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Automotive Vacuum fuel pump problem

  1. May 24, 2016 #1
    Good morning,
    I am new to the forum, so let me get straight into the problem.
    I am working on an old scooter with small displacement (150cc, with about 9.6:1 compression ratio). After rebuilding the engine, I figured I have to put in a much bigger piston 60mm compared to the original 58mm since that's the only oversize piston I have, the cylinder sleeve is pretty worn so that needs to be bored up. In the process, I figure the compression ratio has also gone up since this 60mm has a higher dome. The problem comes from the vacuum fuel pump on this scooter.
    I figured I should change the pump, so I did just that. Since the fuel tank on this scooter is lower than the carburetor, it needs a fuel pump to pump fuel up into the carburetor's bowl. On a stock scooter, nothing bad happens. But when I put in the 60mm piston aforementioned, the carburetor tends to leak fuel out of the overflown tube, a lot. This is a fuel pump that uses vacuum from the intake manifold to push fuel up.
    What is the cause of this problem and how can I solve this? I figured the suction from the intake manifold has to be stronger for the fuel pump to pump too much fuel that the carburetor float cannot handle but if somebody is able to elaborate more, it will be much appreciated. Thank you.
    The small brass tube hanging out of the manifold is where the vacuum line from the fuel pump plugs into.

    This is the aforementioned vacuum fuel pump. The inlet line is on the total left side. the middle line is the outlet (into the carburetor float bowl) and the line on the total right side is the vacuum line that plugs into the small brass tube on the manifold pictured above.

    Attached Files:

  2. jcsd
  3. May 24, 2016 #2


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    Perhaps see...

  4. May 24, 2016 #3
    Thanks but my carburetor is also a new one so I suspect it might be the bigger piston and higher compression somehow making the vacuum pump stronger.
  5. May 24, 2016 #4
    I have read some articles online and taking in new information about fuel flow and fuel pressure. I figure maybe the fuel pressure is too much for the float to handle? Maybe I can try a bigger inner diameter fuel hose to try lower the pressure?
  6. May 24, 2016 #5


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    If the fuel pressure is too high for the carburetor float, changing the diameter of the fuel line will not solve your problem. I suggest you try re-installing your original fuel pump to see if that solves your problem.
  7. May 24, 2016 #6
    Hi. Thanks for replying.
    I did try installing the stock pump but it overflows the carburetor so I ordered a replacement exactly like that from the manufacturer (SYM). Still overflows though.
    Any idea on how to lower the fuel pressure acting on the float and the needle valve? I am using 6mm ID fuel line. If bigger fuel lines won't help then is there some sorts of small size fuel regulator (not the return style) that can lower the pressure?
  8. May 24, 2016 #7


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    I think it will be hard to adjust the fuel pressure so that it is "just right". If you still have the old carb I would try modifying it as per the video I posted, or just try swapping it back on to see if that works.

    Before you do that check that the float in the new carb hasn't got a puncture or similar that's preventing it working.

    Perhaps also check the float needle tip? I think it's rubber. If that was damaged it wouldn't shut off properly.
    Last edited: May 24, 2016
  9. May 27, 2016 #8
    Does your piston have rings? Maybe you can increase the gap to bleed off some of the vacuum. What is your piston skirt clearance? You might be out of spec on the tight side.
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