Datsun Roadster Forum-311s.org

Would an electric fuel pump with too high pressure not allow me to lean out the carbs?

What should the fuel (pump) pressure be for SUs on a U20?

3-5psi would be ample I suggest.
You could try inserting a restricting valve in the supply line to the carbs. if you suspect the pressure is too high

Thanks ... is there a specific PSI number recommended for the SUs?

Did a quick search in Google and it looks like 3psi is the maximum you need

I run an electric pump rated at 2-4 psi, mounted on the frame rail just ahed of the rear wheel. Takes ten minutes to change if it craps out. Works great.

https://www.ebay.com/itm/New-Electronic ... SwABdgBPUp

As Nissanman said, install a restrictor valve/fuel pressure regulator https://www.amazon.com/Universal-Adjust ... ljaz10cnVl

Much more likely to be metering needles, nozzles, or float valves. First check to see if the float level is correct. If the floats are too high then it will run rich. Check the metering needles and nozzles for wear. Needle and nozzle wear will cause the idle mixture to get rich.

Carter electric rotary on the front frame rail @ a regulated 3.5 psi on the Mikuni Solex 44PHH-2s or SUs.

In addition to the above suggestions, check for sunk or soggy floats.

If you are running an electric pump, be sure to include an in-line inertial (high-G shock) power cutout.

A 42S electric fuel pump should be putting out 2-4lbs. There are numerous private label versions that I believe are all the same with a different label slapped on it. I have used them for years and now use one as a lift pump to my EFI surge tank set up. I've only had one fail after many years, but they are cheap and small enough to just carry a spare. I swapped it out on a mountainside road in about 20 minutes.

I do know that you can overpower a Mikuni style carb float with too much pressure which will raise the fuel level in the carb which changes how/when the emulsion tubes operate. I don't think there is a similar condition/design for an SU. With too much pressure I believe the SU float bowl itself will overflow, but I don't know if that puts more fuel into the carb itself... I don't think so though.

Keep posting and we'll see if we can help.

Putting too much fuel into an SU carb will cause it to run rich.

The return line type fuelP use 4-7 psi
And will leak fuel by the float / needle valves

keith0alan wrote: Fri Feb 05, 2021 1:56 pm Putting too much fuel into an SU carb will cause it to run rich.

I stand Corrected! Thanks Keith! My SU history is limited to my old 620 PU... 30 years ago... good Lord...

We decided to go back to the stock mechanical pump for now. Mike the mechanic adjusted the carbs as well as they could be. To get much better will require more time on the carbs & parts & investigation & money than I have right now. The car runs MUCH better (and the fumes don't make me gag). And the car actually requires choke now on start-up. I replaced the spark plugs with NGK BP6ES-11 with .040 gap (one of the jobs Mike did was install the Electronic distributor from East Coat Roadsters...more on my post on all the work later).

Thanks for the help & advice.