Calling All SU Experts

[JLPurcell]

OK, I used my Float Sync tool and a color tune spark plug to set up my SU's today. All went relatively well, but having the advantage of seeing what is going on in the float bowl with the Float Sync caused me to have a question. When off of idle, (hand race engine), the front float bowl level is dropping, the rear is stable. Under load I believe the car will lack fuel in the font carburetor. My first thought is that the fuel pump is not pumping enough volume for both carbs. I am sure someone has a better grasp on this and understands why the back carb would get the fuel it needs as apposed to the front. See Video.  

IMG_0746.MOV

Edited March 5, 2018 by JLPurcell

[Mark Maras]

 My first guess is the filter in the banjo fitting. It's the "last chance" filter where the fuel line enters the float bowl. If that's OK, I'd suggest a fuel pump pressure and volume test. Where is the fuel level compared the the nozzle tops and how many turns down are the mixture screws? 

[JLPurcell]

38 minutes ago, Mark Maras said:

 My first guess is the filter in the banjo fitting. It's the "last chance" filter where the fuel line enters the float bowl. If that's OK, I'd suggest a fuel pump pressure and volume test. Where is the fuel level compared the the nozzle tops and how many turns down are the mixture screws? 

Mark, I did not think about the banjo fitting. I will check that tomorrow. The float levels is set in accordance to the guide that comes with the float sync. 14mm above the body of that tool. It may be 1mm low. The mixture screws are approximately 3.5 turns down. This is where the idle is stable with carbs synced and the color tune has a nice blue with just a slight yellow tone burn in the chamber. The real test will be under load. Thanks for your  continued help with this. JLP

[Captain Obvious]

Banjo filter is a good idea.

Are both float bowl vent nipples hanging open to atmosphere?

[JLPurcell]

41 minutes ago, Captain Obvious said:

Banjo filter is a good idea.

Are both float bowl vent nipples hanging open to atmosphere?

Yes they are.

[madkaw]

Float Sync tool? Glass tubes ? 

Dont let gas sit in them too long or the expoxy gums up that holds the tubes and block the readings- ask me how I know 

[240260280]

Very nice to see real world dynamics of the fuel flow!  BIG THANKS!  Sorry it is for a problem though.

Is this the 72 with the stepped float heights?

Maybe try swapping needle valves between front and back bowls to see if problem moves?

[JLPurcell]

17 minutes ago, 240260280 said:

Very nice to see real world dynamics of the fuel flow!  BIG THANKS!  Sorry it is for a problem though.

Is this the 72 with the stepped float heights?

Maybe try swapping needle valves between front and back bowls to see if problem moves?

;Once, I take a look at the banjo fitting filter, if that does not fix the problem I will order new needle valves and swap them out. Thanks JLP

[Diseazd]

10 hours ago, Mark Maras said:

 My first guess is the filter in the banjo fitting. It's the "last chance" filter where the fuel line enters the float bowl. If that's OK, I'd suggest a fuel pump pressure and volume test. Where is the fuel level compared the the nozzle tops and how many turns down are the mixture screws? 

I agree with Mark.....if you are getting continuous flow to rear carb, the only other variable is a restriction in the route from the fuel rail to the carb. Just had a friend that had the same problem.....new filter in banjo fitting solved the problem. He had just had his gas tank lined at a radiator shop. The red (twizzler looking) liner was dissolving and building up in his filter.

[JLPurcell]

15 hours ago, Diseazd said:

I agree with Mark.....if you are getting continuous flow to rear carb, the only other variable is a restriction in the route from the fuel rail to the carb. Just had a friend that had the same problem.....new filter in banjo fitting solved the problem. He had just had his gas tank lined at a radiator shop. The red (twizzler looking) liner was dissolving and building up in his filter.

I pulled the banjo fitting tonight and the filter is clean. I will order a set for future use but that is not the problem. 

[madkaw]

Am I correct about the type sync tool you are using? 

I was serious about the defect with that tool. You will get erroneous readings . It’s a great tool if it works correctly , but too much time in fuel makes it defectives - at least the one I used 

[JLPurcell]

I have a couple of Uni-sync tools. I always switch between the two to make sure I am getting good flow readings. One is new and the others I have had for over 40 years.