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MG MGB Technical - Fuel Pump Changed - Why is it Doggy?

I not only changed the fuel pump on my 72B, I moved it.

I had one of the little brass block style pumps mounted up by heater motor. Everythin worked fine, except when engine was hot.

I purchased a Carter low pressure fuel pump and installed it back by passenger side battery box, where original pump was.

The carter pump is a P60504 w/pre-filter.

All connections to existing metal gas lines were made with new rubber type gas line. All connections have screw type clamps.

I've observed 2 things (or at least I think I have!).
1. The car feels doggy, like I have to put more pedal to the floor to get equal past acceleration.
2. The car sorta sounds more throaty, like the carbs are sucking more air than before.

Car starts fine, idles fine, pump is steady click. I've checked for loose connection, fuel leak, and all appears just fine.

Is it possible that by changing type of fuel pump and location of fuel pump that I've altered some other dynamics of the fuel supply that requires additional tweaking?

I haven't done a volumn test on fuel delivery prior to fuel entering carbs, but will do that this weekend to determine how much is coming out. I can also pump a pressure gauge on gas line prior to carbs to document actual fuel pressure.

Your thoughts?
R.W Anderson

Unless the old pump was overwhelming the float valves, raising float chamber levels, and causing a rich mixture which you had 'tuned out' with the jet adjustment, then I can't see how just changing a pump would affect driveability. The pump just fills the float chamber, which is a reservoir at atmospheric air pressure. Simple gravity and a lower pressure at the top of the jet is what transfers fuel from the float chamber into the carb throat, i.e. nothing to do with the pump.

If the problem were fuel starvation then pressing the accellerator harder would probably make things worse, as it would just weaken an already weak mixture even more, if the pump were supplying as much as it could anyway. Nevertheless the first thing to do is a delivery check. Remove a pipe from a carb and direct it into a container. With the ignition on the pump should deliver at least one Imperial pint per minute, and in practice closer to two, in a steady stream of pulses with negligible bubbles.

One thing to remember is that your new pump (define 'low pressure') could be delivering too much pressure and overwhelming the float valves now, making things too rich at certain throttle openings where a larger throttle weakens it closer to normal. If that is the case they should eventually overflow with the ignition on but the engine stopped. Disconnect the coil and remove the float chamber vent pipes unless they just drop straight to the ground, then wait a few minutes.
Paul Hunt

If the floats are overfilling it may be that a little dirt got loose during the work on the fuel system and is now holding a float needle open.
John H

The filter before the pump is a bad idea. It can cause starving of the pump and lean out your mixture excessivly. RAY
RAY

What is the pressure spec on the Carter pump? "Low pressure" is different from different manufactures. For a pump mounted at the rear of the car, close to the original mounting point in the MGB the output pressure should be between 2.5 psi and 4 psi (the SU pumps for the same application are either 2.7 psi or 3.8 psi). The OEM pump for all MGBs after 64 should supply 2.4 pints per minute or more (the pumps used in the earlier MGBs supplied 1.1 ppm or more). A filter between the tank and the pump could cause starvation if clogged (so will a filter between the pump and the carburetors) but is not harmful to after market pump as is the case with the SU pumps. For more troubleshooting tips, see the article, Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section on my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

I'll try to respond to this again.... My update from yesterday didn't appear.

The Carter pump I installed is the one recommended elsewhere on the BB or to a link from the BB. It is a 4 psi maximum pump.

The pre-filter comes pre-installed and is required to maintain warranty on pump.

I had to remove factory short fuel line to enable installation of pump to body of car to the side of the right battery box. So the pump is essentially bolted to one of the studs coming from body and the filter is attached by the battery box.

I did find one loose connection, so I'm not sure if that fixed things or if dirt knocked loose in the process has worked its way through system.

In either event the doggy sluggish feel is gone, but the throaty sound remains. Like the carbs are breathing or better able to suck air.

I was thinking about installing an additional clear style fuel filter just in front of the fuel line leading to the carbs, so that I have a visual check on fuel condition/status. But some of the posts above suggest fuel filters (or clogged ones) may starve the system.

Is a 2nd fuel filter over doing it or ?

I still need to double check the numbers on the brass block style pump I removed to make sure it was a low pressure too. And I could still double check pressure and volumn of new pump just so I know.

Thanks for feedback.

BobA
R.W Anderson

A clogged fuel filter *will* likely cause fuel starvation, but that is because it is doing its job i.e. preventing particles getting into the carbs where they could cause more and worse damage. The cure for that is find where the particles are coming from and stop it.

Two filters in series at the carb end would be pointless, only the first one will trap rubbish, the 2nd one will do nothing, and for the same reasons unless somehow the pump is *introducing* dirt then just one at the carbs with one already on the pump inlet is similarly not going to do much. In fact it could simply serve to confuse, filters at the carb end can be anything from almost completely full to almost completely empty, and change from one to the other over time, and yet there be nothing wrong with the way the car runs. An 'empty' filter isn't neccessarily indicative of fuel starvation.
Paul Hunt

This thread was discussed between 24/04/2009 and 29/04/2009

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