I’ve had lots of opportunity over the years to come to grips with MGs
which stopped running when they shouldn’t, weren’t running well or at all,
or which had a hole in need of patching. I’ll be writing about some of these
experiences, and what I’ve learned and applied. I hope you find it helpful.
One impressive fact about our MGs is our wealth of technical reference
material written by devoted MG owners. It’s very hard to find a topic that
hasn’t been touched on before, and there are many experts in the club on
whom we can rely. Often, getting the answer to an MG technical question is
largely about finding a knowledgeable person or article on the topic. My
approach to technical issues follows that pattern.
year, a friend asked me to improve the cold starting and running of his car.
We took a test drive, and noted many shortcomings. The engine started after
a lengthy coaxing, idled roughly when cold, accelerated with less than full
power and idled too slowly when hot. At a hot idle, there was more valve
clatter than one would find enjoyable, and one valve was particularly loud.
After taking the typical tune up steps, I focused on the carburation issues
including the operation of the choke.
After the usual carburetor tuning, the engine continued to be very slow
starting from cold, and would backfire through the carburetors when the
throttle was quickly opened. The backfiring condition, a lean backfire,
caused me to pause and reflect about what was taking place.
Backfiring through a carburetor results from an ignition explosion
occurring when an intake valve is not shut. Or it could possibly be fuel
igniting in the intake manifold rather than the combustion chamber. I
rechecked the ignition timing, found it correct, and decided this lean
backfire was likely to be carburetor related.
a cold engine and the choke on full, why would the fuel mixture be lean?
This seemed incongruous. I went back through the basic settings of SU
carburetors without positive effect, and then concentrated effort on the
operation of the jets. When the choke is pulled, the choke levers lower the
jets to enrich the fuel mixture. Were the jets not fully opening when the
choke was pulled, were the levers and jets operating consistently, or were
the jets sticking or closing at varying levels? Operating the choke levers
by hand revealed the jets appeared to be fully lowering, so why would fuel
I then rechecked the float levels because the level of fuel in the bowls
corresponds to the fuel level in the carburetor. The float settings appeared
correct. However, due to the difficulty I was having with cold starts on
this engine, I wished to confirm the correctness of the fuel level and rule
out this setting as a source of the lean fuel mixture.
My research led me to good advice from Skip Burns and Dave Braun. Their
articles, which helped me solve this issue, were available on the Web and
links are provided below.
Here are the steps I followed for correcting the fuel levels in these
carburetors. They apply to all MGTs.
1. The float levels should be set according to the
standard SU carburetor instructions.
2. Exercise the jet levers and observe the jets in
operation. The jets will move in relative unison if the lever linkage is in
good shape. [If the jets stick open a bit—a condition often not detected—the
engine will have an uneven rich idle. Perhaps this will be the subject of
3. Remove the dampers and pistons from the
carburetors. Both must be removed because the fuel level in each carburetor
must be checked independently.
4. Select a carburetor and look
directly down into the carburetor body at the point where the needle is
inserted into the jet. While doing so, grasp the choke lever from below and
move the jet up and down, observing the jet rising and lowering within the
5. When the jet is fully lowered, you should see fuel
within the jet bearing above the top of the jet. Perhaps you will see the
fuel level is high and near the top of the jet bridge, perhaps it is barely
covering the jet, or perhaps you cannot see the fuel. Check the other
carburetor and most likely, you will find a fuel level varying from the
6. Reset the float bowl levers as needed to equalize
the fuel levels above the heads of the jets when the jets are at their
lowest point. This can be done by eye alone or measured by using a small
screwdriver or slide caliper. Any fuel level above the head of the jet is
satisfactory according to Skip and ideally will be 3/8 inch below the jet
bridge. Also, I have been setting the fuel levels equal to the carburetor
with the highest fuel level. After an adjustment, put the lid back on the
float bowl, and switch on the ignition to activate the fuel pump and fill
the float bowls. Recheck and readjust the float levels until you are
7. Reset the fuel mixture of the carburetors.
I’ve had success with this procedure on four MGs now including a TF, TD,
and TC. You can expect your MG will start faster, particularly in cold
weather, and idle more evenly after equalizing the fuel levels within the
Skip Burns --