ROUND TOP SU CARBURETORS FUEL PRESSURE TOLERANCE

[Johnny wick]

I have several questions regarding the precise fuel pressure both of the 3 screws round SU CARBS on my 240z will tolerate as i am pushing the fuel through an AIRTEX 8012S which is rated 5 to 9 PSI:

1. Should i measure the fuel pressure plugging a "T" before the front carburetor?

2. Should the pressure read no more tan 4.3 psi?

3. What if the pressure reads more than this limit but does not show any spill out of the fuel bowls? Will i need to open both fuel bowls and reset the level. THis was done by Ztherapy in 2014 and i would not like to mess with it.

[Mark Maras]

1. Any spot between the carbs and the and the filter. (if there is one between the pump and carbs) should be fine.

2. 4.3 is probably at the upper pressure limits of the needle and seat (fuel valve) in the float bowl. 3 - 3.5 would be better IMO. (Back in the day, we only ran 3 lbs. of pressure in a small block Chevy dragster.)

3. If the excess pressure isn't over powering the needle and seat valve, you're probably OK for now. The excessive pressure shouldn't affect the float level setting but it could cause premature failure of the needle and seat. A potentially disastrous scenario. Remember, the carbs are placed over the hot exhaust manifold. 

[Johnny wick]

On 19/4/2017 at 10:37 AM, Mark Maras said:

1. Any spot between the carbs and the and the filter. (if there is one between the pump and carbs) should be fine.

2. 4.3 is probably at the upper pressure limits of the needle and seat (fuel valve) in the float bowl. 3 - 3.5 would be better IMO. (Back in the day, we only ran 3 lbs. of pressure in a small block Chevy dragster.)

3. If the excess pressure isn't over powering the needle and seat valve, you're probably OK for now. The excessive pressure shouldn't affect the float level setting but it could cause premature failure of the needle and seat. A potentially disastrous scenario. Remember, the carbs are placed over the hot exhaust manifold. 

Well 3.2 psi is not working for me as carbs are backfiring through intake when car is cold. Anyway to fix this issue without buying a new pump?

Edited February 21, 2018 by jalexquijano

[Johnny wick]

[siteunseen]

5 minutes ago, jalexquijano said:

Well 3.2 psi is not working for me as carbs are backfiring through intake when car is cold. Anyway to fix this issue without buying a new pump?

Let it get up to temp. Mine will do that too if the choke is still on.

[Mark Maras]

 Jalex, what is the fuel height in the nozzles?  Backfiring from the intake is caused by a lean mixture. It's possible the floats are too low. Also possible it's not getting enough choke because of mis-adjusted choke cables. Ignore the fuel pressure (you've been down that road in the past and the problem still exists) and report back on the fuel heights in the nozzles.

[Patcon]

3 psi is more than enough fuel pressure to get your car running right!

[Johnny wick]

1 hour ago, siteunseen said:

Let it get up to temp. Mine will do that too if the choke is still on.

How many minutes of driving with the choke on? 5 to 7 minutes? When its cold it backfires through the intake and hesitates.

[siteunseen]

Don't go by the clock. Go by the TEMP gauge.

Mine is good when the needle lines up with the right leg of the "M". That's where the thermostat opens on my car. 

Use the choke or drive easy until it stops backfiring. once you get the float levels right it shouldn't stumble. Mine did it for a year until  i got so fed up and adjusted them correct level. Now I can not make it stumble.

[Mark Maras]

 It backfires and hesitates because the fuel mixture is too lean, which is what we're trying to fix. We're working on the PRIMARY MIXTURE ADJUSTMENT which is the float (fuel) level. Ignore how it runs for now, it'll get better as we go. What is the fuel level in the nozzles? Yesterday I said to adjust the fuel level anywhere from the top of the nozzles to 1/16" down. I'm going to change that. I think you should set the fuel level as close to 1/16" down as possible. Doesn't have to be exact, just eyeball it. The 1/16"drop below the top of the nozzle should allow for some mixture screw adjustment if we need to richen it up. All we need to know, at this point in time, is where the fuel level is in relation to the top of the nozzles.

[Johnny wick]

Question: How do i know if the needle and seat installed is indeed in good shape and working order? Anyway i can confirm that the following pressure pump which originally rated 4.2 psi did not damage the needle and seat?

https://www.summitracing.com/int/parts/atx-e8012s/overview/

 

[Mark Maras]

 If the needle and seat are leaking the float bowl will fill to over flowing with fuel and flood the area above the nozzles with fuel. One could possibly see fuel spilling out of the intake side of the carbs too.  4.2 psi shouldn't have ruined the needle and seats and since they're relatively new, I doubt they'll be a problem. Where is the fuel height in the nozzles?