Skip the auto shop - Our mechanics make house calls

Q: What type of carb to replace with?

asked by on

I have a 350 gm high performance crate engine in car. I don't know hp or rpm's that it is putting out. I have a 850 edelbrock carb on it now car runs fine, but burns alot of gas. I want to downsize carb. but not sure what size to go with. The carb on it has vacuum secondaries. Car is an automatic 4 speed. I am not racing the car.
My car has an automatic transmission.

A: Picking out the appropriate size carburetor...

Picking out the appropriate size carburetor is a factor of how much air your specific engine build will pump. This is measured in CFM’s (cubic feet per minute). You are most likely correct in needing a smaller CFM carburetor. Your typical 350 only needs a 450-600 CFM carburetor. Contrary to the popular belief, bigger is not better when it comes to carburetion. To enlarge a carburetor slows the air volume that moves through the intake manifold runners. This is an efficiency killer which hurts HP and fuel mileage.

Here is a good page that can guide you in picking the right carburetor for your motor, but you will need to know more about your motor; mainly, the maximum RPM the cam is designed for. Other factors that can affect carburetor choice are part of the cam specifications. Higher lift racing cams will affect your volumetric efficiency and therefore your carburetor choice.

By far, the most important consideration is the CFM rating. It needs to be matched to the engine build. As for what style of secondaries you get or what kind of choke you will get, these details are often decided for you depending on price and brand. If you have a choice, go with what you already have. There is nothing wrong with vacuum actuated secondaries.

You will likely see some improvement in power output and better fuel mileage with a smaller carburetor, but don’t expect too much. A 350 carbureted motor won’t give you much better than 15 miles per gallon. Especially if it is truly high performance.

Choose well.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Obd1 code 45 car miss firing and stalling

The carburetor on these cars have a mixture control solenoid that goes bad and I would suggest replacing the solenoid if you got a Code 23 with or without Codes 44 and 45. Since you overhauled the carburetor, the adjustment...

Q: How do I view the average fuel economy?

The average fuel economy shows you the gas mileage that your vehicle has been averaging. The average fuel economy is directly tied to the trip meters, and will be reset when the trip meter is reset. To view the average...

Q: Engine dies when I take my foot off gas pedal.

Hello. If the engine runs fine when you are holding the accelerator pedal down and dies when you release it then you may have a couple of possible issues. I would first check the vacuum hoses going to the engine....

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.