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Kurbycar32

Fuel shoots into engine bay on backfire

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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 by Kurbycar32
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Interesting. Hard to understand how backfire pressure makes it to the fuel bowl or into the fuel supply lines.

Well one thing I can say with some certainty, if fuel is actually squirting out of the banjo seals, they are loose and need to be tightened. I'd take it apart and make sure there is no crap on the seal surfaces preventing the seal.

Edited by zKars

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My SUs have a vent nipple on the float chamber lid with a hose that connects to the air filter housing.  If I'm thinking right the banjo fittings let the fuel into the float chambers so any pressure should come out that nipple on the lid.

 

I tightened the heck out of mine when I rebuilt them and thinking back there's 2 per banjo bolt, one on each side of the fuel inlet.

 

mk0XlSuO_Fab784UEoHhbvQ.jpg

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Replace the crush washers with new, Make sure that you get soft aluminium crush washers or soft copper.

 

Some " brake banjo " crush washers are made of a harder  alloy to seal high pressures, but they may not seal properly because the Carb Banjo bolts are going into an aluminium boss and you can't tighten them enough. The harder brake washers are for Iron calipers.

 

The reason you are getting fuel blown out of the Banjo fittings is a result of the " Backfire " creating back pressure in the fuel bowl. It will send a positive pressure pulse back through the Jet holder and tubing, directly into the Float Bowl and cause a " Fuel Geyser " like Old Faithful....

 

Edit: As others have pointed out, a backfire can pressurise the float bowl... but it shouldn't cause fuel to squirt out of the Banjo fittings.

Edited by Chickenman

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Replace the crush washers with new, Make sure that you get soft aluminium crush washers or soft copper.

 

Some " brake banjo " crush washers are made of a harder  alloy to seal high pressures, but they may not seal properly because the Carb Banjo bolts are going into an aluminium boss and you can't tighten them enough. The harder brake washers are for Iron calipers.

 

The reason you are getting fuel blown out of the Banjo fittings is a result of the " Backfire " creating back pressure in the fuel bowl. It will send a positive pressure pulse back through the Jet holder and tubing, directly into the Float Bowl and cause an " Fuel Geyser " of sorts...

So would the overflows on the lid relieve any pressure he's worried about?  Is there a difference in fuel bowl and Float Bowl?

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Exactly.  If the pressure is currently exiting the fuel system and i seal that, is it just going to blow a gasket between some of the carb parts?

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Shouldn't need to torque them down that much, if so the washers are bad or wrong, or else there's an imperfection on the mating surfaces. Had to polish mine with 600 grit wet/dry sandpaper and use new washers, after that normal torque was OK.

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Thanks, Site

I didn't have time to try to find that picture. If the vents are open the fuel aught to come out there not out the banjo bolts. Tighten them down

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Ill see if i can find some soft aluminium or synthetic crush washers and re-tighten everything.  Ill update when that happens

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I just bought 2 sets of carb gaskets from my local Nissan parts guy. Google p/n 16455-H2310 and look at kit. They were $3.65 each. Crush washers, float camber lid gaskets and something else?

Cliff

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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?
 

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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 by Chickenman

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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. :huh:

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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

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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

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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.

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