MG Car Repair: 1974 mgb gt

Question
I have a 1974 mgb gt 1.8l and i have had it turned over and running bbut when u hit the gas it wants to die out like its flooding itself out i have the book and have had a guy that works on them all the time but he always sends the carbs out its very exspensive to have the carbs rebuilt so im trying to get it going and it sat for a long time i drained the old gas new fuel filters like i said i have had it running when the gentleman was here helping he was messing with the miixture screw on the side to lean it out or richen the carbs now it will not even start back up u have to hold your hand over the carb were the air filter was and control the air flow and it could keep running but as soon as u pull the gas it wants to die out please help

Answer
Hi Charles,

Your problem does seem to be fuel related but working in MG dealerships for many years I have learned that when an engine does not run right you must go through a set procedure to have a 100% fix rate and in the dealership we had to do that.

The procedure is to first establish which of the systems has failed and there are only three systems that make all gasoline engines run. You must not try to think of the item that is the cause and first establish the system. The three systems are Compression, fire and fuel and they MUST be tested in that order. You should not try to diagnose by symptoms.

First run a compression test on all 4 cylinder. (If you don't have a compression gauge they are not expensive and can be used on all gasoline engines.) Remove all 4 plugs and remove the small wires off of the (+) side of the coil. Have someone spin the starter (with the throttle wide open) and be sure the engine spins and hits the compression stroke at least 5 or 6 times. Test each cylinder the same way. The compression reading must be from 120 PSI to 170 PSI and only a small difference between the 4 cylinders. If it does not have this it is a waste of time proceeding.

If you do have good compression on all 4 cylinders and only a little difference between cylinders then you can test "Fire" (Ignition) It does not require high tech equipment to test this.

Put all four spark plugs on their plug wires and lay each plug anywhere on the metal parts of the engine so you can see the ends of the plugs. Now reconnect the small wire/s on the (+) side of the coil and have someone spin the starter as you watch the plug spark (not in bright sun light) The spark at the plugs should be blue in color and thick as a pensil lead. If you see a hair thin Yellow or Orange spark it is not good enough to start an engine. If you have a nice blue thick spark then proceed to the next test. Move plug around so you can gain access to the #1 plug hole and yet have the #1 plug laying on metal so you can see it. Place your thumb over the #1 plug hole and have someone spin the starter and watch #1 plug spark while you have your thumb over #1 plug hole. As the engine spins it will blow your thumb off of the plug hole with a "Pop" and at the same time you will see #1 plug spark with a "Click". The "Pop" and the "Click" will be exactly at the same time and seem like one is causing the other. This confirms that the ignition timing is close enough for the engine to start. It does not mean the timing is exactly correct but does confirm that it should run.

If the compression and the ignition are as I said, only then can you look at Fuel. Don't skip any of this as it is very important to confirm the Compression and fire are corect first.

Now you can put the plugs back in unless they are very sooted up then you either need to clean and set the gap of the plugs or replace them. (Besure to put the correct wires on the correct plugs.) (firing order is 1342) You have been working with #1 so it is identified and just look on the cap and follow the order going counter clockwise around the cap 1342.

Now get a spray can of "Starting Fluid" form any parts store. Have someone try to start the engine while you give a shot or two into the intake with the starting fluid. If it tries to start but dies, try to keep it running by giving it more shots to keep it running. If you can keep it running on starting fluid but it will not run on it's own then proceed to the next test.

The 74 MGB-GT should have two SU carburetors. You should also have an SU fuel pump in the rear of the car. To test the fuel pump first remove the fuel line from the "Y" in the hose and place the fuel line down in a jar or can and have someone turn "ON" the key (Not start)
The fuel pump should start clicking in the rear and pump fuel into your jar or can. If you get a good flow of fuel into the can then that is all you need at this time. A pressure test is done later. Now put the fuel line back on and remove the air filters and put you finger in each carburetor and lift the piston inside. Each should lift with some resistance but when it gets to the top and you drop it each should hit the bottom with a light "click". This confirms that the pistons are free moving. If you feel no resistance when lifting the pistons remove the large cap on the very top and put a some engine oil in the hole and replace the cap which has a pin and a small piston on the bottom of the pin.

If every thing is correct as I stated so far and it does not run correctly it is time to work on the carburetors and if you want to go into them let me know and I will walk you through the process. It is not very difficult and requires no special tools.

Let me know,

Howard