I just put some pro topline heads on and am experiencing what I think is insufficient fuel volume. I have a stock mechanical fuel pump from an LG4 and an air/fuel ratio meter hooked up. While doing a WOT run, the meter is showing about 13:1 and at 4500-5000rpm or so the meter all of a sudden goes lean, at least 15:1 which is the max the meter will show. About a second later I start to feel a stumble so I let up. I didn't feel this in 1st or 2nd gear but 3rd gear takes longer to wind out so the engine was at higher rpm's longer than it is in 1st or 2nd which leads me to believe that the engine wasn't getting enough fuel and that it's not a carb problem. I should add I have a Walbro intank TPI pump that's not connected and could possibly restricting flow. Question one is, is this a fuel flow problem or could it be carb related? Question two is what should I do about it? I could get an aftermarket mechanical pump or I could get a regulator for the Walbro and run that.

 

Redue the complete fuel system from tank to carb.

You'll need a minimum of a pump that can flow 100GPH
free flow. You'll need a minum of a 3/8" fuel line tank to carb. 1/2" is better.

 

100gph?? I got a high end Walbro and that's only 255lph which is about 60gph

 

Right now the mechanical pump is trying to pull fuel through an electric pump that isn't running - that's gotta be killing you.

Ultimately, redoing the fuel system completely would guarantee more than enough fuel. I think if you can just bypass the electric pump, connecting the fuel feed line to the strainer you will be in a lot better shape.

I'm in a similar situation like you are, I intend to bypass the in-tank pump and use the 12V to drive a Carter or Holley electric pump mounted by the factory filter location.

I believe the TPI pumps are simply the wrong application for a carbed engine - the pumps are all about pressure, we need high volume at much lower pressure.

 

Yeah I've considered pulling out the intank pump but dropping the tank is a pain. Hooking up the intank pump to the carb would definitely work volume wise but with the high pressure the regulator would just be sending back most of the fuel that comes upstream.

 

Returning the majority of the fuel to the tank isn't really a problem in my books, it keeps the fuel moving along the lines under the car, not under the heat of the engine compartment, so it keeps the fuel cooler.

From what I've read in related posts on the board here is that the regulators 'hunt' and the output pressure bounces around trying to pull only 6 psi from a pump that has no internal relief or bypass and will deliver crazy pressures into a deadhead.

I haven't dropped my tank yet, but it looks rather ugly. I only want to do it once.

Interesting that the problem only surfaced with the new heads huh? I take it they're working well for you!

Since you have existing fuel return lines, you could restore the electric in-tank pump so that it runs again, and tee the two lines under the hood together (supply and return). That way you will have fuel pushed from the tank to the engine compartment and it would be allowed to return to the tank through the return line. Take the source (from the tee) to your existing mechanical pump, from the mech. pump to the carb.

This would ensure that the mechanical pump under the hood is always supplied with fuel at low pressure, the low pressure being created by the resistance of pushing the fuel back through the smaller return line.

It's a solution that avoids dropping the tank, takes advantage of the return lines to keep the fuel moving, and you have low pressure fuel supplied to the mechanical pump (you could check it with a fuel pressure gauge, I had to put a restriction orifice inline with the return to get 5 psi).

This would give you lots more fuel than you're getting currently.

 

You could also try hooking up the in tank pump
but restricting the supply voltage to something less than 12v and then contoling the intank pump with a WOT switch and a pressure switch that would cutoff if/when the pressure got too much for the carb (about 6-8psi)

You would need a normally closed Hobbs switch that opens about 6-8 psi.