?? Changed my Fuel pump

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MGB & GT Forum: ?? Changed my Fuel pump
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Sep 04, 2010 05:37:37
Chas74mgb
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I replaced my old SU fuel pump with a new solid state pump from m local NAPA store.
Seems to work well enough but it appears my gas milleage has been reduced, I think my carbs are running a little rich.

Here comes the dumb question from a novice....Would the change in fuel line pump pressure affect my carbs?

Sep 04, 2010 06:12:16
kuz1
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If you got the right fuel pump, you shouldn't have a significant pressure change. if you got a high pressure pump you could have a problem. in theory you shouldn't get richer because the float system should still regulate the bowl level. in reality you may be pumping right past the float valves.





Sep 04, 2010 07:06:59
1980owner
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As Keith said, the float should "turn off" the fuel supply to the carb bowl when it is full.

However, I'll just throw this out there, I assume that the solid state pump is like my aftermarket parts store pump which does not shut off when the proper line pressure is achieved. So I would think it is entirely possible that it is giving a little more pressure to the carb than the SU pump was.

Sep 04, 2010 07:43:22
kevin58
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Ditto... higher pressure than your SU pump. The PO of my car put in an after market pump and like John just said, it does not shut off when proper pressure is reached.

Sep 04, 2010 08:45:20
NNN
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Your pump should be putting out no more than four lbs. of pressure. I just installed a Delphi pump from NAPA on my '79B. Havent had any problems and I can tell the difference.

Sep 04, 2010 08:49:41
kuz1
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Quote: "
Ditto... higher pressure than your SU pump. The PO of my car put in an after market pump and like John just said, it does not shut off when proper pressure is reached."

The cube pumps do not shut off to regulate pressure, they bypass.The fact that it doesn't shut off has little to do with it's maximum pressure.

Sep 04, 2010 09:04:59
Waterfowl
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Can you be so kind as to explain further? Bypass?

Sep 04, 2010 10:31:22
ddubois
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Quote: "
Can you be so kind as to explain further? Bypass?"


The SU fuel pumps use a volute spring to push the fuel (via the diaphragm) out to the carburetor. When the back pressure from the closed needle valve in the carburetor float chamber equals the pressure exerted by the volute spring, the pump stops running until the back pressure is relieved. Some of the after market pumps, such as the Facet, use a piston to push the fuel from the pump to the carburetor. The piston has a "relief" valve in it that is controlled by a spring. When the back pressure from the closed needle valve exceeds the relief valve spring pressure, the valve opens and the piston is free to cycle without moving any fuel until the needle valve in the float chamber opens. Thus the Facet pump will chatter on regardless of whether or not the needle valve in the float chamber is open or closed. The rate and tone of the Facet pump will change slightly between moving fuel, or pumping into a closed circuit or even when it is pumping air, but it is such a slight change that it is hard to head when the car is running.
Cheers,

Sep 04, 2010 15:07:04
golf
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American cars commonly run about 6 psi fuel pressure..
With most after-market pumps a regulator is needed on SUs.
Holley regulator (low press range) price 29.95..
In my experience a pressure of 2-2 1/2 is sufficent and no problems on SUs.
About same with Weber.

Sep 06, 2010 17:23:07
Chas74mgb
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Thanks everyone for your input thus far.
Driving it this weekend I found my car to run well at times, other times appears to stumble a bit. At one point the car " dieseled" upon turning off the key. It appears to me the carbs need adjustment with this new found fuel pressure.

I'm pretty confident in the mechanic who installed it. "The Beard" has been working on these things for years. I trust his judgment.

Any advise on my SU carbs?

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