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.