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Smaller Carb or bigger one or smaller jets

Posted By Tim Quinn 3 Months Ago
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Tim Quinn
Posted 3 Months Ago
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Right now I have a Summit 600 cfm on a 312 that is bored 0.90 w/stock cam, aluminum pistons three speed with Overdrive 3:89. Cruise at 70 mph at 2,000 rpm.
Gas mileage is maybe 18 mpg. The primary jets are .67. Would it help to put smaller jets in the primary and secondary for better gas mileage. Planning on heading to Detroit's Dream Cruise.
Is the 600 cfm to big for the engine.
What do ya think, any advice ?
Thanks in advance,
Tim
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charliemccraney
Posted 3 Months Ago
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Smaller jets will lean it out, which may provide better economy as long as it is not too lean but also possibly at the expense of power.

600cfm is probably more than the engine needs but it is vacuum secondary so it should be fine.  I don't think it is too big.

It's best to do this type of stuff with the help of an O2 sensor so you can see what is going on in real time.  Not something that is good to guess about during a 1000+ mile trip.



Lawrenceville, GA
Tim Quinn
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Right now the primary jets are .67, I'm thinking of replacing them with .64 for the trip to Detroit and put the .67 jets in the secondary position.
Would those jets be too lean ?
Whatda ya think ?
Tim
90 degree Florida
charliemccraney
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Impossible to answer on a forum.  Bare minimum, look at the plugs and make a judgment call based on how it currently looks.  I'm not good at reading plugs so I use an O2 sensor.  No uncertainty with that and you can gather more info in a shorter time, speeding up tuning and also no handling hot plugs on a hot engine on a hot day at the side of the highway for an accurate read..


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Tim Quinn
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Thanks for your time and input.
Tim
Florida
Florida_Phil
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My 292 is running a Holley 1848 465 on an iron 1957 intake manifold.  I have a 4.11 gear and a three speed with O/D.   I like the throttle response of the 465.  I've thought about upgrading to a 600.    From what I have read, I wouldn't be gaining that much.  Like others have said, a vacuum secondary carb is only going to use what it can.  I wouldn't change carbs myself.
Tedster
Posted 3 Months Ago
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Wideband O2 sensors don't cost that much, the fuel savings alone on a Florida to Michigan trip and back would probably pay for itself, assuming there's anything left on the potential fuel economy table.

You can jet down a fair bit to improve cruise fuel economy, but keep in mind jet size also changes the air fuel ratio when under load, under power acceleration. Getting both of these right is critical if you're going to dig in to carb tuning. Typically involves enlarging power circuit fixed orifices with a pin vise. If too lean under acceleration it will probably burn a valve. As Charlie said a wideband O2 sensor is the best way for most people to tune a carburetor. Make sure the distributor ignition curve is optomized to include vacuum advance before getting too deep into the carb adjustments. If the ignition system is weak, leaner fuel mixtures will expose it. A cooler heat range plug may help too, especially on a long highway run.
FORD DEARBORN
Posted 3 Months Ago
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18 mpg is going to be tough to beat, IMO.  At least that's the most I can get after careful tuning  and as mentioned above, too lean and your going to burn parts. If I can remember right, 16 to18 mpg were  figures offered on this forum and those numbers are pretty much what a modern full size vehicle with a V8 will achieve at best also. Unless there are performance issues, your probably in good tune as is.  Hope this helps, JEFF..............


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Tedster
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With the right gearing, a lighter car can do better. Maybe 20 to 24 mpg highway. Though not with corn gas.
62bigwindow
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Put in the jets you want to run then drive it. If you get a surge while at cruising speed then the jets are too small. A wideband is a nice diagnostic tool but not necessary. There is alot of info online on carb tuning. A vacuum gauge is a must in my opinion.


Durham Missouri


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