Technical Improving Fuel Mileage

I recently bought a car with a 305 Chevy equipped with a Edelbrock High-Rise Air-Gap intake manifold and a 600 Holley with vacuum secondaries. I haven't played with jet sizes yet but at idle it seems really rich. I'm wondering if the open plenum intake with no heat crossover and the vacuum secondary Holley might just be a recipe for poor gas mileage for a regular driver? Coming off-idle is pretty disappointing too.

I've considered various other intake/carb combinations and think that going back to a stock 305 quadrajet intake might be a good start. I like that the Holley has an electric choke so possibly using an adapter to put it on the stock 305 Q-jet might be an option. If I found a Q-jet from a 305 with all of the linkage to hook up the automatic choke I would consider going that route as well.

I did some searching for answers whether a combo like I have now is a bad mix of intake & carb for a regular driver but none of the resources I've found really address my complaint regarding terrible fuel mileage..

Lynn

 

While replacing the carburetor is a good idea, you might also check the spark plugs to see if they’re fouled which can affect fuel efficiency. Also consider checking both the fuel pump and filters.

 

An interesting topic. But, not being a ****, I wonder why people prefer the electric choke. I know that's what they sell most of, but don't know why.

 

I like to pull a knob for my choke...no pun intended.

 

My wife might drive it some and even I don't want to add a cable or mess with a manual choke.

Lynn

 

Good point Moriarity. I have a power valve left from when we eliminated the valve on my dragster engine's Holley. I don't know what vacuum rating it is but a good one of any rating would probably be better than a blown valve. Thanks.

Lynn

 

I'd recommend a quadrajet carb and intake. Here's an electric choke conversion for early carbs:
https://quadrajetpower.com/electric-choke-conversion-kit/

 

I agree a Qjet and a factory manifold or after market equivalent will get you good mileage and drivability. For a stock engine and even a mild performance engine the above combo will work well.
Dan

 

Lynn, I had the same intake with a 500 Edelbrock on my 31 truck. It got 24 mpg. Ran great.
Maybe the carb is too big or jetted wrong.
Mine was a 355 c.i. shouldn't be much difference.

 

What's the manifold vacuum read at idle at operating temperature, and when cruising at a steady 40 to 50 MPH? Automatic or manual trans. Is the choke opening fully after everything warms up a bit? Do you have a working vacuum advance on the distributor?

 

The following link is to a Summit Racing cfm calculator that will tell you what size carb to suit maximum rpm and cubic inches of displacement. Guess what it says......

https://www.summitracing.com/newsan...cfoUHqqMbP-W7y5CrYASW1DT4hgxVMiRCB7hYIlUXCnCc

Factory dual plane manifold and a good old Rochester Quadrajet will work very well. That's what I ran on my little 253cui V8 (Australian Holden engine). Had it jetted and tuned on the dyno and it brought it back to life from the horrible single plane intake and Holley 600 setup that was on it.
Way better economy.
Way more down low power.
Never ran out of legs at the top end.
Got rid of the big "Holley hiccup" when you hit the accelerator hard. It used to give a big cough, hesitate........ then go.

 

Buy an Air/Fuel ratio monitor, you won't regret it. It won't pay for itself quickly, but you can use it on all your old cars.

 

Second the Summit Racing cfm calculator. On that size motor your current setup is aimed at generating HP at higher rpm's, less economy, less offline performance. You don't mention any other engine mods so ***uming stock engine and maybe stock gears in the back too which also effects carb/intake selection. For a daily driver and to save $ I'd try to keep the intake and just drop the carb size to 450-500cfm with an electric choke. If you want more offline performance AND fuel economy a smaller carb with a low rise intake would probably get you there.

 

What no one has addressed yet: if it has a single plane hi-rise intake, does it also have a bigger cam in it? If so, that's a bigger issue, for mileage!

 

Thank you all for the input and suggestions. I have a lot of information gathering to do like checking vacuum advance function, vacuum at idle, choke operation and on and on. I might take some time but I'll try to report any changes and progress on my quest to lower my costs at the gas pump. Hopefully drivability will improve as well.

Lynn

 

If its rich fix it.
The open plenum on a stock 305 will not be great in the bottom end.
something like a Edelbrock Performer and that 600 Holley working right should run just fine .
Your not going to get some drastic fuel mileage improvement going to a Rodchester .

Start with the basics on the carb. Float level, adle adjust if still rich jetting and check the power valve .
The 600 is about as basic as it gets .

 

Maybe because, when properly adjusted, they work great.

 

A lot of missing variables including what mpg are you getting that you’re considering it bad?

 

A properly tuned VS Holley 4V can deliver good to great gas mileage along with performance as long as timing is right and all ignition parts are in good working order.

I can’t emphasize this enough it’s not just the carburetor, every system needs to be tuned for optimal efficiency and performance anything less will cost the same.

 

And let me guess, you're also running an auto trans too.

That open-plenum intake is the main culprit as they do very little for low-speed performance. IMO the carb size is marginal and could be jetted to work, but a smaller carb in the 450-500 CFM range will give much better throttle response if better street driving manners is mostly what you're after.

I got a Edelbrock Torquer off a fellow workers kid who had installed it (along with a 600 Holley) on a stock 289/C4/2.80 gear combo. It had all the symptoms you're complaining about. The kid couldn't figure out why it was such a dog after the swap, his dad asked me to look at it. My cost for the Torquer was to furnish and install a stock 2V intake and carb back on it, it ran fine after that. I installed the intake (along with a 450 Holley, the 600 he had was junk) in my '64 Comet 308"/4-speed/3.25 gear and ran fine, but had little power down low. Wind 'er up though, it woke right up. Fuel economy was 'OK' but was sub-20 MPG. I also had a big cam and mildly ported heads, things that required a lot of airflow to work.

The total combination is everything. Vehicle weight, cubic inches, cam, intake/carb, rear gearing, auto vs manual trans, each one affects the other. Going back to a stock carb/intake will definitely help, or at least changing the manifold to something closer to OEM. Knowing more about what you have overall would help in any more precise recommendations we could make.

 

I thought the air gap was a dual plane intake?

 

He's not running a 1050, he's running a 600 Holley with Vacuum secondaries. As in they only kick in with vacuum due to load. Spending a bunch of money on a new carb is very unlikely the answer to his biggest improvement.

 

His information is a bit contradictory. He says it's an air gap but says it's an open plenum. Some single plane intakes are open under the carb like an air gap, so I'm having trouble understanding what he has.

 

From experience on trying to tune a used carb of unknown history, my first step would be to put it back to "factory" Holley settings. Jets, power valve, accelerator pump cam and nozzle, secondary spring, and re-set the idle air screws, float levels, and throttle plate location. Fuel comes from the carb. If it's rich, start there.

Holley carbs are simple to tune.

 

Not necessarily. Carbs need velocity through them to work well, and when it drops off they don't get enough 'signal' to properly atomize the fuel. Open plenum or single-plane manifolds reduce velocity for a given vacuum signal compared to a true dual-plane. Lack of manifold heat further reduces atomization... and requires an electric choke on these. And this is one place where Chevy's much-vaunted interchangeability actually hurts. Nearly all aftermarket SBC intakes are optimized for a 350, when you reduce the cubic inches by 13%, that doesn't help.

 

From what the OP said in post #1, he is after drivability and economy.
Chevrolet (and others) have spent a huge amount on R&D for those exact results. It's what they sell every day.
Best bet without doing all your own R&D is a factory intake manifold and carb.

 

We're talking about a carb that is the next step above what would be ideal. Not some drastically oversized giant carb.

 

Granted. But this is more of a case of 'tolerance stack'. So, you've got a carb that's a bit large for the application. Result is slightly lower velocity through the carb. Next, an open plenum or single-plane intake. Again, a slight loss, added to the above. Intake runner size, yet again a bit big... more loss. Any single one of these could probably be tuned around, but added together he's probably beyond that.

Again, it's the total combination. Does he have an aggressive enough cam to use the extra flow? Automatic trans with a stock stall speed convertor? Tall rear gears?

Years ago I owned a '68 Ford F100, 360 2V, three on the tree, 2.8 rear. Great gas mileage, 20+ MPG at highway speeds, no problem, high teens in town. Then the carb started going bad, leaking around the throttle shaft and mileage started dropping. A quality rebuild was expensive, well over $100 which was real money in those days. One day I'm at the parts house, they've got brand new Holley two-barrels for $50. What a deal, I buy it. On the truck, in town mileage went off a cliff, lucky to get 10, usually less. Highway not much better. The problem? I'd bought a 500 CFM monster, designed for the roundy-round racers who weren't allowed four barrels. No amount of tuning would fix it, the velocity through it in normal driving was simply too low to properly atomize the fuel.

 

Honestly, it's hard to beat a Quadrajet for fuel mileage so long as you can keep your foot out of the secondaries. An Edelbrock RPM Air Gap intake was not designed with optimum fuel mileage in mind.