Section : Fuel and Additives

Leaking Fuel

Posted By : Andrew Parrish, 01-Sep-2006, 08:34pm

• Leaking Fuel

  Posted By : Chris Taylor, 02-Sep-2006, 08:19am

  To cause the carb to leak in this way, unless you have a crack in the floatchamber somewhere, (most unlikely, and it would do it all the time, the problem can ONLY be the needle valve. As long as you have a standard fuel pump, you CANNOT have excessive fuel pressure. Everything that tends to happen to the pump (wear in pivots, leaking valves etc) will tend to REDUCE pressure. Fitting a thin spacer between pump and block will lower the pressure slightly.
  The most likely cause of problems is muck in the fuel line. The best solution to this is an inline fuel filter (one of the plastic ones with paper filter inside you can buy at any motor spares shop will do fine) as close to the carb as possible.
  It is also possible that the needle valve may be a little worn by now. If you do replace it, make sure you use a new sealing washer behind, and a twist of teflon tape on the threads to make sure there is no leakage past the valve.

  • Leaking Fuel

    Posted By : Andrew Parrish, 02-Sep-2006, 10:06am

    Thanks. I'll have a look again. It's all as standard and I've a few spare carbs to play with. It seemed strange tho' how much fuel appeared when I slackened off the union into the side of the carb. Surely there should only be what's in the pipe from the fuel pump and not a continuous stream? I ended up tightening it back up ready for todays challenge. Could the valve be sticking in the fuel pump and petrol forced up to the carb? Can you tell I'm an artist........?

    • Leaking Fuel

      Posted By : Chris Taylor, 03-Sep-2006, 12:04pm

      Depends on what you mean by a continous stream.........................It may help if you understand how the mechanical fuel pump works.
      As the camshaft rotates, a cam pushes against a lever in the fuel pump. This lever pulls the pump diaphragm down against a spring, and sucks fuel into the pump. As the cam rotates it moves clear of the lever and stops the "pull" on the diaphragm. The actual pumping of fuel is done by the spring which pushes the diaphragm back up, at an approximately constant pressure.
      So, when you loosen the fuel pipe, you will get a pump full of fuel under pressure. Then the flow should stop. IF IT DOES NOT, it can only mean that the tank has become pressurised and is pushing fuel straight through the pump. This would normally be because the vent hole in the filler cap has become blocked and the tank has warmed up since it was filled up. A quick check on this is to release the filler cap. If there is any "puff" or rush of fumes out of the filler, then the cap's vent is blocked. As there is no limit to the pressure that can be generated this way, it could overcome the needle valve in the carb and result in flooding.
      The other reason fuel might continue to flow through the pump is if the car is pointing nose down a STEEP slope, and is siphoning out of the tank. It SHOULD siphon as far as the pump, but it should then be "uphill" to the carb. In this case, there should not be much pressure to overcome the float valve.

      • Leaking Fuel

        Posted By : Andrew Parrish, 04-Sep-2006, 06:05pm

        Changed the float and needle valve Saturday, went to a show yesterday and the same thing happened again whilst it was stood. After listening to numerous reasons why, it was suggested that the tank is pressurising because of the fuel cap. I had noticed it 'hissing' a while ago but just put this down to the extreme heat we were having. This now leads me to my next problem..... I don't seem to be able to get a cap that fits properly. The tabs must be worn or bent in the filler neck and when the original is on, it nearly tightens but a few mm turn clockwise it's straight off again! Tried a gentle 'bend' & tried fitting a 'wasso' locking cap off my other Herald and it won't fit either. Finally got a locking cap off Ebay which fits but now I'm wondering if it was for a later model? Should they all have a breather hole? It's strange as the car is a little beaut' with only 14500 miles and totally unmolested. Despite the leaky fuel, I managed a win in the 'best non american car' of the show. Well chuffed! Thanks for your help.

        • Leaking Fuel

          Posted By : Andrew Parrish, 04-Sep-2006, 09:16pm

          Sorry to go on............. Right, been out playing filler caps and no others will firmly stay on apart from this current locking one. The tabs in the filler neck seem way shorter than two others I can compare with. Must have had someone trying to modify it in the past. The caps just spin round clockwise loosely, don't have a definite stop and come off. The problem (hopefully) must be the current cap as I cannot see how it can breathe in comparison to the others. I guess I have 2 options... either swap the tank, which I don't fancy doing just yet as I've only just got it back on the road, or someone has just suggested drilling a small hole on the outside underneath of the filler neck just behind the cap. Think I'll get the hand drill out for that! Do you think that sounds feasible?

          • Leaking Fuel

            Posted By : Chris Taylor, 07-Sep-2006, 07:15pm

            Sorry to say I do not recommend drilling any hole in the filler neck as that will cause fuel spillage when the tank is brimmed full. You MIGHT be able to cut a small nick in the filler cap seal to allow any pressure build up to dissipate. The filler cap however should have some form of internal venting if intended for a Herald.