Carb Fuel Line Size

North Coast Cab · North Coast Cab's Garage

What size fuel lines would I need to feed a set of 46mm carbs?
I have a high volume, low pressure Mallory 4110 pump (recommended by PMO), running through 3/8" hard lines to AN8 flex lines to the carbs. Is this a good set-up? Car is under 250RWHP and I am getting a top end lean condition. Wondering if enough fuel is getting through.

Thanks, John

shbop · 07-19-2007, 07:02 AM

Have you tried increasing the fuel pressure?

gumba · 07-19-2007, 07:54 AM

I'm using 5/16" i.d. on my 3.4 w/50 pmo's. I'm also using a mallory pump. I noticed my motor was starting to run lean at the top as the day progessed. I called the race shop and they suggested raising the fuel pressure, it was set at 4.5 at idle. It seemed to cure it. I'm starting to think the mallory pump is getting too hot and loosing pressure. I have dual a/f gauges and they usually read 12.5-13 throughout the rev range. The a/f went up to 13.5-14 above 5k as the day went on and the outside temps went up.
I added the pmo pressure gauge and a return line to the fuel tank. It was suggested that the pump may have been running hot because it wasn't recirculating the fuel.

North Coast Cab · North Coast Cab's Garage

Interesting. I'd like to find a way to have a fuel pressure gauge in the cockpit so I could see what it's doing. I have the Mallory 4110 as suggetsed by PMO and a recirculating line so I don't think it's getting too hot unless there is something wrong with it. I had my pressure as hgh as 6psi and experienced the same problem right from the beginning. Recommendations are 3.4-5.5psi.

JP911 · JP911's Garage

You can put a fuel pressure gauge in the cockpit. Autometer (and other gauge companies) sell fuel pressure gauges that take their reading from a sender, similar to the senders for oil press and temp. The sender simply replaces the gauge on the regulator. The only drawback is the price.

gumba · 07-19-2007, 09:16 AM

Mine was fine at the beginning, then went up as the temp did. My first mallory pump was defective. Too much fuel pressure isn't good. I would set it back down to 4.5. You might check for any kinks in the fuel line. I just went through this on my 3.2 w/46mm webers. The a/f was fine at idle, but would start to lean out as the revs went up.