Air in fuel lines and sputtering at high and now low speeds

[Mark Maras]

 My experiences with SUs is limited to Hitachi 4 screw, round tops and MGB SUs. Neither used unequal length float pivot arms. I've heard the theories (G forces affecting the fuel level in the float bowls) behind the "improvement" but in over twenty years of very spirited driving I never encountered a problem. If I couldn't source another short eared cap, I'd redrill the long one and use it. The RAF in WWII solved a similar problem with a simple device they named Miss Tilly's Orifice in honor? of the inventor.

[tetert1]

 thank you I appreciate your reply

[tetert1]

 my 72 Z has a short one on the front and a long one on the back I also took apart the mechanical fuel pump while on the engine and check the diaphragm it was good.. started the engine ran good for 10 minutes and then bubbles started appearing starving. Carburetors ..are there any good videos out there showing you how to tune 240Z carburetors the right way ? I might have to take it to a z mechanic anyone know a good one in the Houston area

Edited January 4, 2020 by tetert1

[tetert1]

On 12/29/2019 at 4:03 PM, tetert1 said:

 tomorrow when I have time I'm going to try the clear hose..thanks 

 while I'm running the engine I place my finger on the Overflow tube on the float Bowl it was sucking air on both of them.. when I put my finger on it ,engine try to die. Also check my fuel return line at the orifice tiny pinhole try to stick a pin in it wouldn't go in something very hard like metal is it supposed to be like that or is it supposed to be free of obstructions??

Edited January 4, 2020 by tetert1

[tetert1]

On 12/30/2019 at 7:03 AM, siteunseen said:

Yep, they are small barrell filters in the banjo bolts right before the float chambers.  I personally do not use them.  I have two filters before the carbs that do the job.  Those small filters are hard to find new and actually they cause a little restriction of fuel flowing into the carb in my opinion and my opinion was told to me by many folks on here and at Z Therapy. 

    

 Circled in blue in this picture.

 is it possible that air  is getting sucked in at the Overflow tubes on the float Bowl back down to the fuel line? I stuck a needle in the orifice in the return line and it seems like it's plugged

[siteunseen]

I can't understand how that could be but anything is possible I guess.

I would use a small gas can for the supply hose to the fuel pump then another smaller clear one for the return. 3/16" off the fuel rail at the front of the cylinder head. I can explain better with a picture later this morning when I get back home.

[87mj]

I am very curious about this problem.  I had a very similar issue with a '73 as a teenager.  I never could get it resolved.  '73 had nasty carbs but the symptoms appear the same.  As a kid back in '79, the car would run great and idle great until it got hot.  Then every time I turned a corner or clutched it to slow down for a stop light, that car would die unless I would tap the gas pedal while clutching it.  

I remember getting the engine hot one day and I connected long fuel line to the supply and return on the fuel rail.  Then I placed both hoses in a clear jar of  gas.  The car would not suck gas.  Then eventually it would.  I concluded it was the fuel pump so I replaced that.  It had no effect.  I got the  '73 carbs overhauled by an "expert" at the Datsun dealer.  No effect.  The car really wasn't very old at the time. 

Rather than overhauling the carbs, I suggested they swap the carbs from a '72.  They looked me like I had 3 heads. 

[tetert1]

4 hours ago, siteunseen said:

I can't understand how that could be but anything is possible I guess.

I would use a small gas can for the supply hose to the fuel pump then another smaller clear one for the return. 3/16" off the fuel rail at the front of the cylinder head. I can explain better with a picture later this morning when I get back home.

Like I said my return line on the fuel rail has a pinhole orifice I stuck a needle in it and it's blocked off won't go through is it supposed to be that way

[Patcon]

No, it should be open. That being closed off will raise the effective pressure on the float valves

[siteunseen]

Same here, that's wrong and causing higher pressure in your fuel delivery circuit. Can't think of a reason that was done but it took some effort to do so who knows?

You should get another fuel rail in my opinion. Look in our classifieds, there was a few brand new replated ones for pretty cheap. @jfa.series1 has them if they're not sold yet. He's in Texas too and if I'm not mistaken you are located somewhere around there.

Good luck. Cliff

[Patcon]

I would unhook the lines from the rail and try to blow compressed air into the rail from the return line end. See if it's debris stopping up the end of the rail. The rail may be fuel of garbage or rust even

[tetert1]

9 hours ago, Patcon said:

No, it should be open. That being closed off will raise the effective pressure on the float valves

How would you recommend unplugging the return line.. steam it out add chemicals? Try maybe pressure washing ,maybe a radiator shop might help? Small  drill bit?

Edited January 5, 2020 by tetert1