Standard Carb Jet Sizes

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Re: Standard Carb Jet Sizes

Post by Ollieoldbeard » Wed Dec 16, 2015 10:22 pm

Thanks, ive been trying to study the diagrams but theyre not clear, can i access the two jets you mention from the outside of the carb or does it need taking apart ?

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Re: Standard Carb Jet Sizes

Post by AJL Electronics » Wed Dec 16, 2015 11:22 pm

You need to take the lid off. Whilst there, check and clean the air jets and emulsion tubes. They are on top of the body under the lid. Unscrew the air jet and use something to hook the emulsion tube out. I use an Easyout very gently.

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Re: Standard Carb Jet Sizes

Post by Ollieoldbeard » Fri Dec 18, 2015 12:01 am

Thanks Mr AJL, next on my to do list

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Re: Standard Carb Jet Sizes

Post by philhoward » Mon May 23, 2016 2:57 pm

Vaguely related, I spotted the following on another forum today:
Ever wondered what a 390, 450, 600 etc that Holley use for sizing 4 barrels actually means ?

Well the figure that Holley put on the box is the air flow in CFM at a pressure differential of 1.5" Hg - Hg is Mercury and as it's a pretty heavy doohickey it takes a bit of suck to get it moving 1.5" up a tube - its the same as about of pressure required to move water 20.4" up a tube.

Note - 2 Barrel Holleys are tested at 3"Hg so do not compare the two a 390 two barrel is not the same as a 390 4 barrel.

converting Hg to Wg and flow at 20.4" to flow at 28" is actually quite straight forward.

To the results:

A 390 Holley flows 390 CFM at 20.4" wg which at 28" is - 457CFM

If we want to compare our 400CFM 40 DFA Weber in Holley 4 barrel numbers that is 341CFM

A 390 two barrel Holley - flows 323CFM at 28" - about the same as a stock 32/36 DGAV

Hopefully this has also highlighted that without that crucial "wg figure - flow figures for anything are absolutely meaningless.
The above figures are for the units without chokes fitted. With chokes fitted:
I did manage to get 179CFM out of the 36mm choke so 358cfm for the whole carb.
On the 32mm choke only I measured 151 so 330 together.
In short, a 390cfm 2 barrel Holley flows less than a 40 DFA (and not much more than a bare 38DGAS extrapolating the figures)..and the DFA fits the standard manifold.

Still want a Holley?...

Taken from a thread on Retro Rides of a guy who's made his own flowbench.. ... nch?page=6 (starts halfway down).

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Re: Standard Carb Jet Sizes

Post by backpain » Mon May 23, 2016 3:43 pm

Well that's interesting !

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Re: Standard Carb Jet Sizes

Post by Old and Slow » Mon May 23, 2016 4:27 pm

This might be useful to someone:
The Triple Weber 40DCOEs fitted to 660XYB are jetted as follows:
Idle Jet 45F9
Air Corrector Jet 155
Emulsion Tube F11
Main Jet 120
No air filters, but I recently fitted mesh over the intakes.
I don't know what cam is fitted but it seems to have a lot of overlap, as it spits a bit if you don't drive it hard.

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Re: Standard Carb Jet Sizes

Post by Nick » Mon May 23, 2016 5:41 pm

Out of interest my Coupe with a Webber 40 DF15 runs 200 mains, 32 mm chokes, 115 air corrector ,65 idle and 70 pump jets. At peak gave 140 bhp at wheels at 5500(does not gone this much now!), I suspect the main jet may be a size too big. Burtons seem to think this is match for a 4 barrel Holley (but do not say which one)


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