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MG MGB Technical - Fuel pressure?

Can a SU fuel pump have too much pressure? Having problems with the front carb over flowing. Sometimes the rear as well. Have installed new needles, seats, and floats. All are new SU manufacture. The only thing not new is the SU fuel pump. It was a used spare of unknown vintage. It appears exactley the same as others that I have. The pump on the car did not work, and I used this "spare" it is an SU and either came from a B or a C. Never have had this problem before. Does not always happen, but does happen too frequently for me to feel safe even with correct overflow tubes. As a side note the new needles and seats are different than the old style, new ones are plastic body with metal tips. Old ones were metal body with rubber tips. Both have a spring loaded ball/tip on the float side. Floats are plastic like the old ones, with no way to set float level.

Saftey Fast

Ron
RJS Smith

Ron - The high pressure and the AUF style fuel pumps come in two different pressures, either 2.7 PSI or 3.8 PSI. Unfortunately, there is no way to tell them apart by looking at them, unless the ID tag is still on the pump. It will either be an aluminum tag attached under two of the screws that hold the coil housing to the pump body or a metalized sticker on the end cap. If either of those are present, send me the part number that is on it and I can tell you what the pressure is. The MGB uses the pumps that are 2.7 PSI, but the 3.8 PSI pumps do not normally cause a problem. If you have a pressure gauge that reads 0 -10 PSI (such as a vacuum/pressure gauge), You can attach it at one of the carburetor connection while leaving the other connected and trying to read the pressure that way. This is not always successful, but worth a try. There is no way that the pressure from aSU fuel pump will increase over time unless someone has replaced the volute spring with the wrong one. See my article on SU fuel pumps at: http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm
Good luck - Dave
David DuBois

Unlikely. If only one carb then the float or valve on that carb is almost certainly the problem. Spring loaded balls indicate Gross 'jets' (?) which some swear by and others swear at.
Paul Hunt

Dave thanks, I will look and see if the tag is still on the pump body tonight after work. Paul the needles are not Gross jets, I know what those are and have not had good luck with them. The needles / seats, and floats are new from a known supplier here in the states and were in SU marked wrappings. The car does not always do this, sometimes with the pump on engine not running, other times after the car has run for over 5 minutes.. The car is just coming out of a long re build. I even tried a second set of float lids from my spare set of carbs, same results. Before ordering a new pump I will swap this one with the one on my roadster, I know it is correct.

Ron
RJS Smith

Ron,

I experienced the same problem just after replacing the needle valve with new SU parts. The front carb always over flows after long run.

I opened the float lid and found that the new needle was not moving smoothly inside the new seat, the clearance between the needle and the seat is a little too large making the needle tilt and stucked open. I just replaced the new seat with the old one and the problem never happen again.

Ennio
Ennio Wong

Ennio may have hit on the problem. Check the float drop vs the distance the needle needs to move to open fully. If there is a much larger float drop, then the needle can get jammed and cause flooding (this is a real common problem on the T series cars). The problem can be solved by adding shim washers under the valve seat to move it (and the needle) closer to the float. Good luck - Dave
David DuBois

I checked the float drop, and the needles need to move about 1/2 of the distance of the seat body. (fully open to fully closed)I looked at a couple of other sets of carbs, one set of old HS4's had washers under the seats. Two sets of HS6's did not. The other set of H6's have a completely different style of float bowl / lid. The floats when fully up (needles seated) have very little clearance from the top of the float to the lid. This is both the HS4's and the HS6's. I checked the numbers on the SU fuel pumps, they are AZX-1318 (the pump causing trouble) AZX-1307 spare pump, and the working pump on my roadster, and an other spare AUF-305 which I believe to be a positive ground pump from my A. The car with the problem is a C, but from what I know the float chambers and all parts, and the fuel pump as well are the same as the B parts.

Ron
RJS Smith

Ron - The AZX 1307 and AZX 1318 are both 2.7 PSI pumps with the 1307 being negative ground and the 1318 a positive groung pump. I don't have the specs on the AUF 305 pumps, but they shouldn't be polarity sensitive since the only arc suppression they used was a swamping resistor and a capacitor. The AUF 305 pump may have been a 3.8 PSI pump, but you would have to send a query to Burlen Fuel <http://www.burlen.co.uk/> to verify that. The AZX 1318, being a positive ground pump used in a negative ground car should not cause the problem since all that does is to negate the diode's arc suppression capabilities, but it shouldn't cause an increase in pressure. The possibility still remains that someone in an earlier time in the pump's life changed the volute spring to the heavier one. Cheers - Dave
David DuBois

I was going to say that you can get premature failure with branded SU parts just like with anything else, I had a new float valve start leaking just a few weeks after fitting it, replacing it again has been OK since. But if changing the float lids complete with valves and floats *still* gets a leak from the front carb, then it can't really be the float or valve without a huge co-incidence. I take it it *is* leaking out of the overflow port on the float lid and not from elsewhere e.g. the jet pipe connection, and you have the fuel and overflow hoses on the right ports.

AFAIK using a diode-suppressed positive ground pump in a negative ground car will cause the wiring to cook in very short order as the diode effectively shorts the unfused white to ground.
Paul Hunt

Paul - Reverse polarity on a pump with a diode installed will not necessarily cook the wiring. I have purposely tried it on my test stand to see what happens because I have received pumps with diodes that have been used in cars with the oposit polarity. The diode assembly also has a limiting resistor in series with the diode (I broke one open) which appears to limit the current to safe values. What reverse polarity does cause in pumps with diodes installted is to increase the arcing and cause the points to deteriorate sooner. Cheers - Dave
David DuBois

The pump is working well, despite being in a negative ground car. No wiring issues. Not sure where this one is from (AZX-1318) perhaps a left over from a early B. New parts can be bad, have had that happen before. Tonight I will try and swap the complete lids between the B and the C and see if I still have the overflow problem. Will replace the pump anyway. Carbs are plumbed correctly, and the overflow pipes are installed (a real pain on a C) overflow is pouring out directly from the front carb tube all over the floor...

Ron
RJS Smith

Thanks David.
Paul Hunt

One year ago I bought a brand new set of AUD465 carbs and the front carb overflowed through the vent tube. I found a tiny piece of clear plastic holding the needle open, probably some packing material or something used in the manufacture. After cleaning the needle the carbs have worked perfectly.
Ken T
Kenneth Thompson

One reason why my first step on getting overflow is to disconnect the pump, run the engine till the carbs empty, then reconnect the pump. The rush of fuel through the now wide-open valve can be enough to shift any debris. Mind you, it might only shift it to cause problems elsewhere.
Paul Hunt

This thread was discussed between 12/01/2004 and 15/01/2004

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