First of all, many thanks to everyone who responded with helpful suggestions
to my last message - I'll try to provide some detailed information about
what I have found since then after another weekend of thrashing with this
beast......
To sum up the problem (for those who missed the last episode) I have a 1973
MGB that I purchased in non-running condition. The previous owner (PO) tried
to get it going, but did not succeed and used some questionable techniques.
I have cleaned up all the messes I can find and think of, but the car is still
only running on cylinder #2 (I verified this by removing the plug wires from
the other 3 plugs and there was no perceptible change in running). The car
runs rough on #2, will only run with the choke full-on, and occasionally
backfires out the forward carburetor. After running on #2 for a while,
I checked the plugs and found #1 and #2 dry, #3 and #4 wet with gas.
This problem has been the same with the old parts as well as with the new
parts, so I don't think I have defective new parts (plugs, cap, etc.)
The engine in my MGB is an 18V672Z
The distributor is a 25D, #41491A, the vacuum advance diaphragm is good.
I have SU carburetors, I think they are HS4's, there is a metal # tag on one
that says AUD135R. There is an aftermarket electric fuel pump, the float
bowls are both full of gas.
My compression tests out at 110, 115, 115, 120
I checked my valve clearances and they are all fine, the valves are all going
up and down in apparently (as far as I can see) correct order. There is no
sign that the PO messed with the timing chain cover or the valves.
I have the distributor cap wired 1-3-4-2, counterclockwise with #1 on the cap
at about the two o'clock position. I took out all 4 plugs, grounded them,
cranked the engine, and observed spark on all of the plugs.
New parts: Distributor cap, rotor, plugs (gapped at .025), plug wires, points,
condenser. The dwell is set for a .015 point gap, verified at close to 60
degrees while sputtering along on #2 only.
My car has been "detoxed" as some people put it - the only hose connected to
the intake manifold log is the one for the vacuum advance line - all of the
others are plugged. I don't know what other "detoxing" is recommended....
I went through the static timing ritual from the Haynes manual, and everything
seemed to set up properly. The rotor is pointing at the #1 segment when the
timing marks are aligned and the valves on #1 are closed at this point.
Soooooo, that's all of the information. Now, what the heck is the problem?
I've been through and through the electrical end of things, and the timing, and
the compression - I guess all I have left is carburetion? I was concentrating
on the electrical/timing aspects because I didn't see how bad carbs could
conspire to make only #2 fire. The only scenario I can imagine is: cylinders
intake in 1-3-4-2 order, let's imagine that the forward carb is plugged and
the back one is spitting out mostly gas. This fouls 3&4, then 2 draws across
the bypass tube and gets just enough gas to run, but there is none left for
#1? Ok. That's maybe a long shot, I just made it up, but it leads into my
next question....
What is the best thing I can do with these carburetors of unknown pedigree?
Should I attempt to clean them? How? Rebuild them myself? Is there someplace
that rebuilds them reasonably? Should I give up and get an aftermarket
Weber (if a reasonably priced one exists?) Am I even on the right track?
I'm not trying to turn this car into some sort of high performance monster,
I just want to cruise around the back roads of New England in the summertime...
Sorry to rattle on so long about this, just trying find the best course of
action...
Jon Swanson
swanson@zonker.ecs.umass.edu
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