QuestionMGB----2 door convertible------1.8l OHV 8v------Sept 1976
The SU fuel pump has been replaced with a facet solid state pump and a Brand new fuel tank. The factory points
ignition system has been replaced with a Crane Cams Fireball XR700 optical trigger. The ignition coil, distributor
cap and rotor are new, as are the plugs and leads.The Zenith Stromberg auto (water jacket bimetallic spring) choke
175 CD-2 single barrel carburetor has been re-built at some point in the past although not by me.
When the vehicle runs it runs well, it will run for anywhere from a couple of seconds to several minutes but it
will always die. When it dies it dies gradually by losing power then stalling, after stalling it will not re-start
for several minutes sometimes hours.
I have removed the carburetor and checked the float valve, which is NOT sticking and everything in the carb
itself is clear and free moving, I have tested the fuel pump by leaving it pumping into a gas can for a substantial
period of time (longer than the car has ever actually ran for) without pause in flow. I am at a loss as to what
could be the issue. I do have one concern regarding the the choke which was not secured and completely out of
adjustment but I do not believe this would cause the symptoms I am describing. if you have any input at all it
would be greatly appreciated- Thank you - Paul
AnswerHi Paul,
I learned early when working in MG dealerships that symptoms may lead you to a section but for the most part are useless. Testing is the ONLY method of diagnosis that works 100%.
The easiest engine to fix in the world is one that won't run. All gasoline engines only require 3 items to run. Compression, Fire and Fuel with conditions on each. Forget the symptoms and start from scratch.
Compression -- This MUST be established FIRST. A simple compression test needs to be done first because if it is not correct all the time wasted on checking fire and fuel will net nothing.
A MGB should have 125 to 165 PSI on all cylinders with little difference between cylinders. Be sure to have the throttle open when running the tests. Compression test MUST be run in the failed mode (not starting)
Fire -- Ignition can be diagnosed in a minute on a scope but most people don't have a scope so there are rough tests that will work. First is a simple test. (When it will not start) spray Starting fluid into the intake and try to start the engine. If it starts on starting fluid, You do not have a ignition problem, it is a fuel problem. If it does not try to start on starting fluid take the plugs out and inspect them and put them on the plug wires and lay each plug on a metal part of the engine and spin the starter and watch the plugs. You want to see a thick blue spark (not in bright sun light) Thick as a pencil lead and blue in color. If it is a thin thread like spark that is orange or yellow, it is a weak spark. (Don't for get it must be in it's failed mode when testing.)
Fuel -- Must be the last item tested and only if it passes the first two (compression and Fire) tests. Spray in the intake that made it run is a clear indication of a fuel problem. When you had the plugs out and examined them were they normal looking or were they wet looking? Also, when the engine was loosing power was there any black smoke coming out of the tail pipe? Black smoke is a sure sign of a rich mixture, flooding or the auto choke on. The plugs will look either sooty black either wet looking or flat black powder. If the plugs were clean looking it was most likely a lack of fuel. Jet stopped up with trash or water or a float problem sticking and not allowing fuel in. Or a dying fuel pump. Testing a fuel pump when it is running correctly is not a valid test. You MUST test a fuel pump when the engine won't start. A sure 100% test I use is to put a "T" fitting in the fuel line at the carburetor and a long hose to a fuel pressure gauge (most vacuum gauges have a fuel pump pressure section) and place the gauge under a wiper arm so you can monitor fuel pressure while driving and when it actually dies.
One more test that needs to be done and that is a vacuum gauge test. I do the same by running a long hose out form under the hood and watch vacuum at the time of power loss. Any street car engine will show at least 5 in hg under full throttle at full load so if you see the vacuum steadily go to zero while driving, you need to test for a stopped up exhaust system. (which will not usually show up in the compression test)
let me know,
Howard
