I'm converting to carb and keeping my walbro in tank pump. I'm aware I need a return style regulator, but don't know if I need one with a vacuum port or not. The Mallory unit most people seem to use has a vacuum port but Holley makes a good regulator without a vacuum port (which is cheaper). Is the vacuum port needed/used? If it's not necessary, I'd like to save some money and use the Holley unit.

Thanks in advance

 

Not using the port on my regulator from my carb swap.

 

Thanks for the input. I did some research and found that the Mallory unit says it only increases pressure under boost conditions and decreases pressure 1 psi for every 2 in hg. While that sounds completely unnecessary for my NA motor, I think I may get it anyway since it adds another tuning implement in case I need it. Also the Mallory unit says it holds up to 200 GPH which is way more than my Walbro puts out, so that makes me feel good about it.

 

FP changes w/vacuum are a FI thing. Not useful at all for a carb.

In a FI system, the fuel flow is proportional to the pressure differential across the injector; not just to the pressure in the fuel side. For example, if you had a boosted application, and the pressure in the intake was 3 bar (43.5 psi more or less), and the fuel pressure was also 43.5 psi, how much fuel would get delivered? RIGHT: you would get ABSOLUTELY ZERO fuel under those circumstances. Similarly, if the manifold had PERFECT vacuum in it (-1 bar, or -14.5 psi give or take) then the injector, instead of having 3 bar across it, would have 4 bar... BUTT WEIGHT... at high vacuum, there's NO ENGINE LOAD, so that's the LAST condition that you'd want to raise the fuel delivery artificially, so the FOR instead LOWERS the FP when vac is high, to equalize the flow through the injectors.

Having all that going on while trying to keep a carb bowl full would be a recipe for inscrutable tuning problems... car will only idle for x minutes before it peters out and dies, car runs great at WOT for x seconds then starts blowing black smoke out and fuel goes everywhere under the hood, etc.

Carbs don't work off of fuel pressure though. They work off of fuel flow. Think, a river, as opposed to a garden hose. All a fuel system needs to do for a carb, is keep the bowl full. If it could do that at .0001 psi of fuel pressure, then .0001 psi is all the pressure you need. Of course that's an exaggeration; you'll need more than that; but whatever the # is for your particular setup, then any more than that, is just an opportunity for problems. For most carb applications, about 4 - 5 is plenty; sometimes, for situations where you have restrictive fuel lines, you need more, to assure adequate delivery during times of high demand; but you'd do better to handle that situation by fixing the restriction(s) - increase line size, use sweep fittings instead of sharp 90°s, etc. - than upping the pressure. Aim for a nice steady 5 psi at all times, and you'll be fine. Much better than by complicating the whole thing.