[Mg-t] SU Low Pressure Fuel Pump

[Dave and Liz DuBois]

Stu,

You have two pumps that were made several decades apart.  The pump with 
the brass rollers was a very early design.  Later pumps came with a four 
lobe plastic armature guide with the lobes attached together such that 
is resembled a square.  Later still (in 1985) they started using the 5 
figure eight guides.

> My question, are rollers necessary/important? The six bolts that marry the whole assembly together should align the diaphragm anyway . . . Or am I just assuming too much?

Yes, you are assuming too much.  The holes in the periphery of the 
diaphragm are clearance holes for 2BA screws rather than alignment holes 
and as such will not hole the diaphragm, or more important the armature 
of the diaphragm assembly perfectly centered in the coil housing.  
Unless the armature is perfectly aligned, it can start hitting against 
the core piece in the coil.  If you look at the coil core, you will see 
that it projects beyond the bottom of the core and there is a 
corresponding relieved area in the center of the armature on the 
diaphragm.  If the edge of the relieved area on the armature is 
misaligned it will start hitting on the outside edge of the coil core, 
with will cause the pump to become unstable and eventually quit 
pumping.  The armature guide pieces, whether the original brass rollers 
or the newer figure eight plastic pieces, keep the diaphragm perfectly 
centered until the coil housing/diaphragm assembly is attached to the 
pump body are screwed together.  The guide piece could be removed after 
the pump is completely assembled, except they haven't figured out how to 
do that once the pump is assembled

As far as the plastic covered diaphragm vs. the diaphragms without the 
plastic cover - the plastic cover was an attempt to keep the fuel from 
coming in contact with the diaphragm material.  This was originally 
necessary because the diaphragm would not withstand the newer formulated 
fuels.  The diaphragms on the newer diaphragms (again starting around 
1985) have a different diaphragm material that is resistant to the fuels 
of today.  The older sandwich style diaphragms with the plastic cover 
also had two layers of the elastometer material and over time the two 
layers would fuse together, causing the diaphragm to become stiff to the 
point that the pump wouldn't operate properly.

Another difference in the two pumps that you may not have noticed is the 
diameter of the cores in the two coils.  The early pump has a core that 
is about 1/2" in diameter, while the later pump has a core diameter of 
nearly 3/4".  Looking at the armature of each diaphragm, the older one 
should be flat on the top up to the edge of the relieve area that fits 
over the projecting section of the core.  The late diaphragm should have 
a raised ring around the relieved area of the armature.  The result of 
this change is to produce a pump that pumps much faster than the earlier 
pumps, while putting out the original low pressure pumps.

I would advise against routinely disassembling the pumps to perform 
adjustments and cleaning of the points unless there are other problems 
being experienced.  There are just too many things that can go wrong 
when reassembling the pumps, and it is not necessary.  Among other 
things, the procedure for adjusting the points has changed with the 
advent of the newer points assembly that have adjustment tangs on them.  
If you want to e-mail me at SUfuelpumps at donobi.net, I can send you a 
procedure for cleaning the points that will work much better than just 
dragging a piece of sand paper between them and an adjustment procedure 
to use with the newer points assembly.

Finally, if you are experiencing repeated problems with you fuel pump, 
it is perhaps time to contact me about restoring your pump and 
converting it to solid state.

Cheers,
Dave