I bought these SU carbs used and have not run them yet.  The visual inspection looks fine and I had Rebello do the big bore conversion and give them a good once over.  The motor has never run so a backfire is to be expected while i get things sorted out.  What concerns me is that on backfire fuel is shooting out of the banjo fittings on the sides of the carbs when the car backfires.  My first thought was to replace the banjo crush washers, but if i seal those up tighter where is that pressure going to go? There aren't any leaks with the regular fuel pressure using an electric pump.

Edited June 9, 20159 yr by Kurbycar32

Awesome, ill just order one of those kits.  I stole this from another website that includes parts listing

I'd be hard pressed to believe that a backfire could cause fuel to shoot out of the banjo connection. I don't understand the path of pressure that could possibly make that possible.

 

If you've got atmosphere venting nipples on the fuel bowl lids, it's impossible to build any significant pressure in the bowls. And even if you did, how would that pressure get backwards through the needle valve to the banjo connection?

 

How far is this fuel shooting, and are you sure it's got anything to do with the backfiring?
 

I'd be hard pressed to believe that a backfire could cause fuel to shoot out of the banjo connection. I don't understand the path of pressure that could possibly make that possible.

 

If you've got atmosphere venting nipples on the fuel bowl lids, it's impossible to build any significant pressure in the bowls. And even if you did, how would that pressure get backwards through the needle valve to the banjo connection?

 

How far is this fuel shooting, and are you sure it's got anything to do with the backfiring?

 

If indeed the backfires do coincide with the fuel squirting out of the Banjo bolt fitting, here is what I " Think " could be happening.

 

The backfire introduces a sudden release of gas bubbles in the fuel inside the float bowl.

 

This does two things.

 

1: Causes an immediate geyser like fountain of fuel that sprays upwards from the bottom of the fuel bowl. Via the fuel hose connected to the Jet holder.

 

2: The gas bubbles cause an immediate and dramatic decrease in the density of the fuel because of aeration. Thus the float drops to the bottom of it's travel. This of course leaves the Needle valve open, and that is how the fuel spray can get past the needle valve. The Fuel Bowl vent tube would not hamper the event from happening...although I would expect to see fuel being spayed out of the vent tube as well. However, if drain hoes are connected as per the stock setup, you may not notice the fuel dripping out of the vent hoses as they drain underneath the heat shield. 

 

It's the same phenomena as Oceanographers have discovered in Volcanic regions of the Ocean floor. Sudden massive release of Volcanic gases or Methane pockets can sink a large ship in seconds.

 

I don't know if this is what is really happening... but this could be one possible explanation for this weird problem. More of a " Brain Teaser " than anything else. Just throwing some ideas out...

Edited June 10, 20159 yr by Chickenman

Awesome, ill just order one of those kits.  I stole this from another website that includes parts listing

Hey, I appreciate that diagram!

 

I had no idea where all those smaller washers went.

Good topic, this clears out the problem i had years ago Almost burned the whole car.

Are the float lid vents open and clear?  Nothing should be squiring out of the bowl lid at all.  Be sure to buy extra lid gaskets.........sounds like you will be needing them

That gasket kit did fix the problem and it was dirt cheap, only $7.30 each at Courtesy Nissan.  At the same time i fixed my backfiring issue so the motor runs pretty good and fuel doesn't squirt everywhere

What was ultimately causing the backfire?

Swapping the distributor solved the backfiring issue.  I disassembled the old unit and found the reluctor was all jacked up, the vacuum advance was frozen solid and everything inside was rusty.  Whats crazy is that old dizzy was running on that car before the motor swap, fully daily driver status.