Vacuum Fuel Pump Issues with Oversize Piston

In summary: The small brass tube on the manifold pictured above is where the vacuum line from the fuel pump plugs into. 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.The cause of this problem and how can I solve it is unknown, but the small brass tube hanging out of the manifold is where the vacuum line from the fuel pump plugs into. Perhaps see...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.I have read some articles online
  • #1
Bach Pham Thien
4
0
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.
IMG_20160519_072658.jpg

The small brass tube hanging out of the manifold is where the vacuum line from the fuel pump plugs into.

$_57.JPG

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.
 

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  • #2
Perhaps see...

 
  • #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.
 
  • #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?
 
  • #5
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.
 
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Likes Bach Pham Thien
  • #6
JBA said:
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.
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?
 
  • #7
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:
  • #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.
 

1. What is a vacuum fuel pump?

A vacuum fuel pump is a mechanical device that uses pressure differences to transfer fuel from the fuel tank to the engine of a vehicle or other machinery. It is commonly used in older vehicles and small engines.

2. How do I know if my vacuum fuel pump is not working properly?

Some signs of a malfunctioning vacuum fuel pump include difficulty starting the engine, engine stalling, and loss of power while driving. You may also hear a clicking or whining noise coming from the pump.

3. What are the common causes of vacuum fuel pump problems?

The most common cause of vacuum fuel pump problems is a clogged or dirty fuel filter. Other potential causes include a faulty pump, a damaged fuel line, or a faulty fuel pressure regulator.

4. How can I troubleshoot and fix a vacuum fuel pump issue?

First, check the fuel filter and clean or replace it if necessary. Then, inspect the fuel line for any damage or leaks. If everything appears to be working properly, the issue may be with the fuel pump itself, which will likely need to be replaced.

5. Can I prevent vacuum fuel pump problems?

Regular maintenance, including cleaning or replacing the fuel filter and checking the fuel line for damage, can help prevent vacuum fuel pump problems. It is also important to use high-quality fuel and avoid letting the fuel tank run empty, as this can put additional strain on the pump.

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