SU HS6 fuel pressure
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    1. #1
      Junior Member mathue's Avatar
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      SU HS6 fuel pressure

      I seem to find multiple pressure numbers for these carbs in the net. (B20F converted engine)

      Why do I want to know?

      Currently my mechanical pump is putting out 6+ PSI and that seems high to me.

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    3. #2
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      The 1971 140 factory service manual specifies a fuel pressure between 1.5 and 3.5 psi. Technically, that is not for the HS6; but, the later HI series SU which replaced the HS6; however, the service manual does not differentiate between the SU and the Stromberg on the 1971 cars and I think the mechanical fuel pump remained the same through out the B20 production run. Your 6 + psi does seem a double nudge on the high side. Are you using a low pressure fuel gauge for the measurement or one meant for testing fuel injection systems? I have an FI pressure tester with full scale pressure of 100 psi. Accuracy is probably +/- 1.5 psi which is a significant error at 3.5 psi. If you are using a low pressure gauge, typically -14.7 psi to +10 psi then your 6+ psi reading is probably real and you potentially have an issue.
      A 142 of course. What do you expect? I'm the 142 guy. / 1971 142 E 102 color

    4. #3
      Junior Member mathue's Avatar
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      Quote Originally Posted by 142 Guy View Post
      The 1971 140 factory service manual specifies a fuel pressure between 1.5 and 3.5 psi. Technically, that is not for the HS6; but, the later HI series SU which replaced the HS6; however, the service manual does not differentiate between the SU and the Stromberg on the 1971 cars and I think the mechanical fuel pump remained the same through out the B20 production run. Your 6 + psi does seem a double nudge on the high side. Are you using a low pressure fuel gauge for the measurement or one meant for testing fuel injection systems? I have an FI pressure tester with full scale pressure of 100 psi. Accuracy is probably +/- 1.5 psi which is a significant error at 3.5 psi. If you are using a low pressure gauge, typically -14.7 psi to +10 psi then your 6+ psi reading is probably real and you potentially have an issue.

      Thanks a bunch! I'm using a low pressure gauge 0-15psi and I felt that was way too high. Oddly the fuel bowls are not flooding.

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    6. #4
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      If the needle valves on the float bowl are new and a very good fit in their seats, the surface area of the needle in the seat is so small that fuel pressure is unlikely to force the needle off the seat when the needle is held in place by the floats when the bowl is full. If the diameter of the opening in the needle seat is 1/8" the area of the needle is .012 sq in and the total opening pressure on the needle at 6 psi is only 0.073 lb / 1.17 oz. However, if there is contamination in the seat area or the needles get worn (if they are Viton tipped the Viton can get hard and fail to seal) so sealing against the higher pressure might become a problem. The higher fuel pressure might not push the needle open; but, increase the leakage rate across the sealing surface of the needle. The needle valves might not leak now if they are fresh; but, might become more of a problem with the higher pressure in the future as things age.
      A 142 of course. What do you expect? I'm the 142 guy. / 1971 142 E 102 color

    7. #5
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      Mathue;

      Good calculations and info from 142g...did you measure Fuel Pressure by "T"ing in while under Carb load, or by dead-heading the Pump into Pressure Gauge?...under carb load (even if only Idle consumption) that could drop pressure slightly...probably no news, and maybe I overlooked it, but mechanical Fuel Pump needs to have the spacer in place.

      Cheers

    8. #6
      Junior Member mathue's Avatar
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      Quote Originally Posted by Ron Kwas View Post
      Mathue;

      Good calculations and info from 142g...did you measure Fuel Pressure by "T"ing in while under Carb load, or by dead-heading the Pump into Pressure Gauge?...under carb load (even if only Idle consumption) that could drop pressure slightly...probably no news, and maybe I overlooked it, but mechanical Fuel Pump needs to have the spacer in place.

      Cheers
      This was measured at the front carb with the hose removed and then directly connected to the gauge . Performed at idle with the fuel that remained in the bowls.

    9. #7
      Junior Member mathue's Avatar
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      Quote Originally Posted by 142 Guy View Post
      If the needle valves on the float bowl are new and a very good fit in their seats, the surface area of the needle in the seat is so small that fuel pressure is unlikely to force the needle off the seat when the needle is held in place by the floats when the bowl is full. If the diameter of the opening in the needle seat is 1/8" the area of the needle is .012 sq in and the total opening pressure on the needle at 6 psi is only 0.073 lb / 1.17 oz. However, if there is contamination in the seat area or the needles get worn (if they are Viton tipped the Viton can get hard and fail to seal) so sealing against the higher pressure might become a problem. The higher fuel pressure might not push the needle open; but, increase the leakage rate across the sealing surface of the needle. The needle valves might not leak now if they are fresh; but, might become more of a problem with the higher pressure in the future as things age.
      Sweet calculations!

      The needles are all metal

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