~ So we now come to my question to you. How can I her to idle
~ at say 800rpms? I've recentralised the jet, adjusted the
~ mixture, idle screws, but the rear carb's piston is still that
~ 1/8inch higher. The carbs are SUs with the remote floats if
~ that helps.
Diagnostic:
Turn off the car and look down the throats. Does the rear carb
settle at the same level as the front with the power off and no
air flowing through?
If not, you've got a hangup somewhere in the dashpot; strip the
carb and see what's causing it. But that's unlikely.
More probably, both carbs fall to the bridge when you turn the
power off. That means that you are getting additional air through
the system somewhere, a vacuum leak or a worn throttle plate.
You should also make sure that you are getting full spark at
all four cylinders. I recently got a chance to admire a list
member's '66 MGB and, on the test drive, noticed that it was
only running on three cylinders. (But very smoothly! :-) We
traced it down to a spark plug that had been gapped too close;
there was still a misfire that we eventually identified as due
to worn plug wires, but I haven't checked that one out since
he ordered the new pieces.
The reason I bring this up is that when I pulled the air
filters, it was obvious that the problem was in the two front
cylinders because the rear carb was about 1/8" up from the front
one at idle. In that case, it was because the #2 cylinder wasn't
firing. I did a by-ear mixture adjustment and it ran better, though
in my opinion we still had some ignition playing to do and then it
would be right to go back and re-do the carbs to take the new
ignition components into account.
Problem areas, BTW, include the low-tension lead ("pigtail") from
the coil into the distributor. That's made from very, very fine
wire with a cloth insulator; the wires tend to break after 20 years
or so, and if they break inside the insulator you never see it. This
will cause rough running and eventually make the car stop working
altogether.
Moral of the story: it isn't always the carbs, no matter what you've
heard. In fact, it isn't USUALLY the carbs... When you're sure it's
the carbs, check the ignition system, and THEN do the carbs if only
because you'll need to readjust them now that you've fixed whatever
ignition problem was making you think it was the carbs. (Right, Marcus?)
--Scott Fisher
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