mpg in my 489 BBC w a Holley 850. Could a camshaft really do that?

If you want economy around town and open road cruise you need an engine with good torque down low....basically a 'stump puller' and all the go bits match that rev range
A cam that works between 1200 and 4500 to 5000 rpms
lower duration and over lap with less lift than you have...then even advance the cam another 2 or 3 degs
You need a small carb...to match the cam 450 to 600 cfm
Dump the headers, and go rams horns the old truck ones
And a dizzy curve that is not so fast or advanced...intial around the 8 to 12 gegs all in around the 34 to 36 degs around 3200 rpms with another 6 to 8 degres in the VA working between 12 and 14"

This will give u something that is real quick between interesctions, loose in the rear even on dry roads

Then simply swap the diff ratio from 336 to 255 with give you another 15 to 20% economy....i simple terms less cubic inches per mile.
I have just gone fro a 30 to a 255 couple weeks ago....60mph from 2600 rpms to 2000 rpms...and far more drivable, still loose in the back if need but not accidentaly all the time.

OK now what to do with the current setup
1/ at around town rpms you curve is way to advanced slow it down to the ball park above
2/ get a smaller cfm carb for better bottom end efficiecy and response
3/advance the cam 2 degs
4/ get rid of the headers.

You problem is you have a mix of go components that are out of balance...fighting against each other for want of a better description.

I dont post this as therory, I have spent 20yrs playing around and actually run such a setup.. thu on 105 octane LPG..principles are the same curve/compression different.
Im pulling currently around 12 mpg (LPG) on short trips 2 to 5 miles in heavy traffic and lots of interesctions Since LPG has far less btu /volume than petrol the equivlient is about another 28 to 32% ie petrol 16 mpg in heavy traffic.
Im curently experimenting with diff spring in the vapouisr with realistivc target of 15 mpg lpg....20 mpg petrol.
$/mile Im aready well over the 23mpg equivient.

the easiest way to get better mileage numbers is to make sure that the speedometer is actually correct- you you have a drive/driven gear setup for something like 2.73 gears instead of your 3.36 gears, then it will read more miles traveled than you actually travel, which will mean a lower mpg number.
also, are you actually topping off the tank every time, or throwing $20 in when it gets low? i know people that do this and claim to know exactly what kind of gas mileage they are getting.

4/ get rid of the headers. What is the reasoning behind losing the headers? I've always thought of headers and a low restriction exhaust as a mileage booster. A Q-jet is probably the best mileage carb there is if it's tuned right. How does one calibrated for a Caddy 500 differ from one for a Chevy 350?

Some q jets are real pigs.It could just be that they were set up wrong,but the most extreme example I heard of was a friend of mine who was getting about 7 mpg with his 454 dually full size truck.He yanked the Q jet and put a vacum secondary 600 holley on it and went up over 10 mpg.Headers could cause a car to use more fuel when used with a cam with incresed overlap since it causes more of the intake charge to blow out the exhuast at low engine speeds,but this is usually only the case when you recalibrate the carb for optimum performance after adding the headers.If you dont recalibrate the carb the engine will loose a small amount of mileage but will mostly run lean.
City MPG of course depends on the driving condictions.New York City MPG would be about 1 mpg for a Prius if you spend 3 hours in stop and go traffic to go 1 mile.

headers are truned for different rpm ranges...and off the shelf more often than not full on open throttle hi rpms.
And as mentioned above scanenging ...this is where the little pool of concentated fuel sitting on top of the valve, when the valve opens this is suched straight across and out the exhaust before the exhaust closes...not just caused by vlave overlap/timing , but the efficiency of the headers.

A note here...there have been many a carb way over jetted, becuase the plugs show lean.

Im not a fan of Qjet...and that depends very much on the person who recons and tunes them in...they either go real good or crap....I can dial a holley but not a Qjet as well...so im not fan.

We had a fleet of 70's and 80's chevy pickups. We had to smog them every other year, often the float would get saturated with fuel, which causes the float level to rise. This causes the truck to fail the smog test due to running rich. It is the result of changes in the fuel that attack the nytrofil floats.

I'd try a new float in the carb, they are cheap and widely available, I got pretty good at changing them. Or get the car smog tested and see if it's running rich. High HC means too much fuel/fouled plugs, high CO means too much fuel, but it's being burned by the engine.

David

Thats interesting....we had guy in the hot rod club who had run Qjets swore by them..then in the late 80s/90s things started to go wrong...
we went to unleaded in late 80s.
There where a lot of assumptions, incliding the fuel but nothing one could piont a finger at.
That would explain a lot.
Most of the NZ fleet was english or jappa so it was not a 'main stream; issue.
The big issue in the 'mainstream' was older cars and rubber fuel lines and fuel pump diaphams.

Thanks for all of the responses. Novaderrik hit it right on with the wrong speedo gear. I felt it read a little slow but didn't expect it to be 7mph off. I tested it against another car this past Sunday which I know reads correctly and I was reading 7 slow at each 10 mph increment. So that is where my bad mileage calculation comes from.

By the way, I couldn't think of going with a tall 2.55 gear combined with the 2.20 first gear of an M22. I do much more city driving than highway so I believe a taller gear would only hert me. I don't believe my cam is near big enough to lose mpg with headers. If you want to feel a stump puller the 268 Voodoo has more than enough brute low end torque than you can ask for and perfect all around power.

Thanks for all the help.

With the wrong gear you will be a % off not constant
so 30 mph showing +7 mph at 60 mph will be +14 mph

To work out how many teeth you need you know the % above
count the teeth you have and go up or down to the nearest tooth...keep in mind how worn the rear tyres are

Going from a 308 diff (spot on according to the local police officer radar) with 22 teeth
To a 255 diff meant a 19 tooth gear. and checked with the local constibles radar.

These are different diameter gears...
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