RumRunner

Have you actually flowed the fuel system to see if it's flowing enough fuel?

This combination is not going to make enough vacuum at WOT to worry about closing the Power Valves.

GoFastSonic

No I did not check volume? WHat is the best way to do that?

RumRunner

Check with your fuel pump manufacture so they can tell you how they want it flowed to make sure it's flowing within the parameters it was designed to.

bobl

RumRunner. Please explain how you can have good fuel pressure at WOT and yet be running out of volume. I'm having trouble comprehending that. Seems like if the volume was insufficient the pressure would fall off.

RumRunner

Volume and pressure are independent of each other. You can flow 60 Gallons Per Hour at 6 PSI, just as easily as flowing 600Gallons Per Hours at 6 PSI. Just because you have X amount of fuel pressure doesn't mean it is flowing ENOUGH volume for the engines demands.

bobl

I understand that. But, wouldn't the pressure drop if the volume was inadequate? Isn't pressure generated because you have more volume going in than is being used? If the engine is using more fuel than can be supplied then the pressure should fall, right? I always felt if the fuel pressure was good there was not a supply problem, or if there was a supply problem the pressure would drop at WOT. I'm not trying to dispute what you are saying as I totally respect you as the most knowledgable carb guy around. Just trying to understand and learn something.

RumRunner

You're right and wrong... Within a given "Known" situation that was running properly if the pressure dropped it CAN be an indication that the volume isn't keeping up with the engine, however there are a couple of things to look at.

First the fact that you can use a 100 GPH block mounted fuel pump, and have 6 lbs of fuel pressure, but you can use a 300 GPH electric pump and still have 6 lbs going to the carburetor. So with which one will you get more fuel to the carburetor?

Next there is actual an inverse in pressure and volume meaning the more pressure you have the less volume you're actually flowing. Take the little Holley Blue fuel pumps they flow 110 GPH Free flow (no pressure against them), but they are like 70 GPH at 9 PSI. The same is true of the bigger pumps as well.

Now we get into Pressure at the N&S you can actually raise the pressure going to the N&S and flow more fuel through it (due to pressure differential), but you also run the risk of aerating the fuel.

If you were to take two 5 Gallon buckets, and cut a 1" hole in the center of the bottom of both of them and fill one with 1 gallon, and the other with 5 gallons, and put them over the top of a 1 gallon pale the buck with more volume with flow faster through the same hole and fill the 1 gallon pale quicker.

When it comes to running fuel pressure I prefer to run lower pressure (to the carburetor) with larger N&S so I can control it better.

SB

Real life head scratcher (at that time) issue

Fue System In this order - Pump/fuel flow meter/psi gauge / 3 ft of -8 line/psi gauge / carb.
Psi read fine and exact same on the 2 different gauges.
Carb bowl fuel turned into frothy mess than dropped out of view.
Motor ran f'd up / A/F ratios went up thru the moon.
Removed fuel flow meter.
Carb kept constant fuel level w/ no frothing.
Same psi.
Engine ran good without missing a beat and a/f ratios right on target

So,
Not enough volume - plenty of psi

Real life example that Dyno company called BS too......until after we solved it and reported our exact findings.

Thank god for clear sight glass on some Carbs. It proved what was happening and threw some people's thoughts and theories right out the window.

bobl

So the frothing was something restricting the volume, yet psi was good? Interesting. I'm still having a hard time with this. If the volume is too low, the needle and seat will stay open because the fuel bowl is not full. Therefore the pressure should drop. What am I missing in this picture? Keep the discussion coming.

GoFastSonic

Wow this is getting deep I should have "paid" more attention in school