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harrison28 April 2nd, 2019 11:14 AM

Carburetor question
I'm still quite new to the carburetor game. Right now the 351 has a 4 barrel Holley 600 with a set of 72 jets in it. I bought a 2 barrel Motorcraft carb at a swap meet that currently has a set of 58 jets in it. I want to try the 2 barrel kinda for the fun of it for a couple weeks just see what it does fuel mileage wise. My question for someone with much more carb experience, with it being a 2 barrel, is the jetting going to be smaller?, or will it matter? Should I assume that with 72's in the Holley, I should run something similar in the Motorcraft? I changed jets in the Holley multiple times to insure that the 72's were where it needed to be. I'm interested in any opinion.

glc April 3rd, 2019 10:57 AM

The 4 barrel has both primary and secondary jets. The secondary is either vacuum or mechanically operated. The 2 barrel has no secondary. I don't know if you can translate jet size between 2 and 4 barrels. With a 2 barrel, it's reasonably straightforward, just read the plugs.

This may help:

harrison28 April 3rd, 2019 02:51 PM

I kinda assumed reading the plugs would be my best option, other then using vacuum gauges and O2 sensors. Reading the plugs is how I got the Holley tuned. Thanks for the link.

JSoko April 12th, 2019 09:02 AM

from way long ago I remember
trying the same
result was the 4bbl did better on MPG than the 2bb if you keep your foot out of the secondary's

most 2bbl carbs have higher CFM than a 4's primary

I did a write up about the particulars, but i can't find it

harrison28 April 12th, 2019 09:13 AM

That's good to know. I just a rebuild kit for my QFT 750 with mechanical secondary. I think now that our weather is steady, I'll do some testing with all 3 of my carbs. Maybe do a full tank with each one and see what the differences are. Give me something to do.

JSoko April 12th, 2019 09:19 AM

what I remember is the 2bbls were in the 500 CFM and above
4bbl was less than 500 CFM (primary)

but even though I did this in the mid 70's when gas prices were sky high at half a dollar per gallon
the 4bbl went back on

JSoko April 12th, 2019 09:29 AM

what i did do without switching carbs ('72 Dodge Demon 340 4bbl Thermoquad) was disconnect the link for the secondarys and they would not open
i got much better gas mileage on the hwy traveling, but always wondered if gas was getting sucked in the back from vacuum and it could have been better

this car went a lot faster than it could stop and it did not do well in curves

harrison28 April 12th, 2019 11:18 AM

I bet it was a fun ride though. I hadn't thought about disconnecting the secondary linkage. I'll try that also. I get about 11+ with a 600 cfm and vacuum secondary. This is all really just for info and is gonna just be trial and error. The truck isn't an over the road hauler or everyday driver. Thanks for the tips.

harrison28 April 25th, 2019 10:46 AM

Update #1- Ran another tank through the Holley 600 with vacuum secondaries. Averaged just over 11 1/2 mpg. All in town 10-12 mile round trip driving. I never disconnected the secondary linkage since I knew I wouldn't get into them around my little town. Gonna put the 750 double pumper on it here in the next day or 2. More results coming soon.

JSoko May 1st, 2019 08:08 AM

any up-date

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