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

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 June 9, 20159 yr by zKars

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.

 

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 June 10, 20159 yr by Chickenman

  On 6/9/2015 at 5:23 PM, Chickenman said:

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?

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?

Do your float lids have the small vertical vent?

Like this?

 

 

 

Yup those vents are present

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.

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

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