The Stovebolt Site Forums The Shop Area Engine Shop Fuel Lines for Dual Carbs

When running fuel lines to now two single barrel small throated carbs, does one use 1/4 inch or 3/8 inch lines?

Use whatever size that came on it originally. The two carbs won't use much more fuel and the same size line probably goes from the fuel tank to the fuel pump anyway.

Let me rephase my original line. Does a lower pressure flow work better than a higher pressure flow to the carbs? 1/4 inch should produce a higher fuel pressure after the fuel pump than 3/8.. If post pump line is 3/8 inch then necks down to 1/4 inch to carbs; would that make any difference?

Think the original is 5/16" line. If you had to choose between the 1/4" and 3/8", I would go with the 3/8". It won't appear to be any larger to anyone looking at it and certainly is easier to find than 5/16". If you use 1/4", it will look too small and will probably be to small to supply enough fuel for both carbs.

Just my 2 sense.

Joe

Rattletrap52,

Pipe size has no bearing on pressure, only volume.

I wouldn't be concerned with volume as each carburetor has a giant "well" that would never be drained regardless of a heavy foot.

I agree with 6cylinder, it's all about style points and parts availability. The stock 5/16 line would look 'right' to me.


Stuart

Actually, there are some issues to think about.

A small tube will have slightly (but measurable) LOWER pressure and significantly less volume.

The important issue is velocity of the fuel in the lines.

Too large a tube, and the velocity is very low (larger volume). The fuel sits in the tube and is heated by the surrounding heat sources. This can contribute to vapor lock during periods of high ambient temperature and low fuel useage (stop and go city traffic).

Too small a tube, and the velocity is very high (due to much less volume), which can cause unpredictable operation of the float in the carburetor. A side note to this issue is the Q-Jets of 1967 and 1968. The float position in the Q-Jet was changed in 1969 such that a "windowed" fuel valve seat could be used for higher volume fuel flow necessary for high performance engines. Seemed the fuel coming from the window sprayed on the float, causing a flooding condition. The repositioned float allowed for use of the window.

We would suggest 5/16 for the typical stovebolt.

Jon.

I used 5/16". You can find steel line sizes from 3/16"-1/2" at any parts store.

Longbox55,where did you get your fuel log ? I'm thinking of using one also.

Check out this website:
http://doglipsspeedshop.com/dual_skull_fuel_block.htm

Those are cool,but i'll pass on the skulls .

bowtietim, the fuel log came from Mooneyes, it can't be seen in my pix too well, but it has "Moon" cast on the top of it. The regulator cover came from them, too. No regulator under it, however. It covers the relay for the overdrive.

In my business, there is a huge difference between 3/16", 1/4", 5/16" and 3/8". Those are the sizes we use. For a larger inner diameter, it takes significantly less pressure to move fluid at a specific velocity. The restriction on 3/16" is approximately 2 lbs per ft. For 5/16 it is about 1/2 lb per foot. We balance restriction with pressure to make draught beer pour well. The moral of the story is - size matters!