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jalexquijano

ROUND TOP SU CARBURETORS FUEL PRESSURE TOLERANCE

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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.

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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. 

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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 by jalexquijano

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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.

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 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.

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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.

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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.

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 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.

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 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?

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2 last questions before proceeding:

1. How many minutes should i leave the car on before removing the domea to verify fuel level?

2. Should i take the car for a drive to warm up before installing the clear tubes to the drain plugs to verify the 23mm level?

3. From where to where shall i measure the 23 mm level? From the float lid down?

Screenshot_20180208-085905.png

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 #1 A minute or two should be enough to reset the fuel level in the bowls.

 #2 No need to drive the car or warm it up.

 #3 Yes

 #4 When you're done screwing around with the clear tubes and the 23mm, (which may or may not be correct for both 3 screw carbs) tell us the fuel height in comparison to the tops of the nozzles. The only way you're going to know if the levels are 100% correct is to compare the fuel levels with the nozzle tops.

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16 minutes ago, Mark Maras said:

 #1 A minute or two should be enough to reset the fuel level in the bowls.

 #2 No need to drive the car or warm it up.

 #3 Yes

 #4 When you're done screwing around with the clear tubes and the 23mm, (which may or may not be correct for both 3 screw carbs) tell us the fuel height in comparison to the tops of the nozzles. The only way you're going to know if the levels are 100% correct is to compare the fuel levels with the nozzle tops.

What if i just start with the test by leaving the car on for 5 minutes at the parking and then shut the engine before pulling the domes? After removing the domes i can take pictures and send them for your approval or decline on the correct level. Both carbs are dialed at 2.5 turns exactly.

Edited by jalexquijano

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 Sounds good. Pics will help but I also want your impression on the fuel height too. I'll be gone until early evening.

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6 hours ago, Mark Maras said:

 Sounds good. Pics will help but I also want your impression on the fuel height too. I'll be gone until early evening.

Fuel height seems fine. Almost no fuel over the top of the nozzle. Front carb and rear cab without the dome and pistons:

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20180222_190405.jpg

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  Good job. A little hard to see but it appears the fuel is at the nozzle tops. That would be OK if we didn't have to lower (richen) the mixture screws later. If we have to lower the mixture screws later, the fuel will be puddling on the nozzle tops. Not a good thing. Next thing to do is readjust the floats and get the fuel level 1 1/6" below the nozzle tops. You'll be lowering the fuel level in the float bowls to accomplish that. That will allow for fine tune adjustment later. Go thru the readjustment, start it up run for a minute, shut down and look at the new float levels and report back please.

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Bend the the tap upwards, away from the floats a very minor amount. This will make the valve close sooner and lower the fuel level in the float bowl

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